Color Perception and Cinematography
“VISUAL PROCESSES AND COLOR PHOTOGRAPHY*
Ralph Merrill Evans
Eastman Kodak Company, Rochester, New York
* Paper presented at the meeting of the Optical Society of America held in New York City, March 5-6, 1943. Communication No. 944 from the Kodak Research Laboratories.
I. BRIGHTNESS RELATIONS
In the theory of black-and-white photography, very careful consideration has been given to the relationship required between the luminance** of areas in the object and that of the corresponding areas in the reproduction. [** Throughout this paper, “luminance” will be used to indicate the psychophysical intensity aspect of radiant energy as defined by the O. S. A. colorimetry committee (J. Opt. Soc. Am. 27, 207 (1937)). “Brightness” will be used to refer to the subjective appreciation of luminance, and “lightness” will refer to the subjective appreciation of the luminance of reflecting surfaces.
Readers of The Theory of Photography by C. E. K. Mees should bear in mind that in Chapter XX of that work the recommendations then current were followed. In the former notation brilliance was used for the present brightness and brightness was used for the present luminance.]
Great advances in photography have been made by a combination of theoretical and empirical methods. An exact theoretical correspondence of all luminances in object and print has been found not only impossible but in some cases undesirable. An attempt to reproduce the appearance of a three-dimensional colored object involves more than simply abstracting the luminance relations and excluding the chromaticity.
In many cases, chromaticity is as important a factor in the contrast of a scene as are the actual luminance differences. In the extreme case, two surfaces may be identical in luminance but have sharply contrasting colors. This problem is solved by the use of “contrast filters” which enhance or reduce the effective luminance of the different areas of a particular scene by modifying the relation between energy distribution and reproduction density. Experience has shown that these considerations are of great importance only in those cases where desired detail would otherwise be lost, as in the photography of furniture and the like. The theory of tone reproduction can be developed for any given relation between chromaticity in the object and density in the reproduction, and since these relations all become identical for the reproduction of a subject consisting of white, gray, and black areas only, this is the aspect which is usually discussed.
The aim of black-and-white photography is to produce either a pictorial or an instructional reproduction of an object in a scene. The absence of chromaticity precludes the possibility of obtaining a perfect reproduction in the sense that it might be mistaken for the object itself. Because there is no expectation that the result will look exactly like the object, it is not necessary to consider very carefully all aspects of the differences. In the tone-reproduction theory as developed by L. A. Jones and others,1 little emphasis has very properly been placed therefore, on the subjective phases of the visual processes involved and attention has been concentrated on the problem of luminance rather than brightness relationships. This work has shown that of all possible relations between the luminance of the object and that of the corresponding area in the print, a linear relationship gives the most acceptable result. None of the conclusions which follow affects the validity of this fact.
Color photography is theoretically capable of reproducing all the visible characteristics of a given scene. Except for physical restrictions on the range of luminance and chromaticity which can be reproduced simultaneously in one picture, it is not impossible to obtain a reproduction which may be mistaken for the object itself. This possibility has led many workers to assume that this is the goal of color photography and that if the reproduction were to meet the requirement that every point of the final picture shall match the corresponding point of the object in chromaticity and luminance, the picture would look exactly like the object. Experience indicates that the latter view is merely an assumption since in a particular case this condition is neither necessary nor sufficient to produce this result. It may be the only possible assumption on which to base a theory for the general case.
It is the purpose of this article to consider the subjective phases of color photography in their bearing on this particular point, in the hope that it will provide a more secure footing for theory as well as practice. Present knowledge of the subjects involved will be reviewed and their relative importance indicated by the consideration of the effects observed in the actual operation of color processes. Part I considers only brightness relations. Contrast and adaptation phenomena will be discussed separately.
Luminance vs. Brightness in Photography
One of the basic assumptions which underlies all theories of photographic color processes is that a photograph can be made which looks like the subject. It is usually postulated also that if each area of the reproduction were to match each area of the subject, such a photograph would be obtained. In cases where no such postulate is made it is at least assumed that each area in the reproduction must bear some definite relation to that of the subject.
Restricted to luminance, these assumptions imply that there is a definite expressible relation between the luminance which evokes a given brightness at any point in the scene and the luminance required to evoke the same brightness in the corresponding point of the reproduction.
For the reproduction to appear as a positive rather than a negative, at least the greater part of the brightnesses must come in the right order in the reproduction. Since the assumption states that there is an expressible relation, it would follow that the order of all brightnesses would be correct. Hence, if in the subject one brightness is greater, equal to, or less than another it will so appear in the print. Anticipating the results to be discussed, it was found that such a general relation does not exist except in special cases. Unless a scene is suitably illuminated when the picture is taken, any object in it which appears lighter than another may reproduce as equally light or even considerably darker. In other words, the order of brightnesses in the reproduction does not necessarily appear the same as it does in the subject.
At first sight this may seem difficult to believe. It is true because any satisfactory photographic process produces a fairly accurate record of the relative luminances of a subject but there is no direct relation in vision between relative luminance and relative brightness. One is tempted to say there is no relation at all. The brightness evoked by a given luminance depends on the circumstances at the time of viewing. Since these circumstances are, in general, different according to whether the subject or the print is viewed, the effect is different in the two cases and the order of the two brightnesses may actually be inverted.
In order to understand the failure of photography faithfully to reproduce the brightness of the original, it is necessary to review the lack of correspondence between luminance and brightness in vision. The fundamental question here, however, is why the print does not have the same brightnesses as the subject even when the luminances are exactly matched.
Brightness Constancy in Nature
Various “constancy phenomena” are well established in the psychology of visual processes. They may be summarized conveniently in the statement that objects in nature tend to be perceived by the mind more nearly as they exist in space than as the two-dimensional projection formed by the eye.
“Size constancy” is illustrated by the fact that people at a distance do not look smaller than those close at hand. The process by which this occurs is one of comparison, combined with appreciation of perspective. The magnitude of these objective mental corrections is large. A person at 100 yards forms a retinal image which is one-tenth the size formed at 10 yards and yet he looks scarcely any smaller – he simply looks farther away.
“Shape constancy” is frequently described by pointing out that a penny or a hoop seen at any angle except directly edge-on is perceived as a circle rather than an ellipse. Really this is a single example of the general interpretation of space as three-dimensional through familiarity with the effect of perspective. For relatively near objects, the interpretation is believed to take place through cortical fusion of differing retinal images in the two eyes. For more distant objects, it must occur through perspective. Occasionally the perception is noticeably wrong, as in the apparent meeting of railroad tracks in the distance, but normally it is sufficiently accurate to be of great aid in daily life.
Other phenomena of vision may be considered as constancy effects. Thus, we might speak of a “position constancy” of objects in a room relative to a moving observer or of a “motion constancy” of objects which are seen to be moving at the same speed although they are at various distances from the observer.
These phenomena, which are apparent even to the casual observer, are probably all learned reactions and may be understood readily by their utility to the individual and their relation to known laws of perspective and of relative motion. Perspective relations can be well reproduced by photography and it is not surprising therefore to find that, by the proper choice of the point of view and the focal length of the lens, the size and shape of objects may be reproduced satisfactorily in two dimensions. To secure the effect of depth for near objects requires stereoscopic reproduction. Relative motion phenomena in which the observer is supposed to be moving are not usually well represented by motion pictures because the observer does not have the sensations accompanying motion.
The extension of the constancy principle to brightness and to chromaticity which has been made in recent years2 rests on somewhat less secure ground. A very close parallel can be drawn between the constancy of the chromaticity and the brightness of an object and the constancy of its size and shape. For practical purposes, this is undoubtedly the most satisfactory explanation of the discrepancy between the perceived effect and the actual physical phenomenon.
On the other hand, it has not been demonstrated that it is necessary to assume psychological processes in order to explain the observed facts. In photography we are concerned with the reproduction of the visual effects since they are important factors in the recognition of objects. It does not matter greatly which approach is used in explaining them so long as it will satisfactorily include all the facts.
If a white or a light gray surface is placed in a shadow and a similar surface is placed in full illumination, the observer will have no difficulty in recognizing their identity. White surfaces will continue to look white under both conditions and the lightness of the gray will not appear to be appreciably different. Many manifestations of this fundamental fact may be demonstrated and the magnitude of the luminance difference between two surfaces, both of which may be perceived as white, is startling to the uninitiated. It is not difficult to produce a luminance ratio of 100 to 1 and yet have both surfaces readily recognizable as white. To say that their brightness is the same under these conditions would be an exaggeration since the shadowed white is always noticeably darker. It is, however, much brighter than a gray which reflects the same amount of light but is placed in the illuminated part of the field. It should be noted that brightness and lightness must be distinguished in such a case since the object which appears less bright is also perceived as lighter than the other. An extension of this demonstration is even more startling. Under the above conditions a good black in the illuminated area may actually reflect more light to the eye (i.e., have a higher luminance) than the white surface in the shadow. In this case the black has very low brightness and lightness compared to the white. There is, therefore, no direct relation between brightness and luminance in a non-uniformly lighted scene. In uniform illumination, which is rare and should be considered as a special case, lightness, brightness, and luminance always occur in the same order.
An illuminated scene has two characteristics by which it is seen. A certain percentage of the light which falls on the objects in the scene is reflected and a certain quantity of light is reflected from the scene as a whole. The first of these distinguishes one object from the objects in its immediate vicinity, and the second is the actual stimulus for the eye of the observer. If these characteristics are called reflectance and luminance, respectively, then it may be stated that the perceived lightness of a gray surface having a given reflectance at a given luminance in a non-uniformly lighted scene will always fall between that given by the same reflectance at the high illumination and the same luminance at the low illumination. Thus a medium gray surface in shadow will appear to match a gray in the light which has a lower reflectance but a higher luminance. This is the equivalent of saying that a shadowed (or less strongly illuminated) object tends to be judged by its reflectance rather than by its luminance. This is not strange when it is realized that it can only be distinguished from its background at all if it has a different reflectance.
The degree to which the brightness depends on the one factor or the other is determined almost entirely by circumstances at the time of viewing. The most important of these is the extent to which it is apparent to the observer that the illumination is different on the two surfaces being compared. Scarcely less important, and in many cases essential is a continuity of background so that the surfaces are either seen against a surface which is common to both or against surfaces which do have this relation to a common background.
The role of the observer appears not to have been thoroughly investigated. If conditions are such that the observer can make a quantitative setting, different observers will make matches varying all the way from equations of reflectance to equations of luminance. Some observers can, on request, make two settings, one of which approaches a reflectance match and the other a luminance match. Children and untrained observers uniformly tend to make reflectance matches.3
It is this latter fact, namely, that lack of training and experience tends to produce the closest approach to brightness constancy that sets this phenomenon apart from that of size and shape constancy in which the reverse is true. A psychological effect might be expected to be more complete in an adult.
A somewhat better picture of the subject and one which is directly applicable to photography can be obtained by a more careful consideration of what is meant by the terms white, gray, and black. The variables on which they depend give at least a partial clue to the results of lightness matches.
There has been much discussion in the literature concerning the nature of gray. It has generally been assumed that one or another type of retinal or cortical mechanism is involved. For the present purpose it is not necessary to take any stand on this subject but it is essential to consider carefully the facts insofar as they are known.
It has been noted above that brightness is not directly related to luminance. A moment’s consideration will show that for the same reason there is no relation between luminance and the characteristics white, gray, and black. This follows from the fact that a black surface in the light can be more luminous but darker than a white surface seen at the same time in shadow. Furthermore, this is a general phenomenon of which examples may be multiplied indefinitely. Two neutral surfaces of any reflectance can be made to have any desired lightness with respect to each other if each of them and their surroundings are separately and properly lighted. An intensely illuminated “black” appears white if there is no comparison surface, and a white may be made black or any shade of gray by simply surrounding it with a more highly illuminated white.4
White, gray, and black, therefore, have no physical existence in nature. They are descriptive terms arising from the perception of lightness. The physical property common to them all is nonselective absorption of the illumination. The proper term for the appearance of a particular nonselective body depends on the illumination and surroundings at the time it is seen. In particular, since a body of very low reflectance (black) will be seen as a gray or a white if viewed against a darker background, and a white as a gray or black if seen against a lighter one, white, gray, and black are the terms which describe the perception of relative brightnesses. In the special case of a uniformly illuminated field, they are identical with the perception of relative luminances.
In daylight and in familiar surroundings, a nonselective object with a medium reflectance is seen as nearly the same shade of gray under all conditions. The phenomenon of brightness constancy reduces to the statement that the gray produced depends on the luminance of the gray relative to its immediate surroundings. Surroundings which lie on the other side of a shadow edge or at a distance in non-uniform light are excluded so far as the immediate perception of white, gray, or black is concerned. To what extent a shift of the adaptation level of the eye produced by crossing the boundary or by centering the attention on one side or the other affects this phenomenon is not known. It can, of course, be stated that insofar as the eye tends to divide up the scene into regions of uniform illumination, perception is controlled by the relative luminances within this region. This is not the whole story, however, because in such a case it is not necessary that within that particular, small, uniformly lighted area, either a white or a black be perceived in order to recognize a gray.
The importance of these considerations for black-and-white photography does not require emphasis. Color photography is equally concerned with the reproduction of neutral areas. It is also concerned with brightness relations among the colors themselves. Little work which is directly applicable to the brightness constancy of chromatic surfaces in non-uniform illumination appears to have been published, although Helson and Judd5 have contributed definite information in this field. A little thought or a few simple experiments will quickly convince the curious that the phenomenon is not at all limited in its scope to even approximately nonselective reflectors. It is, in fact, not even restricted to reflecting bodies, being produced almost equally well by luminous areas, provided their luminance is not much in excess of that of their surroundings. Because the general case is so intimately associated with visual adaptation both for luminance and for chromaticity, further attention will not be given the subject here.
Photography of Brightness Constancy Phenomena
The photography of brightness constancy phenomena is usually considered impossible by the psychologists working in this field. Katz2 devotes a section of his book to the subject. He bases his conclusion that “photographs can never give perfectly natural impressions” on the three factors of limited visual angle, decreased size of the reproduction, and monocular viewpoint. He states that “photography provides only a uni-dimensional series of achromatic colors” and explains this on the basis that photography cannot (at least to the same extent) “utilize certain determinants which have measurable influence upon the character of normal perception.” MacLeod,6 in his exceptionally thorough and careful study of the subject, used photographs of his apparatus in at least one case to determine whether or not a true luminance match actually existed. He states that “in the reproduction … the two sides appeared … almost exactly the same” (italics are mine). In this case it is assumed that the photograph itself shows no tendency towards brilliance constancy. Readings of the resulting densities in the reproduction would, of course, have been a legitimate check for a luminance match. The assumption that if two densities in a photograph look the same they are the same, however, requires demonstration for the general case.
This assumption is important in photography because it underlies the theory of tone reproduction, i.e., that there should be an exact point-for-point match of the relative luminances in the subject and the reproduction. If two equal densities in a picture always appear of equal brightness, it is seen that this leads to a false reproduction, since the same statement is not true of objects in nature. The requirement is that the lightnesses evoked by the print be the same as those evoked by the subject, since only in this manner can a shaded white seem white and not gray or even black. The actual densities in the print are of secondary importance except for purposes of photographic photometry.
The identical problem exists in art in those cases in which an attempt is made to reproduce the appearance of a scene. The writer cannot agree with those psychologists who maintain that an artist must learn to overcome brightness constancy effects so that he can make the relative luminances of the objects in his painting match those in the scene. If the artist were to reproduce the exact relations of the luminances in the scene, then, in order for his painting to appear satisfactory, it should produce the same degree of brightness constancy in the observer’s perceptions as the scene. What the artist can and must do is to adjust the relative luminances in his painting so that the perceived lightnesses correspond to his perception of the scene as a whole. He must, therefore, train himself to see and appreciate the naively observed lightnesses rather than luminances. This is true, incidentally, whether the perceived lightnesses are due to brightness constancy or to simultaneous contrast. By a peculiarly circular process of reasoning, probably due largely to Chevreul, it is frequently stated that it is necessary to be able to eliminate simultaneous contrast effects visually in order to paint a scene realistically. Aside from the practical impossibility of doing so without visual aids, an exact luminance match of two areas will not reproduce the simultaneous contrast, if, for example, the areas of the reproduction are different from those of the scene. The requirement again is that the relative luminances be so adjusted that the relative lightnesses do match those being reproduced.
The artist is obviously in a better position than the photographer to make arbitrary luminance corrections from point to point of a picture. The photographer, however, is not entirely helpless in this respect and a photographic reproduction does not always lack brightness constancy effects.
In order to learn the nature of brightness failures in photography and, if possible, to lay the groundwork for techniques which would give more faithful reproduction from this point of view, a brief study has been made of the problem of photographing scenes which show large differences between apparent and actual luminances.
If two pictures which are identical in every respect, except that one is darker throughout the entire scale than the other, are hung against a common background, no amount of light shining on the darker one will make it look lighter than the other. A photograph of the two pictures under these conditions, however, will frequently show the opposite effect, the darker more strongly illuminated picture reproducing as the lighter of the two. If, instead of illuminating the dark picture, a shadow is allowed to fall across the scene so that the lighter picture is in the shadow, a photograph can be taken in which the light picture still looks lighter (higher brightness) in the reproduction even though it is actually darker (lower luminance) just as in the original. (The experiment is a striking one in color photography but does not reproduce well in black and white for a reason to be discussed presently.) An underlying principle is apparent here which seems to hold for all cases in which brightness reproduction is satisfactory. The shadow obviously divides the scene into two areas of different illumination. The extent to which the illumination difference is apparent appears to control the perceptions produced. This illumination difference is shown by the continuous background.
A large wall of yellow tile across which the sun threw shadows at a convenient time of day was chosen for further experiments. Large squares of gray, white, and black papers were arranged in two series. One series was placed against the wall in shadow and the other in sunlight. Photographs of this scene with a person included for reference were taken by single-lens and stereoscopic cameras in both color and black and white. […] The luminance of the shadow was so adjusted at the time this picture was taken that the second paper from the black end in sunlight matched the white in the shadow. The ratio of sunlight to shadow illumination was about 20 to 1. Visually the lightness of the white in shadow was very nearly the same as the white in sunlight.
Visual examination of the reproduction photographed by the various techniques led to the following conclusions:
(1) At least some brightness constancy was shown by all the photographs however they were viewed.
(2) The best reproduction of the visual effect was given by stereoscopic color photography. Distinctly the worst reproduction was given by a black-and-white print from a single-lens camera negative. Stereoscopic black-and-white reproduction was nearly but not quite as good as a single-lens color transparency viewed over an illuminator in a dark room.
(3) Various observers showed more variation in their description of the reproductions than they did of the scene itself.
(4) An attempt to learn to what extent the results were due to simple simultaneous contrast appeared to indicate that a large part of the effect in the poorest reproduction could be ascribed to this cause. The color transparency, however, had a large effect apparently caused by perceptual phenomena only.
This last conclusion is derived from tests made in the following manner. The strips carrying the series of grays were carefully cut out of the reproductions and remounted on neutral backgrounds which had the same density as the backgrounds in the picture. These backgrounds were so arranged that they covered the same areas and were arranged in the same manner as the shadows and sunlight in the original reproduction. The results obtained by a number of observers indicated that the lightnesses of the gray papers in the shadow in the original photograph were definitely higher than on the corresponding gray background. For example, if in the original photograph a given step in the sunlight appeared to match the white in the shadow, then a luminance nearly four times as great was required for the corresponding match on the gray background. This is not considered as complete proof that simultaneous contrast is not largely responsible for the effect, however. Very little is known about the quantitative laws followed by the brightness of adjacent areas. Until such laws are formulated it seems better to note simply that a subject which gave a brightness match with a luminance ratio of about 20 to 1 was reproduced so that a ratio of 16 to 1 appeared to match in the color transparency and that a simple simultaneous contrast imitation of the scene gave a match at a ratio of about 4 to 1.
Other experiments were made on other types of subjects with the same general results. In every case color was better than black and white and stereoscopic photography was better than single lens. In no case was there complete absence of the effect. The greatest effect observed (in a color transparency) had an area which was judged by all observers to be definitely lighter than another area but which actually had a density 1.5 greater (about 1/30 the luminance).
It is felt that such large differences justify the conclusion that brightness constancy plays a large part in photographs on non-uniformly lighted scenes. It also indicates the need for careful quantitative work to attempt to learn the conditions which give the best results and the average which may be expected under given conditions.
Some of the consequences for photography are immediately apparent. The fact that the various techniques arrange themselves in the order found is the equivalent of the statement that the more realistic the reproduction is made the greater the effect becomes.
Tests made indicate that the requirement of realism applies also to the contrast of the reproduction. In black and white, at least, prints made at too high and too low contrast levels appeared to show less effect than those at the normal level. The surroundings under which the reproduction was viewed played a large part, as might be expected. Color transparencies viewed in a dark room were more effective than in daylight surroundings.
Scenes with very contrasty lighting gave poor results in general. It is perhaps this, as much as a misunderstanding of the whole problem that has given rise in the literature of color photography to the ridiculous notion that the latitude of color film is so short it will only handle luminance ratios of 4 to 1. The actual fact is somewhat as follows: In an actual scene the luminance ratios of objects in uniform illumination may be as high as 50 to 1. If a shadow falls across part of this scene and the illumination in this shadow is one-fourth that of the brighter part, the total luminance ratio of the scene will be 200 to 1. This is about the scale of most color films. A shadow with one-fourth the intensity of the main light is also about the darkest which can be tolerated photographically because of failure of the brightness constancy effect in the print. The proper statement, therefore, is that the ratio of maximum to minimum illumination in any given scene should not be greater than 4.
Again, there is much confusion about the colors in which backgrounds reproduce. If a background is to reproduce as the same color which it appears to the photographer, it must be separately lighted to very nearly the same level as foreground objects. The brightness constancy which makes it look normal under all conditions with respect to the walls and the adjoining areas of the studio fails completely when it is seen as an unrelated area in the final picture. A gradual falling away of light from the front to the back of the scene is hardly noticeable in the studio but shows up very strongly in the reproduction.
In black-and-white work, even though the shadows are reproduced at the same relative luminance as in the scene, they appear much darker in the picture. The fault is not in the tone reproduction theory and it cannot be corrected by changing the contrast of the print. The difficulty lies in the lack of realism of black and white as a reproduction medium and the consequent loss of most of the constancy of brightness in the shadows.
The photographer can do much to improve the final result by modification of his lighting. The requirement of increased lighting for backgrounds has already been mentioned. The same principle also holds true for shadows. The principal light source must give fairly strong shadows to be convincing, that is, it must throw shadows with fairly well defined edges. Then in order that objects in this shadow may appear nearly as bright in the print as in the subject, it is necessary that the contrast of these shadows with respect to other illuminated areas be very greatly decreased by auxiliary illumination. This is the basis of the motion picture cameraman’s rule that a set should be considered as a tank to be filled with light just as a tank is filled with water and then the principal light should be added. In the extreme case there is no principal light and no brightness constancy phenomenon. It is this which is meant when “flat lighting” is recommended for color photography.
II. COLOR ADAPTATION AND COLOR CONSTANCY
Sensitivity adjustments of the eye play a large part in vision. For purposes of descriptive discussion these adjustments may be considered as consisting of two types, those for brightness and those for chromaticity. There is good evidence that this division also corresponds to an actual division of functions in the eye. The present article is intended as a brief review of the properties of the eye insofar as they depend on the energy distribution in the spectrum of the illuminant and the light from the objects in the field of view. It is therefore concerned with the adjustments of the eye for color. These are thought of as occurring independently of any simultaneous adjustment for brightness. The justification for the point of view taken comes largely from the recent work of Wright7 and of Schouten.8 Some parts of the discussion must be considered as speculative extensions of their data but an attempt will be made to point out these parts as they occur. It is felt that this new approach to the subject may, in itself, have some usefulness in the field of physiological optics aside from the application of its conclusions to the theory of color photography.
One of the basic problems of color photography lies in the requirement that the reproduction must look sufficiently like the original subject under a wide variety of illuminants and illumination conditions. The fact that such a requirement can be imposed and that to a first approximation it has been met by successful processes implies a visual phenomenon of a remarkable nature. This phenomenon, known as “color constancy,” has been variously described as purely psychological and as purely physiological. The present approach is intended to illustrate the problems which it raises for color photography. For this purpose a combination of these two approaches based on the work of Wright and of Schouten gives a description which leads to concepts useful in predicting and explaining photographic effects. No attempt will be made to make the concepts quantitative beyond indicating the nature of the variables. It is unlikely, however, that a complete theory of color photography can be successful without such an eventual quantitative formulation.
Visual Adaptation for Color
The sensitivity of the eye to light varies over a tremendous range, depending on the conditions under which it is used. It increases in dim light and decreases in bright in such a way as to tend toward a constant effective response. This process is known as brightness adaptation. Maximum sensitivity is obtained after long periods in total darkness.
A similar, although less well-known, phenomenon occurs when the eye is exposed to colored light, the color sensitivity decreasing in such a way that the effectiveness of the light as color is lost in large measure. This is known as color adaptation. The fundamental problem which must be solved before it is possible to lay down rules for color photography concerns the appearance of given color stimuli when the eye has undergone such an adaptation. If a photograph of a scene viewed and photographed by artificial light is to be viewed by either daylight or by artificial light, it is necessary not only that the photograph match the subject, point for point, for color, but also that if there should be any change in the appearance of the scene under the two conditions the photograph must undergo a similar change. It is also necessary that any mutual effects of adjacent areas be the same in the two cases and that they should change with the illuminant color in the same manner in the two cases. Obviously both the characteristics of the eye and those of the particular absorbing materials will play a part.
If the eye is rested for some time in total darkness, any effect due to previous stimulation by light wears off and the eye attains not only its maximum sensitivity but also what may be called its natural sensitivity to color. For the eye in this rested state there exists a certain range of colors which, when presented without previous eye stimulation, are accepted as achromatic. The exact quality of the color most frequently chosen is not known but there is good general agreement even though no international standard of quality has been set up. It approximates closely to what may be called noon sunlight. As a matter of convenience, it may also be considered equivalent either to the sun outside the earth’s atmosphere (having approximately the same quality as the standard I. C. I. illuminant C) or to the stimulus corresponding to the light from an equal-energy spectrum. The sensitivity relations of the color receptor system of the eye which produce an achromatic response when exposed to this color may be thought of as the normal or resting state of the eye. The color itself may be called “absolute white.”
It is not necessary here to postulate a definite receptor system for the eye nor to consider the number or nature of receptors involved. It has been adequately demonstrated that the sensitivity of the eye can be represented by sensitivity distribution functions for three assumed primary sources. Such a set of distribution functions has been standardized in the I. C. I. system of colorimetry. It will be assumed in what follows that when the response of the adapted receptor system can be represented by the same ratios of stimulus-sensitivity products as those calling forth white in the rested eye, the perception will be that of white or gray. A similar assumption is made with respect to stimuli which produce chromatic perceptions, i.e., the color perception is determined by the response due to the products of the sensitivities and the stimulus, the sensitivities being those of the three assumed receptors and the responses being modified by adaptation. For this purpose it will be necessary to distinguish clearly between the sensitivity of the eye to color in the sense of the relative amounts of energy at two different wave-lengths required to produce the same response from a given receptor and the sensitivity of the eye in the sense of the total amount of light necessary to produce the same response under two differing conditions. For the former the word “sensitivity” will be used, the latter, for want of a better term, will be called “responsiveness” but without any implications as to speed of response. That is, “sensitivity” will refer to the manner in which the eye integrates the energy distribution of a stimulus to produce a response, and “responsiveness” will refer to the magnitude of that response with respect to the integrated energy.
Wright7 investigated the adaptation process for the fovea by means of a binocular matching technique. In this work he made use of the fact, which he demonstrated, that one eye may be completely adapted to a given stimulus without interfering with the state of adaptation of the other. By viewing fields with each eye so that they are seen as adjacent, he was able to make direct comparisons between the stimuli necessary to produce identical perceptions from each eye. In brief, he found that the energy necessary to evoke the same perception in the adapted eye as in the unadapted eye is directly proportional to the energy of the light used for adaptation.
If A is the intensity of the adapting field, a is the (very much smaller) intensity of the field as viewed by the adapted eye, and a0 is the intensity of the field which must be presented to the other eye to produce a match at zero time after removal of the adapting stimulus, then A = a/a0 or Aa0 = a = constant. In his results, this relation is independent of the value of A. The equation states therefore that the responsiveness of the adapted eye is inversely proportional to the stimulus causing the adaptation. Wright found that this relation also holds for chromatic adapting stimuli and he demonstrated that the responsiveness to each of the primaries of a three-component mixture was reduced in direct proportion to the amounts of the primaries represented by the adapting light.
Wright was careful to point out that these relations had been examined only for the fovea and that they probably did not hold outside of the region over which the Fechner fraction was reasonably constant. He also found that some discrepancies exist in the blue region. He was concerned primarily with the rate of recovery of the responsiveness after adaptation and this he showed takes place linearly and fairly rapidly. The actual rates of recovery depend sharply on the intensity of the stimulus, and at high intensities the recovery ceases to be linear except in the initial stages. His work was done almost wholly with adaptation times of three minutes.
Schouten,8 using essentially the same technique, investigated the effect on responsiveness of light sources in the field of view but not falling on the region of the retina being tested. He also carefully investigated the rate of decline as well as recovery of responsiveness as determined by both the time and intensity of the adapting exposure.
Schouten’s results confirmed those of Wright except that his data do not show quite as constant a product of A and a0. For adapting radiation not falling on the region tested, he found exactly the same effect but less marked, that is, adaptation was found in adjacent regions of the retina but the extent of the adaptation was less than in that of the exposed region. The actual amount found depended on the distance along the retina and on the intensity.
The rate of recovery of the adapted region was found to depend strongly on the time of exposure as well as on its intensity. Brief exposures caused an adaptation from which recovery was extremely rapid and long times or higher intensities caused proportionally longer recovery times.
The rate of loss of responsiveness was found to be extremely rapid under all conditions, being essentially complete in a time of the order of 0.2 second. This holds equally well for adaptation in adjacent regions and in those directly stimulated.
No adaptations were found which applied to the eye as a whole. In all cases, the local areas of the retina, except insofar as they were near an exposed region, retained their normal sensitivities.
From these considerations and their abundant confirmations in the results of other workers (especially in the field of glare), a clear picture of the process of color adaptation can be assembled.
When an observer looks at a scene, his eyes are in almost constant motion. Detailed vision, however, occurs only when the eyes hesitate in a particular position. Whenever this occurs, adaptation takes place. Transference of the gaze produces a readjustment and this goes on throughout the scene. The speed of this readjustment, however, depends on the length of time that the gaze was held steady. Several times a minute, the eyes are closed for a fraction of a second.
From these facts it is seen that there are three types of adaptation which always exist simultaneously in the eye. The scene as a whole consists of brightnesses that lie between fairly well-defined limits which, in terms of the possible range for the eye, are not unduly separated. The eye accordingly does not receive stimuli outside this range for long periods at a time. Hence, there is an adaptation for the eye as a whole, controlled by something like the average intensity for the scene as a whole, from which recovery is quite slow. This may be called general adaptation. It does not exist in certain special cases, such as protracted viewing of a small isolated field. Superimposed on this are the adaptations due to the immediately effective stimuli and from which recovery is almost immediate when the gaze is transferred. These may be called local adaptations and are not noticed ordinarily unless the eye hesitates longer than usual on a particularly bright stimulus. Coincident with these is the “sideways” effect of local areas on each other, a brighter area decreasing the responsiveness of the eye to an adjacent area. This effect may be called lateral adaptation.
Each of these types of adaptation produces sufficiently different effects so that they are known in the literature as separate phenomena and they will be discussed separately under appropriate headings. All three play very important roles in color photography as well as in everyday life.
General Color Adaptation – Color Constancy
In all cases in which the eye responds to color, it may be considered as a mechanism which always integrates the spectral energy distribution of the stimulus. So far as the resulting color perception is concerned, the actual energy distribution is not a factor but only the integrals of this distribution with respect to the eye sensitivity system. The work of Wright and Schouten has shown that it is not these sensitivities which are affected by adaptation but what has been called earlier in the paper the responsiveness of the color receptors. In terms of these receptors, it is not the relative sensitivities as a function of the wave-length which have changed but the relative outputs of the receptors. This work shows further that color adaptation is the equivalent of a division of the responsiveness or output of the receptors by the integrals of the adapting stimulus with respect to these receptors.
The facts so far presented may be set up formally with respect to a three-color primary system. It will then be found that some modifications are necessary to bring them into exact accord with experience but a valuable technique is obtained by means of which complex viewing situations can be analyzed.
When the light from an illuminant falls on a selectively reflecting surface and is received by the eye, the energy distribution at the eye may be calculated by multiplying the incident energy of the illuminant wave-length for wave-length by the reflectance of the surface. The effectiveness of this light as color can now be determined by multiplying the curve separately by the three mixture curves of the system and determining the integrals under each of these product curves. The values of these integrals specify the amounts of each of the corresponding primaries necessary to produce a visual match. The ratios of these values to the sum of the three, uniquely determine the degree of divergence of the stimulus from any fixed reference point.
In the I. C. I. system as customarily used, “dominant wave-length” is defined as the wave-length at which the line connecting the illuminant point (at least when the illuminant is Illuminant C) with the calculated point cuts the spectrum locus, thereby defining a direction of divergence. The ratio of the distance along this line, from illuminant point to stimulus point, to the distance from illuminant point to the spectrum locus is defined as excitation purity and establishes the amount of the divergence. Similar concepts are used by Judd9 for his uniform chromaticity scale triangle.
Davis10 has used a term “conjunctive wave-length” which defines a spectral wave-length with which any source may be mixed to obtain a stimulus corresponding to any point on the line connecting the two.
It is apparent from the definitions and assumptions on which these terms are based that they deal purely with the psychophysical aspects of the stimulus and are not intended in any way to describe or define the psychical color perception which will result from viewing the stimulus. Two stimuli having identical dominant wave-lengths and excitation purities will match if viewed under identical circumstances. But this does not state what their hue, saturation, and brightness (or lightness) will appear to the eye.
The work briefly outlined, however, suggests the possibility of a first approximation method of calculating the perceived color of a stimulus by taking into account the condition of the eye at the time of viewing.11 From such calculations the mode of functioning of the eye can be made apparent, and calculations can be made of the probable change in appearance of a stimulus when it is seen under different eye conditions. This leads to a direct determination of the condition for “color constancy” of stimuli (again to a first approximation only, since psychological as well as psychophysical factors are involved in many complex viewing situations).
Wright’s work indicates that the relative responses of the eye receptors under a given set of conditions can be obtained by dividing the integrals of the stimulus with respect to each receptor, by the integral of the adapting luminant with respect to the same receptor. On careful consideration, however, it is apparent that there is only one possible set of sensitivity distributions for three primary receptors for which this result will hold. Wright determined such a set and referred to them as the “fundamental” sensitivities of the eye. Walters12 has since redetermined them in the red and green regions and has thrown some doubt on Wright’s values in the blue. The possibility of determining the set has, however, been demonstrated as has their similarity to the “grundempfindungen” curves of König and Dieterici.13
Let X, Y, and Z represent the integrals under the curves obtained by multiplying the red, green, and blue receptor sensitivities, respectively, by the energy distribution of the stimulus. Then, let XSD, YSD, and ZSD, be these integrals for a sample S and illuminant D, and XD, YD, and ZD be these integrals for the adapting illuminant (also D). Under the assumptions, these values are linear functions of the receptor outputs; the relative outputs then become XSD/XD, YSD/YD, and ZSD/ZD.
If a Maxwell triangle be constructed based on X, Y, and Z for “white” light (conveniently the equal energy spectrum), then these values determine a point for any stimulus and adaptation condition (illuminant quality being taken the same as the adapting radiation). This point is calculated in the usual manner:
and similarly for YSD/D and ZSD/D.
For this Maxwell triangle, the illuminant point is always at the center for all illuminants, but a different spectrum locus is obtained for each illuminant.
A line from the center through a stimulus point cuts the spectrum locus for its illuminant at a point which has the same significance as the “conjunctive” wave-length of Davis. Purity, if desired, could be defined as usual by the relative distance of the point and its spectral locus. In these respects, the triangle so constructed is no different from any other possible triangle. The possible advantage arises from the fact that under the assumptions each point of the triangle is invariant with illuminant. That is to say, if two illuminant-sample combinations calculate to the same point, they will be perceived as identical when each is seen under its own illuminant. This property is not possessed by a triangle based on any other primaries and holds for this one, of course, because of the method of calculation.
It accordingly becomes possible, by means of this construction, to tell whether or not a given sample will appear as the same or different under two different illuminants. The sample is simply calculated for both illuminants, and if the points superimpose, the sample will appear the same; if they do not, it will appear different.
Having gone this far, it may not be out of place to point out some other possibilities inherent in this construction, but it must be kept in mind that the method described is inherently a first approximation. For example, the invariance of the illuminant point indicates that under all conditions, when the eye is adapted to an illuminant, it will appear achromatic. This is known not to be true but it does hold to a first approximation for continuous sources of an energy distribution not too far from that of a blackbody (daylight and tungsten light, for example).14
It also is in error within a given illuminated area if there is any great intensity range involved, since under these conditions Helson15 has shown that the brightest achromatic objects appear tinged with the color of the source (as seen under approximate daylight adaptation) and the approximate complementary of this in the dimly lighted regions. These effects, however, are large only for strongly chromatic sources, so that again a fair degree of approximation is indicated.
With these limitations in mind, it is interesting to see how the result could be interpreted if a sample in two illuminants gave points which did not superimpose. The problem in this case is to determine the proper interpretation of the direction and magnitude of the divergence. Neither the constant hue nor the constant saturation lines will be straight in such a triangle and these attributes in general will shift with intensity. However, since every point corresponds to a given perception, each point may be correlated with a known standard as seen under known conditions. A very suitable set for this purpose would be the Munsell system as seen under daylight conditions against a background of a given reflectance. This set has already been correlated16 with the I. C. I. system, and it would only be necessary to obtain the conversion equations for the two systems. It would then be possible to define directly the shift in appearance of the color by specifying the two appearances in the Munsell system.
A further interesting point is the appearance of the monochromatic stimuli under various adaptation conditions. The appearance of these, relative to daylight, shows directly on the triangle, since the plots of the loci for different adaptations can be compared directly with that for daylight. Purities higher than unity are readily obtained in some regions, such as the green, while other points, such as the extreme red, are invariant.
As has been pointed out repeatedly, this whole construction is an approximation, and as such, it must be compared with others experimentally before it can be determined whether or not it is the best one. This is true whether or not it does portray the mode of action of the eye receptors correctly.
Judd has pointed out,* for example, that the work of Adams17 on the x–z coordinate plane (I. C. I. system) has given promise of accurate correlation with the known facts, and it may well be that this will be found to be a better approximation. They cannot be the same since different receptor systems are assumed, and if Wright’s work is correct in the main, the x–z system cannot correctly portray the action of the receptor system.
* Private communication.
The postulated mode of action of the eye and the use of the fundamental primaries permit a mathematical statement of the conditions for complete “color constancy” of a sample-illuminant pair.
Using the same nomenclature, but introducing two illuminants D and D’ and considering the x value only (exactly similar considerations hold for y and z), the “condition” may be set up formally as follows:
It is seen that these expressions both reduce to xS (i.e., the value for the equal energy system here defined as “white”), if XSD = XS · XD and XSD’ = XS · XD’. In other words, if the product of the integrals of the distributions integrated separately is the same as the integral after the products have been performed, then the color appears the same under both illuminants. This condition is obviously met by definition for all nonselective surfaces. For all other surfaces, the surface absorption distribution and the illuminant energy distribution play equally important parts.
For the most important case (D = daylight and D’ = some continuous distribution which is not greatly different), the degree of approximation of this equality is quite good for most ordinary reflecting surfaces since most surfaces have broad, gradually sloping absorption curves. For highly selective surfaces or for discontinuous or sharply selective light sources, the approximation may become very poor indeed, leading to a distinct change from the daylight color. The generally good approximation of most surfaces leads to the concept of “approximate color constancy” in everyday life. The poor approximation under certain illuminants gives rise to the large shifts of perceived color seen, for example, under many of the commercial fluorescent light sources.
Another interesting situation arises when the adapting illuminant is a mixture of two light sources of different chromaticities. Regardless of the chromaticities, the mixture will be represented by the achromatic point at the center of the triangle, and the points representing the two sources will be collinear but on opposite sides of the point. Since the points on this triangle may be interpreted as representing the appearances of the stimuli, and since collinear points on opposite sides of the center are complementary in daylight, it follows that, regardless of the nature of the two sources, they will appear in the customary hues of complementary colors. This is the well-known phenomenon of complementary shadows.18 The effect is so powerful that two-color additive pictures can be made to produce a satisfying greenish blue by projecting one picture through a yellow and the other through a red filter. In this case, the yellow becomes the blue to the adapted observer and the red becomes a complementary orange.*
* Demonstrated by the writer before the Optical Society of America, March 5, 1943 in New York.
A similar consideration holds also for the calculation of local and lateral adaptation effects. The results are obviously similar to these and the effects are well known. Local adaptation to a chromatic surface followed by viewing of an achromatic one leads to a complementary “after image.” If the second surface is also chromatic, the result is an apparent “mixture” with the complementary. These effects, of course, are familiar as “successive color contrast” phenomena. Lateral effects produce the same result. Adaptation of a local area by a chromatic surface leads to a decreased responsiveness of the adjacent areas. If these areas are receiving light from a second surface, the perceived hue is shifted. These effects are mutual and give rise to the phenomena of “simultaneous color contrast.” These effects will be considered in more detail in a later section.
In general, in viewing a scene, first one color and then another affects the eye as it moves about. All the colors will be modified by the illuminant. For most of them, color constancy will be good. The general adaptation condition of the eye, therefore, will approximate that of the light source at an intensity level considerably below that of the source itself. For some special scenes, this will no longer be true. Consider from this standpoint a simple two-part field, in which a small area is seen surrounded by a much larger one which nearly fills the retinal area. If the central area is colored and the larger one is achromatic but the whole field is illuminated by light of non-daylight quality, two extreme cases arise. If the background is black and the eye moves freely about, there will be very little general adaptation and the eye will remain in its normal color state. The patch in this case will have the same appearance as a surface in daylight having a reflectance distribution equal to the product of the reflectance of the surface and the energy distribution of the actual illuminant. That is, it will be seen as a stimulus having the energy distribution of the product curve of the surface and the source. If the background is white, however, the eye becomes generally adapted to the color of the illuminant and the central patch now looks the same as it would in daylight, so far as the color constancy properties of the surface and illuminant permit.
If both the central and the surround patches are colored, the adaptation approximates that caused by the surround. As just shown, this causes the effective stimulus from the small central patch to act as though its product integrals had been divided by those of the surround. This is the customary movement of a color in the direction of the complementary of the surround which is seen in all simultaneous contrast effects.
These points are mentioned to distinguish the effect of general adaptation from that of lateral adaptation or simultaneous contrast to be discussed in the next section.
Lateral Color Adaptation – Simultaneous Color Contrast
There is considerable confusion in the literature concerning the change in the appearance of one color when another is placed adjacent to it. Since almost all color vision involves this phenomenon, it is extremely important that a unified approach be available. These considerations on general adaptation plus the work of Schouten mentioned in the first section make such an approach possible. They do little, however, to explain or define the almost indeterminate nature of any complex viewing situation except to make the effects understandable.
Schouten showed that adaptation of any local area produced a corresponding adaptation of adjacent regions, the magnitude of the effect decreasing with the distance from the exposed area. The responsiveness of the receptors is accordingly depressed nearly as much, immediately adjacent to as in the region of the exposure. The effect, however, decreases fairly rapidly as the distance from this area increases. The effect at any given distance is roughly linear with the intensity of the light in the exposed area. Recovery, as before, depends on both the time and the intensity of the exposure.
There are two general effects to be observed from the point of view of color perceptions. First, because this lateral effect tends to spread out the effectiveness of a bright spot of light over the retina, it combines with the eye movements to produce a more uniform general adaptation. Second, the depression of responsiveness in adjacent areas modifies the perception of the chromaticity of an adjacent stimulus. If two areas are of comparable size and brightness, this effect is mutual. If one is more saturated than the other, or particularly, if it surrounds a smaller area, the effect is seen primarily in the smaller or less saturated of the two.
It will be noted that this effect is exactly the same in many cases as the effect of general adaptation in a two-part field, i.e., adjacent colors tend to move each toward a mixture with the complementary of the other. The lateral effect, however, is purely local and acts quite independently of that due to general exposure. If an achromatic area is seen surrounded by a color, there is a strong tendency for the achromatic part to take on the color of the complementary to the surround and this is independent of colors in other parts of the field. Even in the simple two-part field consisting of a small patch surrounded by a much larger one, lateral adaptation may be demonstrated. Constant eye motion from point to point over the whole field will show a very strong adaptation effect on the central field while fixation of the eye on the central spot will be found to show a much diminished effect which frequently falls off toward the center of the area. Involuntary slight displacements of the fixation point will reveal sharp-edged adaptation regions directly underlying the image of the surround. Under these conditions the diminished induced color of the central area is due to lateral adaptation while the bright edges are due to the local adaptation which will be discussed in the next section.
Lateral effects may obviously be calculated by the same technique used for general adaptation as was pointed out earlier. In this case, the integrals by which the responsiveness of each receptor is divided must be modified to allow for the effects of distance, intensity, etc. Since it has been shown that this approach in itself is qualitative, it is hardly worth while to set up the equations by which such effects may be calculated. They follow the same procedure as was considered for general adaptation but their validity is limited to the instantaneous condition existing after each eye movement. That a colored surround about an achromatic area produces a complementary color sensation in that area follows from reasoning similar to that used for shadows illuminated by two colored sources. If the responsiveness of the eye to green is decreased, an equal energy stimulus will cause a greater response from the blue and red and hence will look magenta, complementary to the green.
This lateral type of adaptation is important to color photography in two ways, first, as it affects a print as a whole, owing to comparison with the surroundings, and second, as a relative area function within the print. Little exact information seems to be available about this latter function as far as color is concerned. Two definite qualitative effects are known but require investigation before they can be stated quantitatively. The effectiveness of a given area of color is greater as such, i.e., the color appears more intense the greater its area. This is probably due to a diminished brightness contrast effect from the surround in most cases and will be discussed in a subsequent article. From the work of Schouten, the smaller the area the more it is affected by the surrounding colors. The combination of these two effects implies that any exact color reproduction of a scene must meet the requirement that the individual parts subtend the same angle at the eye as the original scene or be suitably modified in accordance with these effects in order to give a convincing likeness. No single law has yet been deduced for such a modification but practical experience has indicated the necessity that it be done.
It is important to note that simultaneous color contrast effects can be observed equally well when the intensity of the adapting stimulus is so low that it does not, by itself, produce a chromatic perception. A saturated but dark green surround will make a small central patch of gray look magenta at intensity levels which are so low that the surround appears black.19 It is because of this and similar facts that a separation of brightness and color adaptation is justified.
Although little or no work seems to have been done in this field, it must be true that for relatively large visual angles simultaneous contrast effects vary considerably with the particular area which is being fixated, with the length of time the area is fixated, and perhaps even with the particular aspect of the object to which the attention is directed. It is certainly true for brightness considerations and it seems likely to be true for color in general. A very interesting feature of all simultaneous color contrast work and one which deserves more attention than it has received is the fact that the amount of “induced color” is partly under voluntary control. In this aspect, the subject passes over into the field of pure psychology. One example will be given to make clear the type of phenomenon discussed, although many others could be cited.
If a series of sheets of gray paper are interleaved with green in such a way that only a small strip of the gray sheets is visible, the following situations can be verified easily. If the pile of sheets is laid on a large black background, the gray sheets appear a strong magenta. If the pile is placed on a large gray sheet, this tendency will be much decreased, especially if one of the gray sheets is on the bottom. Even on the black background, however, the effect can sometimes be made to disappear completely by simply laying a narrow gray strip across the pile so that it is available for direct comparison with all the strips. This effect, in common with many others (such as the fact that a coin seldom shows any trace of color contrast regardless of its background), may be summarized by the statement that color contrast may be inhibited if the actual situation is sufficiently obvious.
Local Color Adaptation – Successive Color Contrast
The remaining type of effect produced by color adaptation is that due to momentary viewing of the scene. Since all adaptation is local, the phenomena of this phase of the subject are distinguished almost entirely by the rapidity with which the eye recovers its responsiveness. Fixation of the eye on a particular area for a brief time, followed by transference of the gaze to another surface, gives rise to characteristic “after-images” of the first surface. The phenomenon as a whole is known as successive color contrast. The color sensation produced is indicated qualitatively by considering the second surface from the standpoint of receptors whose responsiveness has been modified by the integrals of the first surface. These effects wear off very rapidly and not at equal velocity for the different receptors, giving rise to a phenomenon known as the “flight of colors.”
Such effects are not ordinarily observed in daily life, partly because they are transient but more perhaps because the blinking of the eyelids permits time for almost complete recovery from all but the strongest effects.
Successive color contrast again gives evidence of the independence of brightness adaptation and color adaptation. A colored stimulus whose intensity is too low to cause the sensation of color will produce a complementary colored after-image when the gaze is transferred to a brighter achromatic surface.19 This fact, incidentally, seems significant in connection with some forms of color-blindness. Green-blind people are sometimes able to see a brilliant red after-image following stimulation by green light which to them appears gray. (This has been found to be true by MacAdam* in the case of an almost complete dichromat, a deuteranope.) This implies that the cause may be a loss of responsiveness of the green receptor for chromaticity while retaining its responsiveness to brightness. It is, perhaps, the responsiveness of the green receptor system to color relative to its responsiveness to brightness, rather than its color-sensitivity distribution as such, which is abnormal.
* Private communication.
Indeterminacy of Color Perception
In the preceding sections an attempt has been made to clarify the bases on which predictions may be made about the appearance of a given color stimulus. It is apparent that for a given material with a given spectral reflectance, in surroundings whose characteristics are completely known, a good estimate may be made as to its appearance, especially with regard to the effect of a change in the illuminant. In the actual perception of color, however, little actual determinacy is apparent. If an observer is given an object with a dark desaturated color in artificial light and asked to describe its probable appearance in daylight, he will have considerable difficulty in reaching a decision. He will ultimately place it in the strongest light available, perhaps next to a white, and then guess. The reasons for this are evident. Few objects show exact color constancy and the exact adaptation condition for the eye depends on its recent history. The brightest possible light and direct comparison with white are the best conditions attainable.
The fluctuation of apparent color with eye movement is well illustrated by small color patches mounted on large white and black backgrounds. This observation has been carefully considered by Judd.20 Because it offers a nice check of the theory that absolute white represents the natural ratios for receptor responsiveness, an interesting experiment was tried. A colored paper showing poor color constancy was chosen. […] A 2-inch square of the paper was mounted in the center of an 11- by 14-inch white card and another on a similar black card. In artificial light, these two showed a strong tendency to fluctuate in appearance as one looked from one to the other while holding the cards in the hand. The piece on white was usually seen as a slightly purplish red and showed the least variation, whereas the one on black varied from a distinct orange through orange-red to an occasional instant during which it appeared to match the one on the white. The causes of this fluctuation have already been discussed in terms of the changing adaptation of the eye. It should follow, however, that if this explanation is true there should be no such chromatic fluctuation when the cards are viewed in light of quality approaching that of absolute white. This was found to be the case. In good daylight, no fluctuation was observed, the two patches consistently appearing to match for color. It is again interesting to note that most observers reported they could see no difference in the relative lightnesses of the two under the two conditions although under both conditions some lightness fluctuations were observed, and of course the one on black appeared the lighter of the two.
This indeterminacy of color vision does not cause us concern; in everyday life we are accustomed to thinking of most colors as not changing at all. This is in large part due to the tendency to remember colors rather than to look at them closely. For the most part, careful observation of stimuli is made only by trained observers. This same tendency also operates favorably in viewing color photographs. It is seldom necessary to obtain exact color reproduction of a scene to obtain a satisfying picture. It is necessary, however, that the reproduction shall not violate the principle that the scene could have thus appeared.
The Problem of Chromaticity Reproduction
The basic first-order problem of color reproduction may now be restated briefly. The effective chromaticities of the subject must be reproduced by areas which have as nearly as possible identical effective chromaticities. For scenes in daylight for which the reproduction is to be viewed by daylight, only the usual chromaticities with respect to the C point are involved insofar as eye adaptation phenomena are concerned. If the print is also to look satisfactory in artificial light, the dye system of the process must show maximum color constancy. This sets a severe limitation on the sharpness of cut of the dye absorptions which may be used and this, in turn, tends to limit the saturation of the colors in the reproduction. The constancy requirement is especially severe in respect to the absorption curve of the mixture which represents gray. For the general case of a process which is to be viewed under any illuminant, the dyes must mix to a neutral which is spectrally nonselective in order that the grays will not change color with that of the viewing light.
If a scene is to be photographed by artificial light so that the reproduction will look as though it had been photographed by daylight, an additional set of requirements are added, not all of which can be met by a three-color system. Achromatic surfaces in the subject must be reproduced as nearly by achromatic deposits as the process permits. In a three-color process, each color being controlled by a separate photographic emulsion, each emulsion has the same pairs of sensitivities as those already considered for the visual mechanism. Each has a wave-length distribution of relative sensitivity and each has a responsiveness to energy ordinarily called the “speed” of the emulsion. In order that an achromatic surface illuminated by artificial light may give a neutral deposit, the relative values of responsiveness or speed of the emulsions must be adjusted so that the effective response is the same to artificial light as would be required to give the same deposit by daylight. The situation is exactly analogous to that in visual color adaptation and produces the same type of result. For a colored surface, therefore, the integrals of the product curve of the light source and the surface are divided by the integrals of the light source itself to specify the ratios of dyes obtained. The integrals in this case are those with respect to the spectral sensitivities of the film rather than the eye. (They would produce the same result if the film had the “fundamental” sensitivity distributions of the eye receptors.) Surfaces which, in general, show good visual color constancy will normally be fairly well reproduced. Colors showing poor color constancy, however, will not only photograph differently than they would appear in daylight but also differently than they appeared in the artificial light. For these colors, therefore, reproduction will be poor. This is, however, the only type of solution possible. The widespread desire to photograph scenes by artificial light makes it necessary for manufacturers to supply two kinds of film “balanced” for the two types of light source. The fact that most surface colors show good constancy plus the various factors discussed later which frequently aid visual acceptance makes such a practice almost entirely satisfactory. An occasional attempt to reproduce a brilliantly colored fabric or flower may be a complete failure in the same process. Viewing the reproduction by artificial light, of course, does not correct the situation since the color constancy failure of the dye system is entirely independent of that of the original subject.
One other case may be considered here. If a scene is to be photographed in artificial light so that the impression of artificial light is given when the reproduction is viewed by daylight, a serious dilemma is encountered. Under artificial light, as pointed out, the source itself does not lose all of its visual color. Artificially lighted rooms (using incandescent bulbs) do not appear to be illuminated by white light but by a distinctly more yellow light than daylight. This may be due to a nonlinearity in the adaptation mechanism, but this same sort of effect could be introduced into the film since the linearity of response with exposure is more or less under the control of the manufacturer. Such film, however, could not also be used to represent effective daylight and would have no exposure latitude whatever. For these reasons, and because it is usually possible to simulate artificial light by means of lamps, etc., in the picture, such films are not manufactured. Where necessary, lamps of lower color temperature than that for which the film is balanced can be used for local lighting to suggest the required effect still further.
This discussion indicates the most serious single limitation of present-day color photography. As the observer shifts from scene to scene and even from point to point in one scene, the color adaptation of his eyes is constantly shifting. He is aware of this only if his attention is called to it and even then can observe it only because of the slight residual source color which the eye does not eliminate, or because of memory of the daylight appearance of some color which shows poor constancy. This is not true of the photographic film with which he takes pictures. Two types of film are available but each of these corresponds to one and only one color adaptation condition of the eye. Unless the observer’s eye adaptation condition corresponds to the illuminant for which the film which he is using is balanced, the reproduction will not correspond at all to what he sees.
The points which have been raised in this section are independent of the relation between the sensitivity distributions of the emulsions used and those of the eye. For the sake of completeness it might be well to point out that in addition to the above effects unless the emulsion sensitivities are valid transformations of eye mixture data with respect to the dye primaries of the process, practically no point for point chromaticity matches will be obtained under any conditions except perhaps in the neighborhood of gray. Chromaticity matches in the subject do not, in general, exactly match in the reproduction in any known photographic process. It is doubtful if such a process is possible.
If the equivalent of color constancy shifts are to be obtained by changing the speeds of the emulsions, then the emulsion sensitivities must be identical with those of the eye.*
* It is interesting that the late F . E. Ives always insisted that the sensitivities of the three emulsions in a three-color process should exactly correspond to the fundamental sensitivity distribution curves of the eye receptors. The present analysis confirms this view for the general case.
Commercially available color processes, however, have been developed empirically to give the best possible compromise under the conditions to be encountered. The success of these compromises measures the commercial success of the particular process. As a matter of fact, the errors due to this cause are far less serious than those caused by the frequent failure of the photographer to use illumination of the correct color.
Reproduction of Simultaneous Contrast Effects
The preceding section considered the factors involved in obtaining a reproduction in which the effective chromaticities of each area, considered by itself, matched the effective chromaticities of the corresponding area of the subject. When each area is considered also in relation to the adjoining areas, the relative lateral adaptation or simultaneous contrast effects must be taken into account. This problem is encountered when it is desired to make a number of reproductions of the same subject of different sizes so that they will all look alike, as well as looking like the original subject.
As pointed out before, only a few directly applicable data are available. Experience with color processes, however, has shown a number of essential requirements. Stated briefly, a small print must have both higher color saturation and higher contrast if it is to compare favorably with one five or six times the size. The required difference is so large, in fact, that if contrasts suitable for small pictures are used for large ones, the quality is inadequate.
At first sight, this is surprising since it is generally accepted that simultaneous contrast effects are greater on small areas than on large, and the intention is to produce the same effect as that produced by the subject. The increase with small areas, however, has been demonstrated only for cases in which the test area is reduced and the inducing area is retained at full size. The fact seems to be that when all the areas are reduced proportionately the effect is reversed. Painters of miniatures have also encountered the same phenomenon. In this type of painting, the maximum contrasts possible with pigments can be used with impunity in places which would ruin a larger picture.
Effect of Viewing Conditions on the Apparent Quality of Reproduction
Isolated Field Viewing
In the preceding section it was implied that a number of conditions under which reproductions may be viewed tend to aid the observer in seeing the reproduction as better than it actually is. The first and, perhaps, the most favorable of all viewing conditions is the familiar projection of a picture on a screen in a darkened room. In such a situation, the eye readily adjusts its adaptation conditions to that of the average of the picture on the screen. The result is roughly the equivalent of a correction in the taking of the picture to the most suitable light source for the material since the same scene also controlled the photographer’s eye when he took the picture. The correction is not complete and a trained observer can judge the daylight appearance of a picture with fair success. The apparent magnitude of any errors present, however, is a small fraction of what would be seen if no shift of adaptation occurred.
Even a person who is familiar with this effect and with the enhancement of quality it produces is often surprised at the order of magnitude of the possible change. In terms of the color temperature of the projection lamp, a picture with a large viewing angle will appear acceptable at temperatures from below 2000°K to above 10,000°K, or roughly, say, from that of a kerosene lamp to that of a blue sky. Under any of these conditions, the observer is aware of the color of the light but the difference from daylight seems small and the pictures are entirely satisfactory. Observable distortions are actually more serious from the standpoint of relative brightness than from hue or saturation. At the blue end of the range, reds appear quite dark and, conversely, blues are quite dark at the yellow end.
The question is sometimes raised as to why it is necessary to have both a daylight and an artificial light type of film if such visual compensations can occur. As a matter of fact, such pictures can be taken but the latitude required of the color process is beyond the range of materials at present available. Such pictures are also useless unless projected in a completely dark room.
The subject has not been investigated thoroughly and acceptable variations are obviously a matter of personal taste. It is certainly true that relatively large errors in over-all color balance in any direction make little difference in the enjoyment of pictures under these conditions. Most observers are not even aware that such differences exist.
The nature and to some extent the magnitude of the phenomenon can be demonstrated by projecting a fairly wide border of light around the picture. This border will tend to control the condition of the eye and the pictures will then be seen under more or less constant conditions. Two phenomena are observed. Any appreciable deviation from correct color balance in the picture is immediately detected by any observer. If the color of the border is different from that of the projection light, all normal pictures look badly off in the complementary direction.
If one picture on the screen is quickly changed to another, the local adaptation caused by the first picture determines the appearance of the second. In many cases, e.g., in motion picture work, this limits sharply permissible variations in color balance. While it is true that the eye quickly adjusts itself to the new scene, the color changes may become so irritating as to spoil the picture for the observer. It is interesting that the requirement is lack of difference from one scene to the next. It is not equally important that the balance be correct for daylight viewing unless a surround is present. In the projection of slides, simply moving the slide carrier slowly rather than rapidly will frequently give enough time for the eye so that a large color difference will not be noticed.
Viewing in Relatively Dark Surroundings
An intermediate case between projection in a dark room and projection with an illuminated border is of considerable interest and importance. Large transparencies, intended to be viewed over an illuminator, are customarily seen under conditions in which the illuminator is by far the brightest area in the field of view. Under these conditions, and to the extent that this is so, the color constancy effects mentioned will correct improper color balance, provided the illuminator is completely covered with the transparency. If part of the bright area of the illuminator is uncovered, the picture will be seen more or less correctly with respect to this color. Hence, if the illuminator has roughly a blackbody energy distribution, its apparent color temperature does not matter over the range of 2400°K to 5400°K, a range smaller than that possible in dark-room projection. The extent of the compensations which occur, even under these apparently adverse conditions, is occasionally difficult to believe. If two large transparencies, for example, are deliberately so adjusted that one is correctly balanced and the other is distinctly green, and these are shown to an observer, one at a time, over the same illuminator, he may be completely unable to tell which one is correct. After the observer has viewed the green transparency, the other will at first appear quite magenta. Careful study will convince him that this one is quite good and the first will now look very green. This change of local adaptation is particularly noticeable if the two transparencies are now seen side by side. Under these conditions, adaptation changes to an intermediate point and neither of them looks acceptable. It may be stated as a general rule that if two pictures with even slightly different color balances are seen, side by side, an observer will state that the correct value lies “halfway between.” This is frequently true even when both pictures are off balance in the same direction.
What has been said of transparencies applies also to large reflection prints, provided they are viewed in a sufficiently isolated position and are more brightly lighted than their surroundings.
Viewing in Normal Surroundings
If the reproduction is to be viewed while held in the hand, or is seen among equally illuminated and familiar objects, almost all these effects disappear insofar as they are beneficial to the picture. The observer then sees the print simply as one of the objects in the field of view and his eye is adapted to the general illumination. Under these conditions, a critical observer can and does note errors in the over-all color of a picture with a precision comparable to that which can be obtained in a two-part photometric field. Provided the “over-all” color or color balance is correct, the observer will be satisfied with the color reproduction of the subject if it meets one very interesting requirement. The process must be so adjusted that the colors are internally consistent. This “consistency principle” cannot be stated rigorously at the present time. Roughly, it requires that no hues which are familiar be badly off color and that the lightnesses and saturations of the colors each bear the correct relation to those of the subject. Neither saturation nor lightness need be equivalent to those of the subject but the rendition of some colors must not be better than that of others. In general, it is less desirable to have good reds and poor greens than to have both poor. If these conditions are met, the eye sees the colors as closer approximations to those of the scene depicted than they actually are. Borders around the picture enhance this apparent fidelity to the original by setting the print apart from its surroundings.
It is significant in this connection that the people who are daily engaged in working with a particular color process are perhaps the worst possible judges of its fidelity of reproduction rather than the best, as might be expected. This does not result from any feeling of partiality toward the process, as is sometimes imagined. Unconsciously, they learn to see the reproduction as though it were correct. Examiners checking the quality of prints in mass production have to pick up a print from time to time and move about the room or carry it to a window to prevent the occurrence of temporary sets in their judgment. This is especially true if all the prints in a given batch are running slightly off color in one direction.
The facts stated above have been obtained empirically by direct experience with color photography in large-scale production. The approach to the visual phenomena involved in color photography which has been used in the first part of the article was developed through an attempt to explain these photographic effects and bring them under control. Postulation of purely mental phenomena to explain apparent discrepancies between stimuli and perceptions has been avoided deliberately. It would be a mistake, in the writer’s opinion, to assume that no such effects are present. It is hard to explain on purely physiological grounds, for example, why familiarity with a poor print may so greatly improve its appearance, regardless of viewing conditions. More information is needed, however, before such effects can be generalized constructively.
As stated at the beginning, brightness has been treated throughout this part as independent of chromaticity. Some justification for this has been given. Since a consideration of brightness involves the subject matter of the previous part as well as introducing much new material, the discussion of this phase of the subject will be presented separately.
Appendix to Part II
The colorimetric discussion in the central part of this article was originally written in terms of the I. C. I. Colorimetry system. Dr. MacAdam kindly pointed out to the writer that this system is not directly applicable, because only one system can meet the requirements laid down, and it is not likely that the I. C. I. primaries constitute such a system since they were chosen merely for convenience in computation. In support of this, he submitted the following demonstration.
If a stimulus matching a mixture of R, G, B units of a certain set of primaries has the same appearance to an observer adapted to RA, GA, BA as does another stimulus R’, G’, B’ to the same observer when adapted to RE = 1, GE = 1, BE = 1, and if R’ = R/RA, G’ = G/GA, B’ = B/BA then this is the only set of primaries for which the tristimulus values of these adaptively equivalent stimuli are connected by equations of this form.
Assume another set of primaries for which the tristimulus values are r, g, b. Then, in general,
Without loss of generality in this discussion:
Then the first adaptation is to:
and the second adaptation is to:
The second stimulus, R’, G’, B’, is specified in the new set of primaries by:
But, by the use of a rule of the form stated in the theorem, the adaptively equivalent stimulus would appear to be
The discrepancies between the tristimulus values of the actual equivalent and that predicted by this misapplication of the rule are:
Since rA is in general not equal to GA or BA, the discrepancy r” – r’ can be zero only if a12 = a13 = 0. Similarly, g” – g’ can be zero only if a21 = a23 = 0 and b” – b’ can be zero only if a31 = a32 = 0. Therefore, the discrepancies cannot be all zero unless the second set of primaries is identical with the original set and r = R, g = G, b = B in general.
As a consequence of this demonstration, Wright’s curves have been taken as the standard for the discussion because by the method of determination they automatically meet the requirements laid down. Dr. MacAdam’s demonstration implies that they are the only ones that can.
III. EFFECT OF ADAPTATION LEVEL
One of the most common visual experiences of everyday life consists of a change in the quality of colors with the amount of illumination. This phenomenon is particularly noticeable in work with color photography, in which the proper illumination may pass through a sharp optimum value. Since the practical aspects of the subject have been discussed very little in the literature of color vision, a qualitative review of the subject, combined with an attempt to link up the known facts with those of brightness and color constancy, may not be out of place.
Many people have observed that colors appear “more brilliant” in summer than they do in winter, especially in the northern latitudes. The same difference is observed between colors viewed on a clear and an overcast day. The effect appears to be due to the difference in the illumination levels. Perhaps the best known example and one which is familiar to all readers is the difference in the appearance of automobile colors (more particularly those used ten or fifteen years ago) when illuminated by sunlight and when seen on dull days. Many of these colors could not be distinguished from black even with a comparison black beside them unless the illumination was fairly high. In sunlight, however, the various colors were seen as well-defined hues, some of them having surprisingly high saturation.
Such phenomena give direct evidence that color as seen depends directly on the illumination intensity aside from all considerations of its spectral energy distribution. It is the purpose of this article to review several aspects of the dependence of color on illumination intensity and try to interpret the facts so that they will explain the observed results in color photography. It is intended merely to separate and indicate the variables in a qualitative way in the hope it may lead to fruitful quantitative work.
Photography, like painting, attempts to reproduce a given scene, not only in form and perspective but also in appearance. This means that, regardless of the nature, color, and amount of the illumination of the original scene (of which only part is recorded), the reproduction should result in the same visual perceptions in an observer, irrespective of the conditions under which it is viewed. Unlike painting, photography attempts to do this with a relatively inflexible technique in that individual areas cannot be arbitrarily modified. Obviously, a complete and rigorous solution of the problem presented is not to be expected, but it is, nevertheless, a practical problem which workers in color photography must face for a public unconcerned with the difficulties.
In the classical tone reproduction theory of black-and-white photography as developed by Jones and others,1 this phase of photography has very properly been considered as external to the usually more important problem of obtaining a point-for-point match between the relative brightnesses of a scene and of its reproduction. Much of the literature on the practice of photography, however, deals with it under the heading “How to produce such and such an effect.” A typical example is the use of infra-red photography to imitate night scenes. It is not usually realized that it is the dependence of the visual processes on the illumination level which is involved.
In color photography with its closer approach to the illusion of reality, the eye also demands a closer approximation to the true appearance of the scene photographed. In addition, the viewing of color prints under different illumination conditions produces greater apparent changes in the prints than is the case in black-and-white work.
While it is undoubtedly true that there is no general solution which meets all the requirements, it is equally true that the problem cannot be disregarded in any complete theory of color photography.
Nature of the Adaptation Process
The sensitivity of the eye to light is highest when it has rested for long periods in the dark. Sudden viewing of a light source under these conditions or sudden illumination of the room produces first a blinding sensation of glare with almost no vision of detail until the visual mechanism gradually adjusts itself to the situation. The time taken for this adjustment or adaptation depends on the average luminance of the scene. The final appearance of the scene to the observer, however, is more or less independent of this value over a wide range of intensities.
The total amount of light entering the eye is variously distributed over its sensitive surface. The greater part forms an image of the scene but considerable portions enter as stray light through the eyeball, are scattered by the media of the eye, etc., and represent non-image-forming light to which the eye also responds.
The eye itself is in almost continuous movement, seldom stopping for longer than a tenth of a second in any one position. Attempts to hold it motionless result in minute quivering motions. Perception takes place only after the eye has been relatively stationary for a brief interval, as can be seen from the absence of apparent detail in the stationary background when the eye follows a rapidly moving object. Continued fixation of the eye on a particular object, however, soon decreases the apparent contrast and, when vision is transferred to another point, produces “after-images” of the first object.
The various adjustments which take place tend to make the perceptions due to light of any luminances fall within a certain range of brightnesses. The range may be described as that from blinding brightness, through white and gray, to black (and correspondingly for chromatic sensations). These adjustments are known collectively as adaptation phenomena. They uniformly tend to produce the most favorable response condition of the eye for the viewing problem presented by the scene. An analogous process exists in photographic printing when the amount of exposure through a negative is adjusted so that the scene best fills the scale of the paper. As Schouten8 has pointed out, the eye is used primarily as a “null-type” instrument to determine whether an object is lighter or darker than its surroundings, not as an absolute instrument to determine the illumination level.
As was seen in the previous parts of this article, three types of brightness adaptation must be distinguished. They are general, local, and lateral adaptation. All three must be considered capable of occurring locally in the eye in the sense that two areas of the same retina may differ in all three. There appears to be no adaptation of the eye as a whole except insofar as conditions external to the eye may produce such an effect.
From these considerations it is possible to piece together a picture of what is meant by the term “adaptation level,” at least as far as the fovea is concerned.
The constantly moving eye itself adapts locally and rapidly, the extent depending on time of hesitation, intensities of the objects viewed, and the angles which the brighter parts of field make with the line of sight. This process and the general nature of ordinary scenes give rise to two general conditions which exist simultaneously.
From the results of a study of such scenes by Jones and Condit21 it appears that the average outdoor scene has an average directional reflectance in the neighborhood of 20–25 percent and that the ratio of maximum to minimum luminance for scenes in daylight varies from 25 up to 750. The eye under these conditions is exposed for long times to luminances which fall within these limits. Unless it is fixed on some particular point of the scene it will be exposed successively to the various luminances and become adapted to some value lying between these limits. From such long time exposures recovery of sensibility is relatively slow. Therefore, since the whole eye is exposed, it will take up a sort of average adaptation condition corresponding to the term “general adaptation.” Its actual value probably lies close to the average reflectance unless the eye for some reason remains for several minutes in a relatively fixed position.
In the process of looking at the scene, the eye views one object after another, stopping for brief intervals at each one. At each of these stops a readjustment of the adaptation takes place locally. From such adaptation recovery is very rapid, the exact rate depending on the time of exposure. If the interval is as long as a second or more, the observer may be conscious of “after-images” owing to the lag in recovery of the local sensitivities. This lag is particularly noticeable when there is a sudden increase in illumination level and before general adaptation has had time to become established. The various areas of the eye are hence in a ceaseless state of changing relative sensitivity.
In addition to the changes in sensitivity directly produced by the retinal image of the scene there is the third effect which plays an enormous part in the appearance of objects. If a moderately dark object is surrounded by considerably brighter ones, the sensitivity is not decreased at the light points in the image only, but this effect extends into the darker part decreasing its sensitivity also. This results in an apparent darkening of adjoining brightnesses and, in extreme cases, to a loss of detail in the darker objects. This “lateral” adaptation may be of large magnitude where considerable brightness differences are involved. Since it depends markedly on the relative areas and positions of the objects concerned it plays a large part in some of the failures of photographic reproduction.
All of these adaptation effects are steeply variable from one person to another. It appears, therefore, that the expression “adaptation level” represents an over-simplification of the situation. In ordinary usage it corresponds to general adaptation, but is frequently used, for example, in reference to the viewing of a small isolated patch of light, or to describe the state of an observer’s eye at some time after conditions have changed. The expression, “as seen by the totally dark-adapted eye” is frequently encountered.
Any attempt to generalize the facts and deduce quantitative relations for the sensitivity characteristics of the fovea at any given time encounters numerous difficulties. At least three factors must be considered as dependent on the time. They are: the average illumination, the effect of small areas of high intensity, and the size of the object being viewed. They must be considered from the standpoint of least perceptible energy and least perceptible energy difference throughout the visible energy range. The ultimately desirable data would deal with visual contrast at all levels. Contrast, however, is a perception and as such may or may not correlate with definite luminance relations. For example, in the brightness constancy phenomena discussed in the first part of this article, what can be said concerning the contrast relations existing when a white object in shadow appears much brighter than a nearby black in sunlight in spite of the fact that the black has a higher luminance?
The usual technique for studying the effect of adaptation level is to surround a small field with a large, uniformly bright area and then to refer to the level in terms of the luminance of this surround. Except over very small fields, the sensitivity of the fovea is not made uniform by this technique, and for these fields lateral adaptation is at a maximum which varies with the degree of fixation of the eye. Schouten has shown, for instance, that for central fixation the sensitivity may be more than twice as great at the center of a six-degree field surrounded by a circle than it is near the boundary. It is doubtless a still higher ratio if points very close to the boundary are considered, although constant viewing of the border would tend to reduce the foveal sensitivity to the proper level.
With these limitations on their extension to everyday vision, certain facts seem well established. Holladay and Stiles22 have shown that the sensitivity of the fovea to luminance differences is markedly decreased by brighter surrounds, a slightly brighter surround producing nearly the full effect. In this work it was found possible to derive an expression from which the “equivalent background” effect of any visible bright area could be calculated.
Cobb and Moss,23 among others, have shown that when the surround has the same luminance as the actual area numerous visual functions such as acuity, luminance sensibility, etc., are at a maximum. This accordingly usually represents the condition of maximum “seeing.”
LeGrand24 has shown that in the absence of any other illumination in the field of view a small light spot produces a light sensation at the fovea even when its image lies some distance from it. He finds that this sensation depends only on the intensity and the angular distance from the fovea. For very small angles the equivalent luminance at the fovea varies as the reciprocal of the third power of the angle, thus producing a sort of spreading of the image. For larger angles it varies with the inverse square and ultimately as the reciprocal.
LeGrand’s work indicates an increase of illumination due to a light source off the line of sight. The work of Schouten, Holladay, and Stiles indicates a loss of sensitivity from the same cause. A photograph involving the perception of a black introduces both functions in an exceedingly complex way.
Abribat25 tried to measure directly the just perceptible luminance in a somewhat complex field. He used a small field, graded in luminance in two directions, and surrounded by a uniformly bright field which completely filled the rest of the eye.
He found that the minimum luminance perceptible under these conditions is a fairly simple function of the surround luminance. Above about 30 millilamberts, the ratio of just perceptible to surround luminance is approximately constant up through daylight luminance. The value of the ratio (computed from his published curve) is about 3X10–3. Below 30 millilamberts, the ratio increases steadily, reaching a value of unity in the neighborhood of 10–3 millilambert, i.e., at this point the just perceptible luminance matches the surround. These figures are in general accord with the earlier, less complete findings of Lowry,26 who used just perceptible binocular differences in a very small field with a very large surround.
These findings indicate that from daylight levels down to about 30 millilamberts the ratio between an intensity which has produced general adaptation and that which is just perceptible is about 300. Below this level, the ratio decreases continuously to the threshold of vision.
It remains to consider the intensity range upward from the adaptation intensity and to consider particularly the situation with respect to white surfaces in a complex field. In such a field the adaptation is almost certainly below that of white. White, however, as has been noted, is a perception which is not confined to surfaces whose reflectance is 100 percent and whose purity is zero, since very different surfaces may appear white under suitable conditions.
Little work seems to have been done in this field and at best only a guess can be hazarded as to the true situation. For surfaces perceived by reflection, brightnesses range through gray up to white and in the same field of view surfaces of many different luminances may appear white. One is tempted to suggest that the perception of white may be produced by any luminance appreciably above that producing the existing general or local adaptation. It is probable, however, that the adaptation level is usually much lower than this would imply and that white is more closely associated with the brightest nonselective diffuse reflecting surface in the immediate field of view.
Jones and Condit21 found that the average reflectance of the scenes as measured was one-fourth to one-fifth the brightness of the average maximum insofar as the average has meaning in non-uniform illumination. Since it is this sort of integration of the light which determines the general adaptation it is of interest to see how far the maximum exceeded the average for some of the specific cases listed. The brightest ratio listed is more than ten times the average reflectance for that scene and the minimum is about one and one-half times. Since the brightest surface in each case could have been and probably was perceived as white, it is safe to say that whites can vary at least up to ten times the general adaptation. Incidentally, in uniform illumination, a reflectance of 10 percent is perceived as a very dark gray while 662/3 percent is very light, these corresponding to the adaptation levels with respect to the highest and lowest maximum brightnesses, respectively.
Taking Abribat’s value for these levels as a ratio of 300 between adaptation level and the deepest possible black, the maximum ratio of white to average gives a possible brightness range of 3000 to 1. This implies that the latitude of the eye is this great without the help of local adaptation. It may, perhaps, be considered as a rough approximation of the possible range in daylight. At low levels it is much less than this, however. Assuming that the same physical conditions can hold at a low illumination level (for example, at one millilambert), then Abribat’s results would suggest a maximum range of 1000 to 1 or less, and at 0.1 millilambert, the value would be of the order of 200 to 1. This is mere speculation, however. What Abribat’s results do appear to show definitely is that, at a general adaptation level of one millilambert, luminances less than 1/100 of this will not be visible and that at 0.1 millilambert, brightnesses less than 1/20 will disappear. At low intensity levels, therefore, the range of visible details below the general adaptation level is greatly compressed causing a loss of details in the shadows. This must be reproduced photographically by an equivalent actual loss if anything like the same effect is to be reproduced by the print.
Nutting27 has shown that the ratio of minimum perceptible radiation to just tolerable glare intensities is greater at low general adaptation levels. His ratios of adapting luminance to just perceptible, however, are of the same order of magnitude as those of Abribat.
These considerations have some interesting consequences with regard to the nature of perception of scenes.
The fact that a scene can be quite bright or quite dark with respect to the same adaptation level, depending on the relation of maximum and minimum luminance to the average, indicates that scene contrast and apparent scene brightness are not controlled entirely by the state of sensitivity of the eye. It suggests, in fact, that apparent contrast may be determined by the relation of maximum and minimum luminance to average luminance and that the brightness of a scene may be almost wholly controlled by the extent to which the brightest area is illuminated above the general adaptation level or by some similar function.
It may be suggested here that the concepts of light and dark as applied to a whole scene, such as a “dimly lighted interior,” a “dark” day, a “bright” day, or a “brilliantly lighted” stage setting, are more directly associated with the contrast of the illumination of the scene than they are with the average luminance as such. This approach affords some promise at least of explaining the frequent total failures of photographic reproduction to recreate the effect of apparent lighting intensity. The brightness, of course, can be imitated to some extent in the printing process by making the print lighter or darker and so varying its level with respect to general adaptation. It cannot be made to appear to have more brilliant lighting (except as a logical fact rather than as a perception) by any technique which does not include local modification of the print.
So far as contrast is concerned, therefore, eye sensitivity data can yield only a factor for the eye which applies under certain conditions, and is roughly analogous to the γ of photographic sensitometry. If the condition is changed, however, the perceptions change according to some quite different function.
At present, this function appears to be unknown. Judging from experience with photographic printing in black and white, a factor would be introduced which would be quite high at the lowest intensities and quite low at the highest. A photographic print seen in the low levels of a photographic dark-room appears considerably more contrasty than at ordinary levels and is, in turn, more contrasty than it appears in full sunlight, especially if it is a low contrast print. If the print itself has an exceedingly high ratio of reflectances, the reverse may actually be the case. Contrast then appears to be about equal at all average luminances if good detail is just visible in the deepest blacks and both a good black and a good white are visible, in other words, if the “scale” of the eye is just filled. If detail is obviously lost in the shadows and they appear black, contrast will appear higher; if brighter whites or deeper blacks are obviously possible, contrast will tend to appear lower.
Wright’s work, as well as Schouten’s, shows that the various brightness adaptation phenomena of the fovea apply to colored as well as to neutral stimuli. Before discussing the relation of this phase of the problem to color photography, we shall consider the various ways in which colored stimuli are modified in their appearance by their physical intensities, both in absolute measure and in relation to their environment. Actually, the question is how is the perception of color by the eye modified by the stimulus itself and by its environment.
There are only three fundamentally different ways in which a colored area can occur in a scene. It may occur as a completely isolated patch, as in an optical instrument, with no stimulation of the eye in the region around the image. It may be surrounded by fields of any color under any color of illumination, and it may occur in a portion of a total field which is subject to a different illumination from the remainder of the field. Depending on the exact conditions, these three fundamental situations give rise to numerous differing series of color perceptions. The two cases with which we are chiefly concerned are as follows.
Case 1. Isolated Color Patches With No Surround Illumination
As the luminance of an isolated chromatic stimulus increases from very low values to very high values, the perceptions produced vary in all the attributes of color. Over a short region of intensities there is no hue. This is followed by a region of gradually increasing saturation and brightness in which hue is not, in general, constant. Above this range, hue and saturation change more slowly than brightness for a short time, followed by a region in which saturation decreases, again with considerable shift in hue as the intensity becomes more and more painful. At the lower end of the range in which color is seen, the brightness can properly be described as weak. Increases in intensity make the brightness higher and higher until it becomes unbearable. There is no luminance under these conditions in which the color appears dark, in the sense that a pure pigment becomes dark when it is mixed with black. Neither is there any region in which the color appears to change in the amount of white which it contains. At very high intensities there is a tendency for the hue to diminish and this diminution is described as a loss of saturation. The sensation, however, is that of being blinded for color rather than of admixture with white.
Case 2. Colored Area Surrounded by Daylight
The statements just given apply only to a single isolated stimulus under conditions (either monocular or binocular) in which there is no other stimulus present in the field of view. The presence of another stimulus of any area or nature changes this whole series more or less completely. Perhaps the simplest case, and the one having the greatest bearing on the present problems, consists of a two-part field in which a relatively small area subtending a few degrees at the eye is surrounded by a much larger field, both areas being uniform throughout, and the dividing line between the areas being sharp.
In such a two-part field, color perception takes on two new attributes which are usually described as “contrast effects.” These attributes consist of apparent admixture of black with the less intense part of the field and of admixture of white with the more intense part. With the relative sizes of field mentioned, however, these effects are best seen when the central small area is made darker or brighter, respectively, than the surround and when the surround is made of white material. If the luminance of the surround is fixed at a certain moderately high luminance and is of daylight quality, then the following sequence of perceptions takes place when the intensity of the central area is increased from zero. The area first appears black. This black is a darker and a more positive perception than absence of light without the surround. As the intensity is increased there is no change in the appearance of the black until a certain level is reached which is different for each surround luminance. At this level the black is slightly tinged with color. As the intensity increases further, this color increases and the percentage of black decreases until, at the same brightness as the surround, the black has disappeared completely and the stimulus is seen at its maximum color. Above this luminance, the effects depend markedly on the nature of the color, the size of field, etc. If the central field is fairly large, increase in its luminance above that of the surround appears to decrease the saturation of the color up to a certain point, above which saturation remains constant and further increases have the same result as though no surround were present. If the color sensation from the stimulus is saturated when the brightness matches the surround, it will retain most of this saturation when the level is too high to be affected by the surround. If the color is very desaturated, it may lose all trace of hue before it reaches this point. In this case, it does not look as though it were white or mixed with white but rather as a brilliant achromatic source of light. In general, hue varies noticeably from the point at which it appears admixed with black through to the highest luminances.
The limits of the range, from just perceptible hue in the black up to a brightness match with the surround, and from this point up to independence from the surround, vary tremendously with the luminance of the surround. For low luminance, both ranges are relatively short, the ratio of brightnesses being of the order of 10 or 20. For luminances of daylight magnitude, the lower range may be 1000 to 1 and the second shorter than this. At very high surround luminances, it may not be possible to tolerate the brightnesses necessary to make the central part independent of the surround.
It is important to consider more closely the changes in the appearances of the stimuli of various hues as they are made to pass through these and other series.
The simplest series from the standpoint of color attributes is the case of the isolated field which is varied in luminance only. Starting from zero with dark-adapted vision, the sensation is first achromatic, then gradually gains hue which then shifts with increasing luminance (the Bezold-Brucke effect). In general, throughout such a series the saturation changes very little except at the extremes of the range over which vision is possible. The series differs fundamentally from the others in the almost complete absence of the sensation of gray throughout the normal visual range. As the intensity changes, there is a change in the brightness of the field only except for the hue changes, which are usually hardly noticeable. The field at all times has the appearance of a chromatic color mixed with some white. Throughout most of the range, for example, an orange stimulus looks like some part of the spectrum in the region between yellow and red but varies in brightness. When an achromatic comparison field is introduced, the sensation series varies in a new attribute, that of admixture with gray. With some stimuli the presence of this new attribute produces sensations which are sufficiently different from any in the first series to have received special names in everyday speech. Perhaps the most distinctive of these is brown. Brown is the sensation produced by a stimulus corresponding to the orange-red end of the spectrum when the eye is at a higher general or local adaptation level than that produced by the chromatic stimulus itself. Hence it can be seen only when there is a brighter comparison field. The same thing holds for all colors containing gray. There are no dark colors in an isolated field, no “olive drabs,” no khaki, no “navy blue” etc. All of these are a combination of the sensation produced by an isolated stimulus plus the sensation of gray, and the amount of gray depends not on the stimulus itself but on its brightness relative to that of another stimulus. In colorimetric terminology, the variable is relative luminosity. For chromatic stimuli, then, gray is the perception produced by relative luminance just as was found for neutral stimuli. The series of sensations generated by the stimulus field when the white surround intensity is varied is a series in relative luminosity. At very low values relative to the surround, the perception is that of a black; at moderate values, color plus gray; and at a relative luminosity of one, no gray is seen. When the value exceeds unity, two possible types of change occur. Either the stimulus field looks like a light source, i.e., “glows,” or it takes over the control of the adaptation of the eye and the “gray sensation” is transferred to the surround. In the first case, there is a transfer to the condition produced by an isolated field usually with a noticeable loss of saturation; in the second case, the perception is better described as an apparent admixture with white. Obviously, the two perceptions are simply different ways of looking at the same thing. The type of perception in a particular case is determined by the spatial and temporal characteristics of the fields as well as by the saturations and relative luminances.
The foregoing discussion involves a concept which has been mentioned previously but which is not customarily stated in these terms, namely, that of the identity of gray with relative luminance at a given adaptation.28
For the present study, this fact may well be worded as follows. Under conditions in which white paper or its equivalent is to be considered as white and the illumination is uniform, relative reflectance with respect to this white corresponds to a definite gray. Non-uniform illumination or the absence of high reflection surfaces will shift this value to an unknown extent but in a predictable direction.
Brightness Perception under Color Adaptation
In Part II of this article, careful consideration was given to the phenomena accompanying general adaptation of the eye to a colored stimulus. It was stated that brightness adaptation might be considered as a separate phenomenon and some evidence was given to support this contention. The relative brightness of chromatic stimuli was not considered although it was mentioned that the appearance of a blue surface was noticeably darker relative to red in artificial light than in daylight. This is a general phenomenon which may be summarized as follows. The relative brightness of colors in artificial illumination differs from that of daylight in the same way in which the light source differs from daylight. If the source is relatively weak in blue, the blues will be similarly darkened and similarly also, for any other region of the spectrum. So far as color adaptation is concerned, relative luminance may accordingly be determined directly by the usual I. C. I. calculations. The integral of the spectral energy distribution curve of the stimulus with respect to the standard luminosity function gives the relative luminance (the Y of the I. C. I. system) directly.29
A photographic print viewed by artificial light will then show the same loss of brightness in the blues as did the subject, regardless of the particular spectral absorption of the two blues, since the effect depends only on the source of light used. A color film which corrects artificial light so that the photography appears to have been done in daylight produces correct results. Again, however, it is not possible to reproduce the effect of artificial light unless the print also is viewed by artificial light.
Possible Calculations of Color Perception
The possibility of calculating the appearance of a given stimulus can now be considered in the light of the foregoing resume. It is exceedingly important to the theory of color photography to be able to make such calculations, because it is only by the deduction of general laws which film may be made to obey that corrections can be introduced. Needless to say, such deductions have yet to be made in complete form. Their nature, however, can be suggested.
Any color stimulus may be specified by three variables, such as those of the I. C. I. system. The general or local color adaptation may also be specified by three. The general brightness adaptation level requires one (the average luminance of the scene at the moment). The spatial distribution of the stimulus with respect to its surround requires three relative tristimulus values to introduce the effect of lateral adaptation. To these it may be necessary to add the luminance which the observer considers to be white.
From these ten variables the appearance of a stimulus should be calculable in terms of a known and standard group of stimuli (an area of definite size with a given surround at a given general intensity level, for example), provided the functions connecting the variables are known. Judd2 has attempted an empirical solution of this problem and has deduced an equation by which the hue, saturation, and lightness of a given colored paper may be calculated under certain circumstances.
Fortunately for color photography, however, the problem presented is simpler than this. If a color process can be made to give a point-for-point match of a given scene considered as a group of physical stimuli with respect to the eye, then it is the relative effect of the remaining seven pairs of variables in viewing the scene and the reproduction which must be considered. Because of the phenomenon of color constancy, a good approximation for most cases can be obtained by eliminating the three variables of relative color adaptation, although the illumination must be considered from the standpoint of the relative luminosity of the colors. The phenomenon of lateral adaptation is then governed by the relative sizes (visual angles) of the scene and the reproduction. If sufficient data on this function were available, it might be possible to change a process to allow for it. The effective adaptation level of the eye, however, depends on the scene as a whole and on the way the observer looks at the scene. In like fashion, it depends on where and how he looks at the print. Only by assuming that the observer will view the reproduction under conditions which fall within certain permissible ranges can any worthwhile modifications be introduced. The requirement that the luminance being taken as white must be specified is easily met but places a definite limitation on possible processes. Objects which are to appear white must be white, i.e., must match a piece of paper which is obviously white. In the case of a paper print, such a comparison can be supplied in the form of a white border.
No attempt will be made to deduce any of these relations, if indeed, they can be deduced from existing data. There is no fundamental reason, however, why they cannot all be determined experimentally and the consequences for reproduction studied with respect to possible inclusion in the theory of color photography.
Experience over some years in the mass production of color photographs both for general amateur use and for the more exacting demands of the professional field has indicated the relative importance of many of these variables. Many effects which force themselves on the attention are based on visual processes which have been very little discussed in the literature of vision but which have been considered here at some length. The remainder of the article will discuss the observed photographic effects, pointing out either their connection with the foregoing discussion or stating the problems involved in such a correlation.
Photography at Different Luminance Levels
In black-and-white photography in which brightness is the chief variable, the contrast of a picture may be modified by changing the contrast factor or gamma of the process. This can be done in several ways, such as increasing the time of development of the negative, choosing a more contrasty printing paper, etc. Since relative brightness is the variable chiefly affected, this procedure is entirely satisfactory. In color photography, however, a change in the contrast factor or gamma of the process modifies not only the brightness relations but also the saturation of the colors. It will be noted from the preceding discussion that the contrast of a scene as a visual phenomenon varies independently of saturation. Hence, any particular color process can be used only at one gamma, the actual value being determined primarily by the level of saturation reproduction required by the particular process. This fact, coupled with the fact that subtractive processes in general have a higher gamma for their reproduction of neutral scales than for colors, has important consequences. The first of these, of course, is that all subjects must be handled by means of a single contrast factor, i.e., by a fixed photographic color process with exposure as the only variable.
The range of exposures to simple stimuli which can be reproduced by a given photographic material is limited, perhaps of the order of 300 to 1. By means of the time factor, however, and because of the fact that in photography the product of intensity and time may be made roughly constant, any given brightness in a scene may be reproduced at any desired density within this range. It is seen at once that the time of exposure in photography plays a role which, with respect to the photographic material, is very similar to that played by adaptation processes in the eye with respect to the range of customary perceptions. Both processes place the effect of the stimulus within a certain range. There are also other similarities. Underexposure of photographic materials causes loss of details in the shadows in much the same way that the eye loses them through loss of low brightness sensitivity at low levels. Overexposure causes a loss of detail in the highlights similar to the effect of glare in the eye. Here, however, the similarity ceases.
It has just been shown that in vision any particular scene ranges from a brightness which will be seen as white to one which will be seen as black. Although, in any one scene these have definite relations to the general state of sensitivity of the eye at the time, this relation is not general, that is, it is different for every scene. It is also probable that it is just these relations that determine the general impression of brightness experienced by the observer. Since this impression, among other things, must be reproduced by the photograph the problem may be stated as follows: How can any scene be so photographed that the brightnesses which were perceived as white and black by the observer under one set of conditions will again be so perceived under any viewing conditions?
Since reflectances of the order of 80–100 percent and approximately zero colorimetric purity are seen as white under nearly all conditions, the first requirement which must be met is, as noted before, that whites in the scene must be reproduced as white areas on the print. If it can be assumed that all prints are to be viewed under normal illumination levels, i.e., not lower than 15–20 footcandles, then the density which will be seen as black may be approximately specified. All scenes, then, must be fitted to these values with respect to the observer at the time the picture was taken. In black-and-white work, this can be and is done by modification of the contrast factor. In color work, this variable cannot be used in the same way. Before discussing what can be done, however, we shall consider further the relation of maximum and minimum brightness to the average.
If the impression of general brightness is produced by the relation between the luminance of white and the average luminance, this relation should be retained in the reproduction. However, if the luminance range of all scenes is brought to a constant value, this condition can only be met by a distortion of the relative brightness reproduction scale, unless the observer views the print under conditions such that the average for his surroundings has the same relation to white as it did when he viewed the original scene. There are two possible solutions of this difficulty. Either the print and its mount may be made to control the general adaptation level of the observer, or distortions can be introduced based on some assumed average viewing condition. A point-for-point relative luminance reproduction must be seen under the same average brightness conditions as the original subject to give the same impression. Conditions under which the reproduction does control the observer’s general eye sensitivity will be discussed under “Effect of Viewing Conditions”; the introduction of equivalent distortion for an assumed viewing condition will not be discussed here.
Returning to the problem of fitting any scene into fairly definite density limits in color photography, it should be noted that this must be done without modifying the contrast factor of the color process as such. Although the saturation of the colors is controlled by the contrast of the process, this is no longer true after the reproduction has been obtained. By superimposing a neutral image (black-and-white) of the desired contrast (positive or negative), the brightness reproduction may be modified to any desired extent without changing the chromaticity of any of the colors. Such superimposed images are known as “tone correction masks” and are of wide possible application. Since the relationship of density to exposure may be varied at will, it is possible by this technique to control the “curve shape” of the color process completely without affecting its color reproduction. The use of these masks, however, is limited by the fact that they have been successful only over color transparencies and, of course, are of use only during a copying process. They do not, in general, improve the appearance of an original but only of the print made through the original and the mask. Numerous practical difficulties are also encountered. With these masks, it is possible, however, to meet the requirement of printing any transparency so that any two densities in the transparency correspond to black and to white in the print and at the same time to make the average reflectance fall at almost any desired point without making any change in the final printing process. Their use is almost essential for some types of work but the control which can be utilized is limited by economic considerations and by the fact that a more serious difficulty demands a somewhat different type of mask.
Even after attaining the best possible brightness masking, the white of the print is still no brighter than a white in the surroundings. Special viewing conditions, then, are necessary in order to give the desired effect in this case also.
In order to reproduce the effect of general adaptation level, therefore, it is necessary first to expose the picture so that shadow detail is represented only to the extent that shadow detail was visible at the time of viewing and then to print the transparency so that one end of the brightness scale is black and the other end of the scale is a true white.
This statement, however, introduces another complication because the eye is capable of rapid local adaptations. The effect of these adaptations is seen most clearly in large shadowed areas. There are three ways in which such areas may be perceived. They may be perceived (1) as part of the scene as a whole; (2) with the intent of seeing as much of the detail as possible within the shadow; or (3) overlooked completely as in the perception of the continuity of a single surface in shadow. In the first case, the eye does not rest on the shadowed area, the perception of it as an area is largely peripheral, and the sensitivity utilized is that of the general adaptation level. Careful study of the shadowed area involves long direct viewing with consequent relatively large foveal local adaptation to the level of the shadow and hence a greater perception of detail. The mechanism for the effects of brightness constancy referred to in the third case is not clear. The shadows appear much brighter than their relative luminance would lead one to expect.
In order for a reproduction to produce exactly the desired local adaptations in the observer it is probable that not only would it have to appear under identical visual angles with the subject but might also have to match the subject for luminance. The usual solution of this problem, however, is either to illuminate the shadows in the original subject so that after loss of effect the reproduction looks like the subject, or (what amounts to the same thing) to control the exposure locally in the printing operation (“dodging”) to produce the required density difference.
Relative Brightness Reproduction Errors
In addition to the relative brightness errors which have been mentioned and which are due to the intensity and geometrical distribution of the illuminant, there are several others which are due to the defects of the color processes themselves or to the conditions under which they are used.
The first of these, as already noted, is the failure of any process to record the differences in relative brightness of colors when seen under chromatically different illuminations. The defect so far as brightness is concerned may be summarized as follows: When a color process is adjusted to correct for an illuminant, colors in which the source is relatively deficient will reproduce as too bright compared to their appearance to an adapted observer. For any single case, brightness corrections can be made which depend directly on the color by using colored light in making the brightness masks mentioned earlier. They then become what might be called “color brightness” masks.
More important for practical purposes because of their magnitude are the errors of all subtractive processes which are due to the general absorption of the eyes used. If all dyes in a color process contained the same amount of black, there would be a higher contrast factor for neutral stimuli than for colors, and neutral areas would reproduce darker in color than in the subject. In the usual process there is more “impurity” in the cyan dye than in the magenta and least of all in the yellow. The result is that blue, cyan, and green colors tend to be much darker than the reds, oranges, and yellows. Under certain conditions this effect can be almost eliminated by the use of a “brightness” mask exposed to a definite color of light and developed to a definite gamma. This is known as a “color-correction” mask. Under certain favorable conditions, it may be combined with relative color brightness and contrast masks but the limitations are rather severe. In practice, this mask with or without modification to improve highlight contrast is the only one used.
The theory of masking for the various brightness and color defects of processes is exceedingly complex and difficult to explain. Only part of the theory has been published (Miller30), and space does not permit further consideration here. Experience has shown that the relative brightness errors are among the most serious produced by subtractive processes. Masks, or techniques which give the equivalent of masking, are almost necessary for their operation.
Effect of Viewing Conditions
It is interesting to find that many of the general defects of color processes may be overcome by the proper choice of viewing conditions. Although it is hardly possible to take advantage of this fact with a process designed for general use, anyone desiring to display a reproduction to the best advantage should take such factors into careful account.
The most favorable condition under which any type of picture is seen is exemplified by projection of transparencies in an otherwise completely dark room. Under these conditions, the screen acts as the equivalent of an isolated patch of color with a completely dark surround. The eye accordingly adapts locally, and to some extent generally, to the brightness and average chromaticity of the image. This adaptation produces two results, both of which are desirable: (1) A maximum amount of gray is removed from all the colors and (2) any slight lack of color “balance” in the process is corrected by the eye. Both of these effects make the reproduction appear more correct than it really is.
Under dark-room projection conditions, the over-all density of the transparency makes little difference, so far as faithfulness of reproduction is concerned. Most projection, however, is done by lenses which do not give a black of as low luminance as the rest of the room. That is, the darkest part of the projected picture is not as dark as the luminance perceived as black by the observer. Under these conditions, improvement in the contrast of the pictures can be obtained by decreasing the projection intensity or by surrounding the picture with a border of uniform brightness.
The effect of a surround on a projected picture is to decrease the sensitivity of the eye by affecting both the lateral and local adaptation processes. This decrease in sensitivity raises the intensity which is required to appear just brighter than black and gives a much more convincing reproduction of dark scenes which, without surrounds, permit such a great rise in sensitivity that they do not appear to contain black areas at all. The surround also acts in interesting ways as a fixed reference point both for chromaticity and for brightness. In theory, both of these effects are desirable, since they tend to extend the possible range of average brightnesses which may be projected. However, the presence of a fixed reference point also eliminates all the visual correction for light source and process balance errors. The use of a low intensity surround, therefore, while ultimately desirable, does not always give better results in projection.
Transparencies viewed over an illuminator in a partially darkened room are similar in appearance to projected images except for the generally greater luminance range due to absence of projection lens flare. The beauty of transparencies viewed in this way is usually ascribed to the greater density range available in transparency materials as compared with that of reflection prints. It is apparent that this is only part of the story. If an illuminator is so adjusted that the luminance of a white in the transparency exactly matches that of a sheet of white paper lying in room illumination, the transparency will usually appear less satisfactory than a reflection print. Transparencies appear better primarily because they are customarily seen in such a way that their whites are very bright compared to the whites in the surrounding objects, and hence have a high white brightness relative to general adaptation. This permits the reproduction of scenes of much greater average brightness, removes the greatest percentage of gray from the colors, and, in general, greatly extends the usable range of the process. In these respects, however, it is not superior to projection in a darkened room except insofar as the greater effective luminance range plays a part. The color temperature of an illuminator is more noticeable as such, and transparencies so viewed must be more nearly correctly balanced.
A transparency viewed over an illuminator in a well-lighted room represents a transition case between dark-room projection and the viewing of reflection prints. Provided the highlights of the transparency on the illuminator are brighter than a white in the same position with respect to room illumination, there will be enhancement of whites in the reproduction and a longer scale of visible detail. The general adaptation level, both for color and for brightness, however, is now set by the room-lighting conditions rather than by the picture or by the illuminator (assuming that the whole surface is covered). The color of the illuminator must now approach rather closely therefore to that of the room illumination and the transparency must not be off-color by any great amount. The fact that the white is represented by a luminance which is usually considerably higher than is possible in a reflection print means that the picture as a whole has lost a considerable amount of the added gray owing to the general absorption of its dyes.
There is, finally, the most important case of straightforward reflection color prints viewed in normal surroundings but under all the very great variations in luminance and chromaticity which the word “normal” implies. Provided the illumination is uniformly distributed, all the visual effects operate in the direction of recognizing with maximum precision any faults which the print may have. Experiments indicate that under such conditions differences in hue which are just perceptible in the best two-part fields are easily perceived.
In a reflection print to be viewed “in the hand,” then, everything in the process must be as nearly correct as possible. Whites in the original must be reproduced as white in the print. The average reflectance must be appropriate to such comparison surfaces as the observer’s hand, etc., and color balance must be exactly correct or at least consistent with the subject matter of the picture. Needless to say, these conditions are not all met by any existing color process. The chief failure is that prints are too dark when hue and saturation are satisfactorily presented. This fact, coupled with the difficulty of maintaining exact control of color processes, is at present more important in making reflection prints than any of the other variables discussed. There are several ways of viewing prints, however, which markedly improve their appearance.
A dark print, by definition, is one which contains too much black in the reproduction of each color. The effect of this black content, however, is to decrease the “brilliance” of the print. In particular, such a print lacks what is known in the trade as “carrying power.” This interesting term is difficult to define accurately and yet expresses a valuable property of a reproduction. If an observer stands close to a large print, the general and local adaptation of his eyes is controlled by the reflectance of the print. The eye then tends to see the print with as little gray content as it would in dark-room projection and whatever is represented as white tends to appear satisfactory. As the print is seen at greater and greater distances, however, the print has less and less effect on adaptation and the brightnesses are seen more with respect to their general surroundings. The print will “go dark” if the average of the surroundings is appreciably brighter than the print. Such a print is said to have “poor carrying power” because its effect does not carry to as great a distance from the print. For obvious reasons, the term applies equally well to the illumination changes which the print will tolerate. A dark print with poor whites decreases in brightness faster than its surroundings as the illumination decreases, at least at low illumination levels. A print with low carrying power, therefore, does not show up as well in a poorly-lighted part of a room. Carrying power of a given process could probably be expressed in the form of a function containing the reflectance of maximum white, average reflectance, and saturation for the best print that could be obtained by that process. It is one of the few bases on which two fundamentally different color processes could be compared.
From a fixed viewing distance, the size of a print may have several effects. In general, the larger the area of a given color the more brilliant this color appears to the eye. In general, therefore, the larger the print the more brilliant will be the appearance of its colors. This is due partly to the increased control of adaptation given by the larger visual angle mentioned above, and partly to the decrease in the simultaneous contrast effects in the large areas from the usually less saturated surrounding color.
It is the usual aim in making a color print to approximate as closely as possible to the relative saturations and relative brightnesses of the colors of the original, provided other conditions have been met. In the original scene, the relative brightnesses were controlled very largely by what has been called “lateral adaptation” effects. These same effects take place in the print, their magnitude being controlled by the actual relative luminances and the visual angles subtended. Most processes give lower saturation and lower luminance on a point-for-point basis. The print then should be seen at a viewing angle which is at least as large as that subtended by the subject and larger angles may be necessary.
Some improvement can be made in the brightness of the print itself by placing it on low reflectance surfaces such as dark gray mounts and the like, the effect being similar to a local change of illumination. Great changes in the amount of gray in the colors may be produced by such mounts. It is thus even possible to obtain a fairly convincing reproduction of scenes containing brightnesses above those of the viewing conditions (sunlight on the snow, etc.). In spite of this fact, however, most people prefer white borders around prints rather than black. Presumably, association and similar phenomena are involved since a person stating preference for a white border will usually admit that the color reproduction is better on the black.
It is thus seen that color photography treads a precarious pathway through the visual processes. It is hoped that sufficient evidence has been presented to show that more is involved than the spectral distribution of the color and the relative sensitivities of the emulsions employed in its reproduction by a given dye system. Little has been added to the existing theory of color photography except to point out that much must be added before it will result in a workable scheme of operation. It is hoped that the visual phenomena to which color photography has thus called attention will receive more of the consideration which they deserve.
In closing, the author wishes to express his great debt to the workers in many fields who have made this article possible. Particular acknowledgment is made of the valuable comments and criticisms of the manuscript made by Dr. D. B. Judd of the National Bureau of Standards and by Dr. D. L. MacAdam of these Laboratories. Many others have discussed parts of the paper as well as the lecture and demonstrations given before the Optical Society on March 5, 1943, and considerable modification of the text has been introduced because of these talks.
1 C. E. K. Mees, The Theory of Photography (Macmillan, New York, 1942), Chapter XX.
2 D. Katz, The World of Color (Kegan Paul, Trench, Trubner, and Co., London, 1935); K. Koffka, Principles of Gestalt Psychology (Harcourt, Brace and Co., N. Y., 1935); A. Gelb in Bethe’s Handbuch der normalen und pathologischen Physiologie (Julius Springer, Berlin, 1929); E. R. Jaensch, Zeits. f. Sinnesphysiol. 52, 165 (1921).
3 D. Katz, reference 2; R. H. Thouless, Bull. J. Psychol. 21, 348 (1931).
4 R. S. Woodworth, Experimental Psychology (Holt & Co., N. Y., 1938), p. 14.
5 See, for example: H. Helson, J. Exp. Psychol. 23, 439 (1938); H. Helson and V. B. Jeffers, J. Exp. Psychol. 26, 1 (1940); D. B. Judd, J. Research Nat. Bur. Stand. 24, 293 (March, 1940).
6 R. B. MacLeod, Arch. Psychol. 21, 1–101 (1932).
7 W. D. Wright, Proc. Roy. Soc. 115B, 49 (1934); Ibid. 122B, 220 (1937); Brit. J. Ophthalmol. 23, 51 (1937).
8 J. F. Schouten, Dissertation, Utrecht, 1937; J. Opt. Soc. Am. 29, 168 (1939).
9 D. B. Judd, J. Opt. Soc. Am. 25, 24 (1935).
10 R. Davis, Bur. Stand. J. Research 7, 659 (1931).
11 Although the present writer was led to this concept chiefly by the work of Wright, it was suggested in part, or in whole, by several previous writers. So far as he can determine, it was first proposed by H. E. Ives as a pure hypothesis to explain his observations on artificial daylight: H. E. Ives, Trans. Illum. Eng. Soc. 7, 62 (1912). The following references to the literature using the same approach have been kindly supplied by Dr. Judd: E. Noteboom, Zeits. f. Instrumentenk. 55, 317 (1935); W. Ströble, Das Licht 9, 149 (1939).
12 H. V. Walters, Proc. Roy. Soc. 131B, 27 (1942).
13 A. König and C. Dieterici, Zeits. d. Psychol. u. Physiol. d. Sinnesorgane 4, 241 (1892).
14 L. T. Troland, J. Exp. Psychol. 4, 344 (1921); H. Helson and D. B. Judd, ibid. 15, 380 (1932).
15 H. Helson, J. Exp. Psychol. 23, 439 (1938); H. Helson and V. Jeffers, ibid. 26, 1 (1940).
16 See, for example, the Symposium on the Munsell system in J. Opt. Soc. Am. 30, December (1940); especially J. J. Glenn and J. T. Killian, “Trichromatic analysis of the Munsell Book of Color” and K. L. Kelly, K. S. Gibson, and D. Nickerson, “Tristimulus specification of the Munsell Book of Color from spectrophotometry measurements.”
17 E. Q. Adams, J. Opt. Soc. Am. 32, 168 (1942).
18 See, for example, J. Plateau, Bibliographie analytique des principaux phenomènes subjectifs de la vision jusqu’à la fin du xviiie siècle (Royal Academy of Science, Bruxelles, 1877) and more recently, J. P. C. Southall, Physiological Optics (Oxford University Press, New York, 1937), p. 398.
19 W. von Bezold, The theory of color (L. Prang and Co., Boston, 1876), p. 163, S. R. Köhler, translator.
20 D. B. Judd, J. Opt. Soc. Am. 30, 2 (1940); Nat. Bur. Stand. J. Research 24, 293 (1940).
21 L. A. Jones and H. R. Condit, J. Opt. Soc. Am. 31, 651 (1941).
22 L. L. Holladay, J. Opt. Soc. Am. and Rev. Sci. Inst. 12, 271 (1926); J. Opt. Soc. Am. 14, 1 (1927); W. S. Stiles, Proc. Roy. Soc. 104B, 322 (1929).
23 P. W. Cobb and F. K. Moss, Trans. Illum. Eng. Soc. 23, 1104 (1928); P. W. Cobb, ibid. 11, 372 (1916).
24 Y. LeGrand, Rev. d’Optique 16, 241 (1937).
25 M. Abribat, Réunions de l’lnstitut d’Optique 6, 3 (No. 3, 1935).
26 E. M. Lowry, J. Opt. Soc. Am. 18, 29 (1929).
27 P. G. Nutting, Trans. Illum. Eng. Soc. 11, 1 (1916); ibid. 11, 939 (1916).
28 D. B. Judd, Am. J. Psychol. 54, 289–294 (1941).
29 In a private communication, Dr. D. B. Judd points out that for intense adapting stimuli there is evidence that changes are produced in the luminosity function. See also N. T. Federow and V. J. Federowa, Zeits. f. Physik 57, 855 (1929); ibid. 62, 834 (1930).
30 C. W. Miller, Principles of Photographic Reproduction (The Macmillan Company, New York, 1943).”
(Evans, Ralph Merrill (1943): Visual Process and Color Cinematography. In: Journal of the Optical Society of America, 33,11, 1943, pp. 579-614.)
“LES COULEURS DE LA GUERRE
Université Toulouse II – École Supérieure d’Audiovisuel
The images of WWII in colour were brought to public knowledge in France for the first time in 2002. They show among other things images of the German occupation and of the Liberation of Paris, and mostly sequences of the American D Day in Normandy, which literally questions the imaginary myth which has been mainly elaborated for the last fifty years by images of “archives” in black and white, or by great Hollywood feature films. Colour restores the continuity of a destiny the events of which, circumscribed, synthetized, even fantasied, by History (WWII: 1939–1945), do not represent breaks, cultural gaps, but the shared evocation of daily life which revives them under various forms.
La propagande militaire s’est emparée très tôt de l’outil cinématographique pour réaliser sur le terrain même des batailles, des bandes d’actualités en noir et blanc, qui constituent aujourd’hui des témoignages dont nous sommes loin, me semble-t-il, d’avoir épuisé tous les apports. Ces documents, c’est ainsi que les nomment les historiens, sont avant tout des films, et supposent conjointement, à l’origine de leur fabrication, un dispositif et un opérateur sur le terrain pour le faire fonctionner.
Quoique la plupart de ces bandes aient été tournées en noir et blanc, il existe, dès la Première Guerre mondiale, des images en couleur, (dés images fixes, les autochromes Lumières, à base de grains d’amidon colorés sur des plaques de verre), initiant le développement de procédés expérimentaux dont la mise en œuvre dans le cinéma de fiction sera tardive – le premier film en technicolor trichrome date de 1935: Becky Sharp, de Rouben Mamoulian. Ces images, obtenues par des procédés relativement au point – Kodachrome et Kodacolor aux États-Unis, Agfachrome en Allemagne – le public attendra la fin du siècle, voire le début du siècle suivant pour les découvrir.
Pourquoi si tardivement? La question est légitime, et l’on peut imaginer pour y répondre dans un premier temps, des raisons essentiellement techniques ou commerciales: instabilité des couleurs, fragilité des supports, complexité des procédés de restauration… Ces images, collectées par les Américains (George Stevens, notamment) et par les Allemands, n’étaient de toute manière pas destinées à être diffusées, et, paradoxalement, n’étaient pas soumises à la censure.
Peut-être a-t-il fallu aussi préserver la spécificité du genre, en réservant la couleur au cinéma de fiction, le noir et blanc continuant de caractériser les images d’actualités. À ce titre, c’est en 1962, en plein âge d’or du Technicolor, que Darryl F. Zanuck réalise sa grande fresque historique relatant l’histoire du Débarquement allié en Normandie, le 6 juin 1944, osant le noir et blanc en Cinémascope, et n’hésitant pas, de surcroît, à intégrer au film des fragments d’actualités. S’est-il agi en cela de correspondre à un imaginaire collectif conditionné par le monochromatisme des documents filmés ayant valeur de vérité historique, de sorte à ménager ainsi un plus grand effet de réel? Pratique fort répandue depuis…
Quoi qu’il en soit, la non-diffusion (on ne parlera pas encore de dissimulation) de ces images en couleur de la propagande militaire soulève un certain nombre d’interrogations. Peut-on penser que leur réception soit susceptible d’induire, de la part des publics populaires, une appréhension de l’histoire, sociologiquement, culturellement, idéologiquernent non désirée ou non souhaitable? Exclure la couleur des bandes d’actualités, ou a contrario, en imposer ultérieurement la profusion, n’est-ce pas là l’un des traits manifestes d’une manipulation de l’image documentaire, dans le sens d’un monolithisme contrôlé de la signification?
“Image d’archive”: l’expression en elle-même semble une condamnation! Une forme de ségrégation d’un certain type d’image qui aurait fait son temps, et ne serait désormais valable que pour la mémoire. L’expression sollicite d’emblée un imaginaire qui lui concède un certain usage, celui de nous parler de l’ancien temps. Et encore, quand on le lui demande.
La première qualité d’une image d’archive, c’est le noir et blanc. Avec le temps, sans doute, la couleur devient acceptable, mais il faut qu’elle soit altérée, sinon les repères chronologiques sont brouillés. Le noir et blanc dénote, sinon l’ancien, du moins l’avant, par rapport à une contemporanéité qui commence, disons à partir des années 70.
Mais le noir et blanc dit aussi une réalité brute, le minimalisme du dispositif, et la neutralité du point de vue, avérant le caractère “objectif” du document. L’archive est remisée dans les armoires, hors du monde et du temps, usée (confusion manifeste avec son support), poussiéreuse, figeant de surcroît l’événement dans un au-delà qui n’est pas nécessairement le passé ou l’ailleurs, mais une sorte d’espace-temps singulier, celui que l’on réserve communément à l’histoire, avec toutes ses variantes.
L’image d’archive semble, de fait, réticente à toute forme d’actualisation, assertion que ferait mentir un film du cinéaste biélorusse, Sergueï Loznitsa: Blockade, réalisé en 2005 à partir des images du siège de Leningrad, en 1941, sonorisées sans voix ni dialogue, et acquérant par là une force et une présence étonnantes. La remarque de François Niney résume assez bien cette singularité de l’image d’archive, dont “certaines, à force d’être vues, finissent par être ‘réellement’ l’événement qu’elles symbolisent fort partiellement et partialement”1.
A contrario, c’est à partir des années 50 que la couleur devient l’attraction du film à grand spectacle et en Cinémascope. Couleur ornement, couleur de l’exotisme, des fastes, de la féerie, inséparable d’une tendance à la saturation imposée, sans doute, par la technique (le Technicolor), mais aussi, prise en charge comme nouveau matériau de la construction du sens, par les réalisateurs2. Précisément, les contraintes du Technicolor supposent une représentation en a-plat, en raison de la nécessité d’un éclairage frontal. L’image, devenant tableau, ne se prête plus à l’enregistrement, à la collecte documentaire que, par ailleurs, le dispositif rendrait quasiment impossible.
Mais une fois l’étonnement passé et les procédés stabilisés, que devient le statut de la couleur? Il ne renverse pas pour autant les a priori relatifs aux minimalismes ou aux altérations chromatiques de l’image d’archive. Et il convient, en outre, de ne pas confondre au cinéma la couleur et les usages de la couleur: distinction essentielle, dès lors que la couleur qui procède d’une mise en scène, autrement dit du choix des matériaux colorés, ne produit pas les mêmes effets que les couleurs de l’émulsion.
Qu’en est-il du réalisme? À lire et relire les textes d’André Bazin3 sur le sujet, on ne sera guère plus avancé pour tenter de résoudre la polémique autour d’un surcroît éventuel de réalisme apporté à l’image cinématographique par l’émergence de la couleur. Et pourtant, le sujet est sensible, quoiqu’en définitive, assez peu pertinent. Bien sûr, avec la couleur, c’est le rapport d’analogie de l’image avec le réel qui se trouve enrichi. La couleur est perçue, non pas comme un apport décisif ayant manqué à l’origine, mais comme un complément à l’arsenal esthétique déjà disponible.
On ne saurait dire, par exemple, que le film en noir et blanc aspire à la couleur (ce que le marketing, quant à lui, assène depuis longtemps, avec la colorisation). Par ailleurs, les mouvements dits “réalistes” ou “néo-réalistes” ont majoritairement œuvré en noir et blanc: inaccessibilité des procédés couleurs, mais aussi, mimétisme de l’esthétique de l’archive, focalisation sans ornement, ou forme de contestation militante qui passe par une revendication d’austérité, de brutalité des contrastes, et d’une conscience de la duplicité du monde. D’autant que, comme pour la couleur, il y a ici le noir et blanc, et l’usage du noir et blanc dont les mises en scène renvoient aux mondes obscurs et souterrains, au chaos, ou à une forme d’exotisme des bas-fonds ou de la pègre… Cela suffit pour faire écrire à l’historienne d’art américaine Anne Hollander: “L’avènement de la couleur eut en fait pour conséquence un recul et non un progrès dans la qualité du réalisme”4.
Alors, que faire de la couleur? Pourquoi ne pas l’interroger plutôt sur la manière dont elle sollicite le rapport au monde du spectateur? Veut-on l’impliquer ou le tenir à distance? Ce qui importe, ce n’est pas le réalisme ou l’irréalisme d’une représentation, c’est son animisme: est-elle habitée par l’humain, auteur, personnage, spectateur? Qu’y a-t-il de moins réaliste que les univers de Zola? Et pourtant, tous sont habités jusqu’à la plénitude d’une obsession paranoïaque qui les fait organiques et monstrueux, plus proche du Brazil de Terry Gilliam, que du Gervaise de René Clément.
Venons en maintenant aux images du Débarquement, le 6 juin 1944, sur les plages de Normandie. Certaines de ces images ont été filmées en couleur, et nous allons en étudier également une en noir et blanc. C’est un monde homogène qui s’instaure, un monde où le détail s’évanouit sous la prédominance des contrastes. La perception dominante ramène tout à des oppositions de blocs, et anéantit les tons de gris, en les rejetant vers les extrêmes. De fait, le noir et blanc suppose une dimension imaginaire s’appuyant précisément sur des oppositions duelles, à la duplicité et au manichéisme significatif lorsqu’il s’agit d’images de guerre et de propagande militaire: jour/nuit, plein/vide, début/fin…
L’image noir et blanc, loin d’un assemblage, figure un monde en elle-même: elle s’autoréférence, construit sa propre réalité. Tous les matériaux de la collecte font l’objet d’une synthèse, d’un procès de focalisation à l’élaboration d’un monde singulier. La lumière, par exemple, s’y confond rarement avec son origine: elle reste avant tout un outil de la “mise en lumière“, livrant au spectateur des contrastes dramatiques à l’origine d’une mythologie tragique: le blanc de la plage vs les corps noirs des combattants, le ciel et l’eau noire, la masse des navires, comme autant de membres d’une destinée organique monstrueuse – on peut citer à ce propos l’apparition progressive des navires alliés, surgissant de la brume, comme des fantômes, vision d’horreur à en juger par l’air hagard et terrorisé du soldat allemand, comme d’un possédé mis en face du surnaturel.
Le noir et blanc empêche la distinction d’individualités au sein des groupes, et renvoie les visages du côté des blancs, de la statuaire, du diaphane – anges ou cadavres – selon la perspective que l’on choisit. À l’opposé, les corps, l’eau, les bateaux sont du côté du noir, opposant à l’espoir, la fatalité, au salut, le sacrifice – nous sommes là dans l’ordre du tragique – à l’humanité admise de ces combattants, la mécanique froide et déterminée de cette inertie qui les projette vers l’avant, vers le danger, vers la mort inéluctable. L’image noir et blanc produit alors une entité qui renvoie premièrement à sa signification dramatique, et non à une réalité immédiate, modulaire et complexe.
En couleur, c’est l’hétérogénéité qui domine. Le regard devient plus mobile, mieux disposé à l’errance, en quête du détail. Le spectateur se surprend à collecter lui-même ces fragments familiers, communs à l’expérience contemporaine du monde, pour reconstituer une réalité à portée de raison. Plus exposé au réel que véritablement impressionné par une scène tragique et monstrueuse, il saisit soudain l’origine de la lumière: car on fait aussi la guerre sous le soleil, aussi anachronique qu’en paraisse la présence dans ces images – Truffaut estimait anachronique la présence du ciel dans l’image d’un film en costume. La couleur procède par additions successives de fragments, lesquels sont toujours susceptibles de renvoyer à une réalité totalement extérieure, selon le degré de concentration du spectateur. A contrario, le noir et blanc élèverait le concept (un groupe, un navire, l’océan) à la puissance X. La couleur actualise: la caméra est là, et les liens entre les fragments se relâchent. C’est une perception plus autonome, voire libérée des orientations déterministes et de la force tragique de l’image noir et blanc. La raison prend le pas sur l’imaginaire… Ce n’est sans doute ni mieux ni pire. Mais idéologiquement, ce peut être décisif.
Quand, dans l’image, il y a de la couleur, le regard s’attarde, résolument, à la recherche du moindre détail, puisque le détail se pare d’une nuance singulière qui lui donne de la visibilité. Le regard fait du corps de l’image une représentation comparable à ces planches d’anatomie où l’on pourrait observer un réel démembré, fragmenté, ces planches dont la couleur manifeste, en son jaillissement, un symptôme de véracité. En ce cas, le spectacle de l’image d’archive en couleur ne sollicite pas immédiatement le potentiel créateur et l’imaginaire du spectateur, mais suscite, au demeurant, un plaisir qui implique le corps physique, le corps tout entier. Sans doute est-ce un plaisir morbide, celui que le corps éprouve aux aventures du corps d’autrui, mis en danger, altéré, en lutte contre les convulsions de la douleur, et abandonné parmi les territoires de l’agonie, entre la blessure et la mort.
L’image en couleur provoque une résonance affective, excluant une transcendance passant par la mise en scène, dont le noir et blanc donne quelquefois l’illusion. Ce sont là les ingrédients du voyeurisme, terme dont l’usage galvaudé mérite bien ici une courte explication: pour le spectateur, il s’agit d’assister à la représentation des possibles de sa propre condition, sans pour autant les subir de plein fouet. Le réel d’une humanité déchirée, littéralement atteinte dans sa chair et son intégrité, s’oppose à la proposition du noir et blanc qui, a contrario, opère par une sorte de sublimation: c’est la force d’une impression tragique, un ordre dramatique apparent, la référence implicite à un réseau de métaphores qui frappent l’imaginaire et témoignent de l’universel, autrement dit du mythe, autrement dit, du sacré… En noir et blanc, on ne parlera pas de mort mais de sacrifice.
Le tournage et la projection d’images en noir et blanc ont indéniablement servi la propagande militaire. En maintenant une distance, que la couleur n’aura de cesse d’abroger, le monochromatisme entend sauvegarder l’unilatéralisme du discours, et la disponibilité du public à la validation des choix officiels. Les hommes, temps, lieux, événements, y gagnent la force de l’hyperbole, et l’image est habitée de caractères et de matériaux monstrueux. Omaha Beach, avec ses contrastes, ses obstacles, opère comme un no man’s land qui s’anime, se remplit, jusqu’au surpeuplement, comme l’espace de l’épopée. En ce sens, c’est un espace exemplaire où se joue, en résumé, le destin de l’humanité.
Mais la distance n’est pas seulement morale, elle est aussi, idéologique, voire politique; on soulignera le caractère sacré des mondes du noir et blanc: mondes clos, ceints de barbelés que le spectateur n’est, en aucune manière, invité à franchir, car il n’y a pas de place pour lui. Resté en dehors, empêché d’agir, le voilà disposé à acquiescer, sans jugement: les faits sont les faits, indéniables, uniques, sans précédent et sans similitude. Le noir et blanc rationalise le temps et l’espace, en les restaurant à l’intérieur d’un cadre et d’une chronologie, délimités et organisés. De fait, notre spectateur se trouve préservé de toute tentation à l’assomption des destins. Sa vision est une vision disciplinée, idéologiquement contrôlée, n’étant autorisée, ni au jugement, ni à la contestation.
Tout en abrogeant la distance, la couleur, on l’a vu, devient indissociable d’une implication physique, d’un engagement du corps du spectateur dont elle sollicite brutalement la mémoire de l’expérience sensible et sensitive. La dictature du réel qu’elle suppose convaincrait volontiers que la blessure, la souffrance ou la mort y sont devenues, non plus admirables, mais inacceptables. La couleur focalise l’attention autour du partage de l’expérience individuelle, au détriment de l’intérêt porté à l’héroïsme collectif. La couleur, c’est un brouillage lumineux de la raison d’État, l’avènement incontrôlé des idéologies de résistance, de contestation, de suspicion et de pacifisme. La mise en scène, soudain, semble se lire comme telle, et c’est son auteur qui s’incarne: état-major, dirigeants politiques, classes sociales… Et la contestation qui trouve là un adversaire. Observant l’émergence d’une technologie nouvelle et riche de promesses, nous voici relevant les mises en danger du nationalisme! Le sacrifice des guerriers a débordé les frontières du mythe où l’on voulait le maintenir. Il n’est plus désormais dans l’ordre de la destinée sublime, mais dans celui de l’injustice et de l’inacceptable.
En outre, les rapports chronologiques se sont relâchés avec la couleur. Moins tyrannique, la notion d’événement s’efface au profit d’une réalité plus sensible, le continuum de l’histoire. Si le noir et blanc enferme l’événement à l’intérieur d’un cadre et d’une chronologie rigoureusement datée – le Débarquement a bien eu lieu le 6 juin 1944, à l’aube – en refoulant le spectateur aux frontières géographiques et temporelles de cet événement, la couleur ouvre une brèche en avérant des similitudes entre l’image projetée et l’expérience contemporaine du spectateur. Par ce phénomène d’actualisation, c’est bien la réalité autonome et pérenne de l’océan, du ciel, du soleil, du vent, de la houle, de la durée d’une traversée, des gestes, des regards, des mouvements, que l’on se met enfin à considérer.
L’histoire-événement, exposée comme un récit, cède la place à une durée qui ne le contraint plus à débuter et à finir. La couleur brise le cadre, et légitime le flux où, naturellement, le spectateur a sa place, devenant à son tour, et fort logiquement, un acteur de l’histoire. A contrario, d’ailleurs, de la fiction cinématographique classique, qui ne tolère d’engagement du spectateur qu’à l’intérieur des cadres maîtrisés de l’identification aux personnages.
Au-delà des débats et des polémiques sur le réalisme, augmentés ou non par la présence de la couleur dans l’image, il a fallu s’attacher au cas particulier des images documentaires, et plus précisément des images de la propagande militaire de la Seconde Guerre mondiale. Passant du statut en partie maîtrisé d’archives en noir et blanc, à celui, plus inattendu, d’une actualisation par la couleur de la chose filmée, l’image documentaire allait s’accompagner d’un certain nombre de stratégies visant à l’annulation pure et simple des effets incontrôlés de contre-propagande. Ayant fait de l’histoire un monde familier dont le spectateur ne saurait être exclu, l’autorisant, a contrario de ce qu’aurait souhaité l’idéologie dominante, à juger ou à contester, l’image en couleur se manifeste désormais sous la forme d’une profusion, à la puissance X, comme jadis, le noir et blanc. Que faut-il lire dans cette opulence? Sinon la tentation d’une propagande toujours sous-jacente pour fabriquer du déterminisme et détourner – divertir – le spectateur d’une histoire qui lui appartient, et dont il ne revendique pas suffisamment la propriété. Réinventer des cadres en guise de clôture, du récit en guise de destin. Et décider arbitrairement des lois et des procès de cette fabrique de héros…
Plus que jamais, pour réinvestir les mondes de l’image couleur, pour que chacun redevienne ce qu’il ne doit jamais cesser d’être, à savoir un acteur de l’histoire, il faut aujourd’hui se donner la peine de réfléchir sur ce trop plein des images, sur les images en série. C’est un peu le rôle du documentaire contemporain que d’inventer de nouveaux modes d’investissement de l’image en couleur, de nouveaux protocoles de réalisation favorisant, à partir du réel, des postures d’enthousiasme, littéralement parlant, et d’engagement.
1 François Niney, L’Épreuve du réel à l’écran, Paris: De Boeck, 2002, p. 253.
2 Guy Chapouillié, Couleur Vienna, in Revue Entrelacs, “La Machine”, ESAV/Lara, Université de Toulouse II: Toulouse, 2002.
3 André Bazin, Qu’est-ce que le cinéma?, Paris: Cerf, 1975, p. 3.
4 Cité par Jean Loup Bourget, “Esthétiques du Technicolor”, dans La Couleur en cinéma, dirigé par Jacques Aumont, Paris: Mazotta, 1995, p. 110.
Collectif, La Couleur en cinéma, dirigé par Jacques Aumont, Mazotta: Paris, 1995, p. 110.
Collectif, Revue Entrelacs, “La Machine”, ESAV/Lara, Université de Toulouse II: Toulouse, 2002.
André Bazin, Qu’est-ce que le cinéma, Paris: Cerf, 1975, coll. 7e art.”
(Arbus, Pierre (2009): Les couleurs de la guerre. In: Raphaëlle Costa de Beauregard (ed.): Cinéma et couleur. Paris: M. Houdiard, pp. 73–92.) (in French)
“Der Stand der Farbenphotographie1
Von John Eggert und Gerd Heymer.
Mit 8 Abbildungen.
I. Die Zurückführung aller Farben auf drei Grundfarben.
Die Aufgabe, die unendlich vielen Farbtönungen der Natur auf photographischem Wege wiederzugeben, ist auf zwei grundsätzlich verschiedene Arten angefaßt worden. Der wissenschaftlich exakteste Lösungsversuch geht davon aus, daß jede Farbe der Natur durch ihr Absorptions- oder Reflexionsspektrum, im Fall der Selbstleuchter auch durch das Emissionsspektrum, gegeben ist. Dementsprechend müßte auch der Vorgang der photographischen Wiedergabe in einer Anordnung bestehen, bei der zunächst das von jedem Bildpunkt ausgehende Licht spektral zerlegt, das Ergebnis photographisch aufgezeichnet und schließlich dem beobachtenden Auge wieder ein Strahlengemisch zugestrahlt wird, das nach Intensität und Wellenlängenverteilung genau dem Original entspricht. Das Verfahren von Lippmann (Erzeugung stehender Wellen in der photographischen Schicht) kommt dieser Forderung weitgehend nach, wegen zu geringer Lichtempfindlichkeit und zu großer Umständlichkeit ist es aber nur in ganz besonderen Fällen anwendbar.
Die größte technische Bedeutung hatten dagegen von jeher solche Verfahren, die auf der Erkenntnis aufbauen, daß man die spektrale Zusammensetzung fast aller in der Natur vorkommenden Farben in hinreichender Genauigkeit mit den Spektren dreier Grundfarben nachahmen kann, die je das langwellige, mittlere und kurzwellige Drittel des Sonnenspektrums umfassen und den Farbtönen Rot, Grün und Blau entsprechen.
Weshalb gerade diese Grundfarben Rot, Grün und Blau mit besonderem Vorteil gewählt wurden, wird am anschaulichsten, wenn man ein Gitterspektrum unter kleinem Gesichtswinkel oder bei geringer Intensität betrachtet: Unter diesen Umständen scheint das Spektrum überhaupt nur aus einem roten, einem grünen und einem blauen Teil von einheitlichem Farbton zu bestehen (Bezold-Brückesches Phänomen), und erst unter größerem Gesichtswinkel werden auch die schmaleren Gebiete der Zwischenfarben Orange, Gelb, Gelbgrün, Blaugrün, Violett usw. deutlich. In erster Annäherung kann man also das Spektrum durch ein Farbenband ersetzen, das nur aus je einem homogenen Streifen in roter, grüner und blauer Farbe besteht.
Nun weiß man aber, daß man den gleichen Farbeindruck auf verschiedene Weise erzeugen kann. So kann man den Farbton einer Spektrallinie für das Auge völlig übereinstimmend durch ein größeres Wellenlängenband ersetzen. Zum Beispiel erzeugt das Gemisch aller Strahlen vom langwelligen Ende des Spektrums (700 mµ) bis zu einer Wellenlänge von etwa λ = 600 mµ den Eindruck eines reinen Rot. Ähnlich erzielt man durch Mischung aller Strahlen mit Wellenlängen von 600 bis 500 mµ den Eindruck eines reinen Grün, während der Rest des Spektrums bis zum kurzwelligen Ende als reines Blau erscheint (s. Abb. 1 a). Das Schirmbild dreier aneinandergelegter Filter mit den obengenannten Durchlässigkeitsgebieten kann daher auch in erster Annäherung das Bild des Spektrums ersetzen, da die Spektralkurven der 3 Filter aneinander anschließend das gesamte Spektrum überdecken. Gleichzeitige Projektion aller 3 Spektralgebiete auf die gleiche Schirmstelle ergibt daher auch Weiß. Die Nachahmung von Farben, deren Absorptionsgebiet gerade eines der Grundfarbengebiete umfaßt, ist ebenfalls einfach: Übereinanderprojektion der beiden anderen Grundfarbengebiete läßt ein Strahlengemisch der gleichen Zusammensetzung wie die Vorlage entstehen. Abb. 2 zeigt als typischen Fall, wie das von 700 bis 500 mµ reichende Durchlässigkeitsgebiet eines gelben Farbstoffs durch die Grundfarben Rot und Grün gemeinsam ersetzt wird. Nun verlaufen die Spektralkurven fast aller Naturfarben in mehr oder weniger flachen Kurvenzügen; nach den bisherigen Erörterungen muß es daher auch ohne größere Fehler möglich sein, eine derartige stetige Kurve durch eine “Treppenkurve” zu ersetzen, deren einzelne Stufen jeweils das Gebiet einer der drei obengenannten Grundfarben umfassen, während ihre Höhe so zu bemessen ist, daß das ersetzende Spektralgebiet auch intensitätsmäßig an die Stelle des gleichen Wellenlängenbereiches der nachzubildenden Kurve treten kann. Abb. 3 zeigt den Ersatz des Spektrums eines Orange, entsprechend der Spektralkurve 3a, durch die Treppenkurve nach 3b. An die Stelle eines Orange, das alle roten Strahlen und von den grünen die langwellige Hälfte enthält (a), tritt hier ohne Änderung des subjektiven Eindrucks ein solches, das ebenfalls alle roten Strahlen, außerdem aber auch alle grünen, diese jedoch in halber Intensität, aufweist (b). Der einzige Unterschied liegt also im Grün, wird aber deshalb nicht merkbar, weil eine Farbfläche aus einer Mischung der Strahlen von 600 bis 550 mµ Wellenlänge sich von dem eines Spektralbandes von 600 bis 500 mµ, (s. das Gitterspektrum) nicht erheblich unterscheidet. Die Mischung anderer Farbtöne aus den 3 Grundfarben Rot, Grün, Blau ergibt sich entsprechend. Wichtig ist, daß auch die grauen Töne durch gleiche Mengen der Grundfarben darstellbar sind. – Sämtliche hier aufgeführten Tatsachen stellen eine Anwendung der bekannten, besonders von Young und Helmholtz ausgebauten Dreifarbenlehre dar.
Ist aber einmal erkannt, daß man durch Mischung der 3 Grundfarben alle anderen nachahmen kann, so ist damit das Problem der Farbenphotographie grundsätzlich gelöst: Man muß zuerst für jede Farbfläche des Bildes jeweils den Gehalt an Strahlen, die in das Gebiet der Grundfarben Rot, Grün und Blau fallen, getrennt ermitteln. Dies geschieht praktisch, indem man drei konturengleiche getrennte Bilder erzeugt und dabei durch Farbenfilter mit den in Abb. 1 a angeführten Durchlässigkeitsgebieten oder durch Auswahl der Empfindlichkeitsgebiete der verwendeten photographischen Schichten dafür sorgt, daß auf jedem Bild nur die in je einen dieser 3 Spektralbereiche gehörenden Strahlen aufgezeichnet werden. Man erhält hierbei zunächst einen “Rot-” , einen “Grün-” und einen “Blauauszug” als Negativ. Nach ihrer Umwandlung in die Positive kann man dann das farbige Bild aufbauen, indem man das bei der Aufnahme durchgeführte Verfahren umkehrt. Man muß also die Rot-, Grün- und Blaupositive auf die gleiche Bildwand übereinander projizieren, so daß die Umrisse sich genau decken, und dabei in den Strahlengang jeweils ein Filter der gleichen Farbe bringen wie bei der Aufnahme. Dann läßt jedes Teilbild an jeder Bildstelle das Licht der betreffenden Grundfarbe in der gleichen Intensität auffallen, die der Helligkeit dieser Grundfarbe bei der Aufnahme entsprach; das Bild muß also in den richtigen Farben erscheinen.
In manchen Fällen kann man schon mit zwei Grundfarben eine entfernte Annäherung an die Naturfarben erzielen, wobei das Spektrum sogar nur in 2 Teilen verwendet wird. An die Stelle aller “warmen” Farbtöne von Rot über Orange bis zum Gelb tritt dann ein einheitliches Orange, während die “kalten” Farbtöne von Grün über Blaugrün zum Blau und Violett durch ein einheitliches Blaugrün ersetzt werden; gleiche Mengen beider Farben liefern die farbtonlosen Graustufen vom Weiß bis zum Schwarz, da die Teilspektren, aneinandergelegt, das gesamte Spektrum des weißen Lichtes ohne Überdeckung ausfüllen. Auf diese Weise können niemals rein gelbe, blaurote oder warmgrüne Farbtöne wiedergegeben werden.
1 Sammelreferat über Vorträge vor der Physikalischen Gesellschaft, der Deutschen Gesellschaft für technische Physik, der Deutschen Chemischen Gesellschaft und dem Bezirksverein Groß-Berlin und Mark des Vereins Deutscher Chemiker in Berlin am 11. XII. 1936 (Eggert), vor dem Verein Deutscher Chemiker und der Gesellschaft für technische Physik in Jena am 26. I. 1937 (Eggert), vor der Gesellschaft der Freunde und Förderer der Universität Heidelberg am 7. IV. (Eggert) und vor der Deutschen Gesellschaft für technische Physik in Leipzig am 16. IV. 1937 (Heymer) .
1. Eggert, J. und Heymer, G., Naturwiss. 25 (1937) im Druck.”
(Eggert, John; Heymer, Gerd (1937): Der Stand der Farbenphotographie. In: Veröffentlichungen des wissenschaftlichen Zentral-Laboratoriums der photographischen Abteilung Agfa, pp. 7–28, on pp. 7–10.) (in German)
“Woran im Grunde nie ein Zweifel bestand: daß durch Farben ein metaphorischer, oft auch ein intellektueller oder emotionaler Hintersinn in die Filme kommt. “Farben”, schrieb schon Béla Balázs, haben “eine außerordentlich große symbolische, assoziierende und Empfindungen erweckende Kraft.” 4 Die Versuche im nachhinein, dieses Symbolische, Assoziative, Emotionale präzise zu entschlüsseln, sind inzwischen Legende. Übers Rot, das für Aktivität, Erregung und Wärme stehe, bis zum Blau, das stets Passivität, Sicherheit und Kälte signalisiere.5
Dennoch ist es mit der Farbe so eine Sache. Eigentlich scheint alles ganz einfach zu sein. Wie Farbe ins Auge fällt, meinen wir sie auch zu sehen. Andererseits wissen wir inzwischen, daß den Objekten selbst, physikalisch gesehen, keine Farbe anhaftet, daß nur verschiedenartige Oberflächen das Licht verschiedenartig absorbieren und brechen. Wer vermag also zuverlässig zu sagen, wie Farben an und für sich sind? Zumal anerkannte Kriterien dafür, wie Farben sich individuell niederschlagen in Eindrücken und Assoziationen, es gar nicht gibt. Klar ist nur, daß wir auf Farben spontan reagieren, emotional und im Augenblick. Wir bekennen quasi Farbe vor der Farbe.
4 Béla Balázs, Der Film. Werden und Wesen einer neuen Kunst, Wien 1972, S. 226
5 vgl. dazu: Gerold Behrens, Das Wahrnehmungsverhalten der Konsumenten, Thun 1982; Faber Birren, History of Color in Painting, New York 1965; Gert Koshofer, Color. Die Farben des Films, Berlin 1988; Thomas Langhoff, Am Rande der Straße: Über das Rot, das Weiß, das Blau und das Grün in Jean-Luc Godards Pierrot le Fou, Magisterarbeit an der FU Berlin 1990; Tom Porter/ Byron Mikellides (Hrsg.), Colour for architecture, London 1976″
(Grob, Norbert (1991): Farbe im Auge, Ausdruck im Kopf. Hein Heckroths Farbdramaturgien für Powell & Pressburger. In: Katharina Spielhaupter (ed.): Hein Heckroth. Frankfurt/M.: Filmmuseum, pp. 57–78, on p. 60.) (in German)
“Mit der Farbe aber ist das, wie gesagt, so eine Sache. Jeder meint zu wissen, wovon die Rede ist. Und doch meint jeder etwas ganz anderes. “Niemand kann je sicher sein, daß seine Mitmenschen eine bestimmte Farbe genau so sehen wie er selbst.”8 Das impliziert: Auch Wirkung und Eindruck von Farbe sind bloß variable Größen, abhängig von LichtSorte und LichtStärke. Der Effekt, den die Farben machen, variiert je nachdem, ob sie bei hellem Tageslicht oder unter Neonlampen, ob sie bei starker oder trüber Beleuchtung betrachtet werden. Das Rot von Schwester Ruths Kleid in Black Narcissus oder das Dunkelrot von Lermontovs Hausjacke in The Red Shoes sind dafür schöne Beispiele. Cardiffs Kamera spielt da mit Licht- und Schatteneffekten, so daß das Rot in bestimmten Momenten zum Schwarz wird. Dazu kommt zum einen, daß ein bestimmtes farbliches Licht verschiedene Farben ganz unterschiedlich aussehen läßt; das monochrome Rot, das in Black Narcissus Schwester Ruths Anfall signifiziert, nivelliert die Farben, die zuvor so prächtig leuchteten. Zum anderen wirken Farben wechselseitig aufeinander; ein tiefes Blau etwa, das neben einem strahlenden Rot steht, sieht anders aus als ein Blau neben einem hellen Gelb.9 Farbe ist also ein Wert, der auch festgelegt wird durch das, was um sie herum ist. “Eine Farbe, die im Zusammenhang mit ihren Nachbarn gesehen wird, verändert sich, wenn sie in eine andere Umgebung gestellt wird”, konstatiert Arnheim10. “Die gleiche Farbe in zwei verschiedenen Umgebungen ist nicht die gleiche Farbe… Das bedeutet, daß die Identität einer Farbe nicht in der Farbe selbst liegt, sondern durch den Zusammenhang bestimmt wird.”11 Daß dennoch Konventionen für Farbwahrnehmung entstehen, feste Übereinkünfte für Interpretation und Verständnis, hat wohl eher religiöse, kulturelle oder soziale Gründe. Etwa das Purpur der Macht (als Farbe für Kardinäle oder früher für weltliche Herrscher), das Rot der Gefahr (als WarnFarbe im Straßenverkehr) oder das Weiß der Reinheit (als Farbe der Ärzte oder der Braut vor dem Traualtar). Dennoch bleibt Vorsicht geboten, wenn Farben allzu sehr auf eine Wirkung festgelegt werden: “Symbolsysteme, die auf Farben aufbauen, sind nur in seltenen Fällen (sub)kulturübergreifend… Solche Systeme sind oft nicht geschlossen, besitzen unterschiedliche Strukturen und Organisationsmechanismen, machen also vom Medium Farbe in verschiedenster Form Gebrauch.”12
Ganz allgemein sind viele Farben durch ganz einfache Vergleiche definiert, zuallererst durch Vergleiche zur Natur. Schwarz wie die Nacht und Weiß wie der lichte Tag, Rot wie das Blut und Blau wie der Himmel, Gelb wie die Sterne und Grün wie das Gras. Betont gängig sind diese Vergleiche, dazu in unterschiedlichen Kulturen noch unterschiedlich akzentuiert. So bleibt genügend Raum für individuelle Imagination. Die Assoziationen zu den Farben erscheinen darüber als integraler Bestandteil der Farben selbst.
Gerade da, wo die Filme durch Farbe besondere Akzente setzen, wo sie den Menschen und Dingen ein ganz eigenes ‘Gesicht’ geben, schimmern jene “märchenhafte Kraft und Pracht” auf (Kandinsky), die dem Visuellen eine neue Dimension eröffnen. Ein anderes Reich von Magie tut sich auf, das – jenseits irgendeines Bezugs zum Alltäglichen – in erster Linie auf den ästhetischen Ausdruck zielt: auf den Bereich in den Bildern, der ungebändigt bleiben soll von konventionellen (oder gar begrifflichen) Begrenzungen. Von einem “Wahrnehmungsluxus” sprach Frieda Grafe, als sie Farbe im Film näher charakterisierte. Farbe verleihe “den Bildern eine Aura”, sei “eine Erfahrung, die nicht aufgeht in Funktionalität”, sie lasse sich nicht “auf Information reduzieren.”21
8 Rudolf Arnheim, Kunst und Sehen, Berlin 1978, S. 325
9 vgl. auch dazu Rudolf Arnheim, a.a.O., S. 341–343 (Arnheim vergleicht dabei die Wirkung von Farben mit der geometrischer Formen: “Wenn man ein Dreieck neben ein Rechteck stellt, zeigt sich, daß sie bleiben, was sie sind, obwohl sich die Formen gegenseitig ein wenig beeinflussen. Eine blaue Farbe jedoch, die unmittelbar neben einem starken Rot liegt, nähert sich einem Grün, und zwei Bilder, die nebeneinander an der Wand hängen, können die Farben des jeweils anderen stark beeinflussen.” [S. 341/342])
10 Arnheim, a.a.O., S. 342
11 Arnheim, a.a.O., S. 359
12 Joachim Knuf, Unsere Welt der Farben, Köln 1988, S. 18 f.
21 Frieda Grafe, Licht im Auge – Farbe im Kopf, Süddeutsche Zeitung, 20./21.8.1988″
(Grob, Norbert (1991): Farbe im Auge, Ausdruck im Kopf. Hein Heckroths Farbdramaturgien für Powell & Pressburger. In: Katharina Spielhaupter (ed.): Hein Heckroth. Frankfurt/M.: Filmmuseum, pp. 57–78, on pp. 60–61.) (in German)
“Wenn man sich einen Film ansieht, so wird man sich kaum dessen bewußt, daß man es nur seinem Erinnerungsvermögen zu verdanken hat, wenn man die auf einer Leinwand sich bewegenden Lichtflecke mit der Wirklichkeit identifiziert und dadurch die Illusion eines Geschehens erhält, das an sich gar nicht existent ist. Noch weniger wird man sich dessen bewußt, daß auch unsere wirkliche Umwelt, wie wir sie mit unseren Augen unmittelbar wahrnehmen, eigentlich nichts Reelles darstellt. Es sind lediglich Bilder, die auf unserer Netzhaut entstehen und die wir in jahrelanger Erfahrung unter Zuhilfenahme anderer Sinnesorgane, insbesondere des Tastsinnes, mit der Umwelt zu identifizieren gelernt haben. Es ist ja bekannt, daß Blindgeborene, die erst in späteren Jahren sehend geworden sind, mit den optischen Netzhautbildern zunächst gar nichts anzufangen wissen, da ihnen jeder Vergleich mit der Wirklichkeit fehlt. Eine einmal gemachte Erfahrung genügt aber nicht. Die gewonnenen Erkenntnisse dürfen nicht verlorengehen, sie müssen im Gehirn in irgendeiner Form gespeichert werden. Ein weiterer wichtiger Faktor ist deshalb das Erinnerungsvermögen, das jede optische Wahrnehmung erst mit einem reellen Inhalt erfüllt.
Wenn man jetzt die Frage stellt, welche Bedingungen für eine befriedigende Wiedergabe irgendeines Gegenstandes zu erfüllen sind, so kann man zwei Forderungen stellen:
Erstens braucht die Wiedergabe nicht genauer zu sein, als sie durch die physikalisch-physiologischen Gegebenheiten unseres Sehapparates bedingt ist, dem gewisse Leistungsgrenzen gesetzt sind. Sie erstrecken sich im wesentlichen auf seine Fähigkeit, bunte und unbunte Farbreize zu erfassen sowie auf die Möglichkeit, Helligkeitsabstufungen und Einzelheiten des Objektes wiederzugeben.
Zweitens muß die Wiedergabe aber so sein, daß sie nicht im Widerspruch steht zu den Vorstellungen, die wir dank unseres Erinnerungsvermögens von den Gegenständen haben. Diese Forderung ist außerordentlich wichtig, da sie manchmal mit der ersten Forderung nicht vereinbar ist. Ein bekanntes Beispiel ist das weiße Papierblatt, dem wir aus Erfahrung das Attribut “weiß” beilegen, da es diesen Eindruck bei Tageslicht auf uns macht und das bei gelblicher Beleuchtung uns trotzdem als weiß erscheint, verursacht durch die Fähigkeit des Auges, sich umzustimmen. Es kann auf die vielen Erscheinungen, die hiermit im Zusammenhang stehen, nicht im einzelnen eingegangen werden. Es ergibt sich jedenfalls, daß neben physiologischen Problemen ganz besonders solche psychologischer Art auftreten, deren Beachtung von entscheidender Bedeutung ist.
Die technischen Probleme des Farbfilms sind heute so weit gelöst, daß eine im allgemeinen befriedigende Wiedergabe möglich ist. Dagegen müssen die psychologischen Momente noch weit mehr erforscht und berücksichtigt werden, als dies bisher der Fall war. Mit diesen Fragen hängt auch die Tatsache zusammen, daß es viel leichter ist, eine befriedigende Schwarz-Weiß-Wiedergabe zu erzielen als eine Farbwiedergabe. Beim Schwarz-Weiß-Film liegt eine Abstraktion vor, die sich deswegen nicht störend bemerkbar macht, weil schon nach ganz kurzer Zeit eine psychische Umstimmung eintritt, die uns das Fehlen der Farbe vergessen läßt. Ja, es tritt sogar ein Ergänzungseffekt auf, in dem auf Grund unseres Erinnerungsvermögens das projizierte Schwarz-Weiß-Bild mit dem Vorstellungsbild verschmilzt. Es ist klar, daß diese Ergänzung harmonisch sein wird und nicht im Widerspruch zu unserem Erinnerungsbild steht.
Die Abstraktion ist an sich ein in der Kunst allgemein angewandtes Verfahren, um wesentliche Merkmale eines Gegenstandes hervorzuheben und nebensächliche zu unterdrücken. Damit soll natürlich nicht gesagt werden, daß der Farbinhalt nebensächlich sei, sondern nur, daß ein farbiges Bild leichter Gefahr läuft, in Widerspruch zu unserem Erinnerungsbild zu stehen. Aus diesem Grunde ist es auch fraglich, ob dem plastischen Farbfilm die Bedeutung zukommen wird, die viele von ihm erwarten. Je mehr Merkmale eines Gegenstandes wiedergegeben werden, desto größer werden einerseits die technischen Schwierigkeiten und desto größer auch die Gefahr, in Widerspruch zur Wirklichkeit zu geraten, andererseits fehlt die für eine künstlerische Wiedergabe wohl nie ganz zu entbehrende Möglichkeit der Abstraktion. Es ist sicher kein Zufall, daß die Plastik in ihrer reinsten Form auf Farbe verzichtet und ihrer auch gar nicht bedarf.
Man kann die vorstehenden Überlegungen auch auf die Filmhandlung selbst übertragen. Es wurde schon hervorgehoben, daß der Blindgeborene nach Erlangung des Sehvermögens mit den optischen Eindrücken zunächst nichts anzufangen weiß, weil ihm jede Erfahrung fehlt und es noch nicht zu einer Aufspeicherung von Erinnerungsbildern im Gehirn gekommen ist, die ihm die Möglichkeit geben, das optische Bild auf der Netzhaut mit der Wirklichkeit zu identifizieren. Ebenso würde auch ein Mensch, der zwar im vollen Besitz seines Sehvermögens ist und von dem wir annehmen wollen, daß er die Gegenstände als solche auch identifizieren kann, einer ablaufenden Handlung verständnislos gegenüberstehen, wenn er bis zu diesem Zeitpunkt noch nie einen solchen Ablauf wahrgenommen und empfunden hätte. Voraussetzung ist hier ebenfalls, daß die Wahrnehmung vieler Vorgänge zu einer Aufspeicherung entsprechender Korrelate im Gehirn geführt hat, und daß die Möglichkeit besteht, diese mit den Eindrücken des gerade ablaufenden Vorganges zu vergleichen. Dieser Vergleich wird meist unbewußt erfolgen, gelegentlich können aber auch die Erinnerungswerte zum Bewußtsein kommen.
Ganz analog den oben aufgestellten Forderungen nach der Naturtreue bei der Wiedergabe eines Gegenstandes in den Grenzen der Möglichkeiten des Wahrnehmungsapparates und der Widerspruchsfreiheit zu den Erinnerungsbildern könnten wir ähnliche Forderungen auch für die Wiedergabe von Handlungen aufstellen. Zunächst brauchte die Handlung nur so viele Merkmale zu enthalten, als sie den Grenzen des Auffassungsvermögens angepaßt sind, und dann dürfte sie nicht in Widerspruch zu den Erinnerungsvorgängen stehen, die auf Grund einer längeren Erfahrung aufgespeichert wurden. Es ist klar, daß sowohl das Auffassungsvermögen als auch der Bestand an Erinnerungsvorgängen bei den einzelnen Menschen in sehr viel höherem Maße Schwankungen unterworfen ist als etwa das durch den Sehapparat begrenzte Wahrnehmungsvermögen und der Bestand an Erinnerungsbildern. Außerdem kommt hier noch als zusätzliches Moment die ethische Bewertung von Handlungen hinzu, die nicht nur eine Funktion der jeweiligen Erfahrung, sondern auch der Veranlagung ist. So erklärt sich die oft außerordentlich verschiedene Beurteilung eines Filmes durch Menschen verschiedenen, ja sogar desselben Personenkreises, je nach ihrer Einstellung dem Leben gegenüber. Auch heute gibt es noch kein allgemeines Rezept, die Wirkung und den Erfolg eines Filmes im voraus zu bestimmen, und es ist daher dies nach wie vor eine interessante und lohnende Aufgabe der psychologischen Filmforschung.”
(Narath, Albert (1952): Psychologische Probleme bei der Wiedergabe von Farbfilmen. In: Kino-Technik, 6, pp. 131–132.) (in German)
“Wenn man denselben Gegenstand in verschiedenen Einstellungen aufnimmt, ändern sich naturgemäß die Hintergründe und damit der gesamte Farbenaufbau.
Die Frage der Hintergründe spielt für den Farbenfilm eine ganz erheblich größere Rolle als beim Schwarz-weiß-Film. Vor allem müssen sich die Gegenstände des Vordergrundes bzw. die im Vordergrund befindlichen Personen genügend von den Hintergründen abheben. Man muß möglichst vermeiden, einen Schauspieler direkt vor einer Wand aufzunehmen, da die Bildwirkung hierdurch völlig flach wird. Bei genügender Entfernung der Schauspieler vom Hintergrund erzielt man sofort eine gute Raumwirkung.
Die Unschärfen des Hintergrundes können beim Farbenfilm unter Umständen äußerst störend wirken. Ich meine nicht die durch Unschärfen auftretenden Farbensäume bei Linsenrasteraufnahmen, welche aller Voraussicht nach gänzlich vermieden werden können, sondern jene Farbenklexe, die sich durch verschwommene Konturen des Hintergrundes ergeben. Regisseur, Kameramann und Architekt müssen sich daher bereits vor der Aufnahme über den Einfluß der Hintergrund-Unschärfen im klaren sein.
Wofern eine Unschärfe störend wirkt, wird man einen einfarbigen Hintergrund bzw. einen einfarbigen Hintergrundausschnitt wählen, oder aber einen Hintergrund mit wenig ausgesprochenen Konturen verwenden; oder man muß die Schauspieler soweit vom Hintergrund entfernt aufnehmen (gegebenenfalls unter getrennter Ausleuchtung der Hintergründe), daß keine Störung durch Unschärfen mehr eintritt.
Aus ästhetischen Erwägungen ist es oft zweckmäßig, die “warmen” Farben (purpur, karmin usw.) in ihrem “Schwerpunkt” auf die linke Seite des Bildes zu verlegen, eine Tatsache, die von vielen berühmten Malern bereits berücksichtigt wurde.
Wenn das Bild auch naturgemäß eine harmonische Farbverteilung aufweisen soll, so ist doch eine rein bildhafte Farbkomposition möglichst zu vermeiden.
Man muß durch die Farbenverteilung bereits irgendeine dynamische Komponente hereinbringen, um das bildhaft Stagnierende zu vermeiden und eine filmgemäße Bewegung zu erzeugen. Wenig bewegte Szenen sind im allgemeinen etwas kürzer zu schneiden, da das Auge das farbige nahezu ruhende Bild schneller erfaßt als das entsprechende Schwarzweiß-Bild.
Zweifellos wirken Farbenfilmaufnahmen, insbesondere mit Tiefenschärfe aufgenommene Bilder erheblich plastischer als Schwarzweiß-Filme. Durch geeignete Lichteffekte kann man die Plastik noch erhöhen. Eine im Hintergrund stärker erhellte Szene (z. B. Blick aus dem Wald auf eine Lichtung, Aufnahme eines freien Platzes von einem dunklen Säulengang aus) ergibt einen außerordentlich plastischen Bildeffekt.
In diesem Zusammenhang sei darauf hingewiesen, daß man auch bei einem an sich idealen Farbenfilm-Verfahren neu “sehen” lernen muß. Das gesunde Auge, welches je nach Blickrichtung sich scharf einstellt, die Blickrichtung ungemein schnell wechselt und nur das sieht, was es sehen will, muß sich erst mit der Tatsache abfinden, daß die den wirklich plastischen Dingen zugehörige Farbe auf einmal mit dem zweidimensional flächenhaft projizierten Bild kombiniert ist. Dieses neuen Sehenlernen geht jedoch verhältnismäßig sehr schnell vor sich, und bald kann man einen Schwarzweiß-Film kurz nach einem Farbenfilm überhaupt nicht mehr sehen. Der “Gestalt” des zweidimensionalen Bildes fehlt das neue zwangsläufig assoziativ verbundene Charakteristikum: die Farbe.
Im allgemeinen ist unaufdringliche natürliche Farbgebung für den künstlerischen Farbenfilm Vorbedingung. Wo es aber not tut, z. B. bei Naheinstellungen, welche zum dramatischen Aufbau des Films gehören, muß man jedoch auch den Mut zur stärkeren Farbgebung besitzen.
Wenn der wirklich einwandfreie Farbenfilm zu einem marktfähigen Verfahren entwickelt sein wird, wird das Publikum ihn kategorisch als farbigen Tonfilm verlangen und den schwarzweißen Tonfilm ebenso ablehnen, wie man im allgemeinen den stummen Schwarz-weiß-Film nicht mehr sehen kann.”
(Conrad-Alberti, Victor (1933): Die technischen und künstlerischen Voraussetzungen für die Herstellung farbiger Kulturfilme. In: Film-Kurier, 112, 13.5.1933.) (in German)
“THE UNIMPORTANCE OF CORRECT COLOUR REPRODUCTION
We have seen that the departures from exact colour reproduction inherent in colour photography are of a fundamental and more or less incurable nature. It is necessary, therefore, to consider how important such departures are in practice, and in particular whether errors in some directions are more important than those in others. Before classifying errors under various headings, however, and weighing their relative importance, the way in which colour rendering is judged must be considered for a moment.
Colour photographs are practically never compared side by side with the original scene. Moreover, very few colour photographs are taken with a view to their being seen only by persons who were present at the time of exposure. So the majority of the criticisms of a colour photograph come from persons who never saw the original scene, and whose judgment must be based on some mental comparison between what the picture looks like and what they think it ought to have looked like. The precision of such mental comparisons will depend entirely on the precision of the mental standard used. For instance, if in a colour photograph there was depicted, amongst other things, a pillar-box, then its colour would be mentally compared with one’s impression of the usual colour of pillar-boxes. The impression will be the average of those given by a large number of different pillar-boxes seen on different occasions. And, of course, the colour sensations received will have been subject to the considerable variations caused by differences in the colour, intensity, and the direction of the lighting, whether the surface was wet or dry, dusty or clean, whether the pillar-box had been recently painted or not, etc. The impression, therefore, cannot be precise; and hence, provided that the reproduction of the pillar-box in the colour photograph compares favourably with what a pillar-box could look like, it will generally be acceptable.3
Similarly, all objects of well-known colour give rise to colour sensations which are not always the same and which exhibit quite wide variations. It is these variations, therefore, which govern the tolerances available in colour photography. Let us now consider some of these variations under the headings, lightness, saturation, and hue.
Variations of Lightness
Lightness is probably the attribute of surface colours which varies most from point to point over their surfaces. Any fabric tends to hang in folds, and the troughs will be much darker than the crests. Foliage, and grass, are subject to wide variations in lightness due to the shading of one leaf or blade by another, and due to the difference in orientation with respect to the direction of the incident light. Simultaneous contrast between a patch of colour and its background will also affect its apparent lightness. Thus a given colour will appear much lighter when seen against a black background than when seen against a white background. Thus one would expect errors in lightness reproduction to be relatively unimportant in colour photography, and this is borne out by the fact that there is a considerable range of contrasts over which a colour photograph may vary without detriment.
Variations of Saturation
Saturation also exhibits variations from point to point over a surface, especially on any surface having some sheen or gloss. Such surfaces can also show large increases in saturation when the type of lighting is changed from diffuse to directional, and it is well known that a scene always looks more colourful when the sun is out than when the weather is overcast. The saturation of all colours of distant objects is likely to be decreased by atmospheric haze, and sometimes the effect is strong enough to remove all sensations of colour completely. Reference has already been made to the presence of dust or dirt on surfaces, and while this may result in some changes of lightness, the change in saturation will be considerable. Wetting a matte surface often results in startling increases in saturation. The saturation of the blueness of the sky varies enormously with the direction of viewing relative to the sun, and similar variations occur, of course, in the case of the blueness of seas, rivers and lakes. The apparent saturation of colours also varies with the intensity of the illumination; for instance, at dusk colours are far less saturated than at noon, and by moonlight colour vision has almost ceased, all colours appearing almost completely desaturated.
The use of illuminants of different colours, such as tungsten light and daylight, also results in variations in apparent colour,4 and in the case of blues and yellows the differences in saturation can be considerable. Similar effects also occur, of course, with different phases of daylight, such as noon sunlight, north sky light and late evening sunlight. Again simultaneous contrast can alter the apparent saturations of colours. A pale colour seen against its complementary colour appears more saturated than when against a saturated colour of the same hue. It would thus be expected that errors in saturation would not be of very great importance. This is borne out by the appearance of many water-colour paintings, in which the colours are usually quite pale, but which as pictures are often highly successful. It seems that rather than requiring exact reproduction of saturation, all that is necessary is a reasonable saturation-maximum for each hue, and a uniform desaturation of colours of all hues and saturations, since this is what normally occurs in the conditions mentioned above. In terms of the purity characteristic curve, suggested by Wright,5 this means that the purity gamma can have a value substantially below unity, but that it should have the same value for all hues, and that the curve should be linear with saturation.
Variations of Hue
Let hue as a variable in surface colours now be considered. Simultaneous contrast can cause apparent changes in the hues of colours, but it is clear that most of the phenomena described above, which give rise to changes in lightness and saturation, do not give rise to any changes in the hues of colours. The hues of some objects, however, are quite variable. For instance, foliage varies in hue with time of year and with type of tree, and nearly all fruits change hue with degree of ripeness, as well as being different for different varieties. The colour of flesh varies with type of skin, and, of course, with amount of sunburn. There are, of course, other objects the hues of which vary, but, generally speaking, variations in hue, while important, would seem to be more restricted in surface colours than variations in lightness and saturation. It is, for instance, easier to think of a pillar-box which is a light or a dark red, or a pale or a deep red, than to think of one which is an orange- or magenta-red.
By this type of argument, and by experience, an approximate order of priority in the requirements, as far as colour is concerned, of a successful process of colour reproduction is arrived at, as follows:6
1. Correctness of hue.
2. Approximately equal desaturation of colours of all hues.
3. Approximately proportional desaturation of colours of all saturations.
By way of illustration of these principles it is a well-known fact that in colour reproduction the variable with the least tolerance is the overall colour balance of the picture. If the picture is slightly off balance, pale colours will undergo violent changes of hue, and it is these which make off-balance pictures so intolerable.
It is concluded, therefore, that, owing to the way in which the colours in a photograph are judged, and owing to the large changes in colour which well-known objects so often undergo, the discrepancies inherent in present-day methods of colour photography can be tolerated. That is not to say, of course, that improvement is not desirable, and in certain types of process special devices have to be resorted to in order to overcome some of the discrepancies because they have exceeded the admittedly very wide tolerances.
Most of the effects described in the paper were demonstrated during the lecture, either by actual experiments or by means of colour transparencies.
3 Phot. J., 91B, p. 2, 1951.
4 G.E.C. Journal, 18, Apr., 1951.
5 J. Brit. Kine. Soc., 13, p. 1, 1948.
6 Phot. J., 91B, p. 107, 1951.”
(Hunt, R.W.G. (1951): Colour Cinematography and the Human Eye. In: British Kinematography, 19,6, pp. 173–180, on pp. 176–178.)
“Color photography must always start and end with the mechanism of color perception by the human eye, that is, the color process must first “see” the scene in a manner approximating that of the human eye. It must then reproduce that scene in such a manner that it seems plausible to the eye.
It has long been known that all colors could be matched by mixing amounts of three so-called primary colors. With a given set of three primaries taken from the spectrum, each of the other colors of the spectrum can be duplicated by a mixture of certain intensities of the original three. There is an important reservation in this generalization, for it will be seen from Fig. 1 that in certain regions of the spectrum negative amounts of the three primaries must be permitted. Since negative quantities of the primaries cannot exist, the equivalent is obtained in practice by adding the third primary to the colors which cannot be matched.
If another set of three wavelengths had been chosen as the primaries, a similar but different set of curves would have resulted.
It is frequently desired to express the color-mixture data obtained with one set of primaries in the equivalent amounts of a different set of primaries. This process is illustrated in Fig. 2.
The orange-yellow color at the top can be matched by the primaries, R, G, and B. The squares show the unit amounts of these primaries and the rectangular arrangement at the right shows the amounts of the three required to match the color. If we wish to express this color in terms of another set of primaries, R’, G’, and B’, we can first find the amounts of R’, G’, and B’ which are required to match exactly the original unit amounts of R, G, and B. By using R’, G’, and B’ in these ratios, the total amounts of R, G, and B shown in the upper right can be matched. Then the sum of the three values of R’, the sum of the three values of G’, and the sum of the three values of B’ will match the original color.
This operation is usually carried out mathematically. The equations which are involved are shown below. There a color, C, is matched by an amount, r, of the primary, R, an amount, g, of the primary, G, and an amount, b, of the primary, B. These primaries can, in turn, be defined in terms of the amounts (a11, a12, a13, a21, etc.) of a second set of primaries, R’, G’, and B’. If the unit amounts of R, G, B, and R’, G’, B’ are defined by the amounts required to produce a white of a certain brightness, then the substitutions and factoring give the last form of the equation.
In a similar manner, the amounts of the primaries, R’, G’, and B’, which are required to match the various spectrum colors can be calculated from the color-mixture curves of the primaries, R, G, and B. This then gives us a new set of color-mixture curves. The change from one set to the other is a linear transformation. There are an infinite number of primaries and corresponding color-mixture curves which describe the color-vision characteristics of the human eye, and all of them tell exactly the same story. These of course include as primaries purely hypothetical colors which cannot exist in practice. Proper choice of hypothetical primaries can lead to mixture curves with no negative regions.
Another important characteristic of color vision is the relative brightness of different colors. Equal energies of different colors are not of equal brightness or luminosity. If the relative luminosity of spectrum colors of an equal energy is measured, the curve in Fig. 3 results.
For many years there was a good deal of confusion in the field of color measurement because a variety of workers used different primaries in determining the color-mixture curves and different methods of measuring the luminosity of the different spectrum colors. Different types of equipment led to slightly different results, because of certain inaccuracies and also because of the fact that the individual observers vary in their characteristics. For this reason a standard system of color specification became necessary for all the various workers in the field of color.
This standard system was set up by the International Commission on Illumination and is called the ICI system. This group selected the previously adopted luminosity curve as a standard and defined its three primaries to meet certain requirements. First, all real colors should be matched with positive amounts. Second, one primary should be such that one of the mixture curves would be identical to the luminosity function. By using the best available color-mixture data, the primaries, X, Y, Z, and the corresponding mixture curves were established to define the “standard observer.” Obviously the primaries do not represent real colors. These standard color-mixture curves are shown in Fig. 4.
Maxwell, in 1855, suggested that positives made from black-and-white negatives which, in turn, were made through red, green, and violet filters, could be used to control the amounts of red, green, and violet light transmitted by filters and that these, when superimposed, would give a color reproduction of the original scene.
All additive systems of color photography are modifications or applications of this invention made more than 90 years ago.
This system of Maxwell’s was an additive method. Shortly afterwards this principle was extended by du Hauron, who showed that images made through the red, green, and blue filters and printed in superposition in cyan, magenta, and yellow dyes or pigments would also give a fair reproduction of the original scene. This extension of Maxwell’s system is the basis for all the subtractive color systems. However, it was many, many years before the sensitizers, dyes, and photographic materials in general were available for the application of these simple principles.
For many years there were heated arguments as to the exact requirements for the sensitivity distributions of the three emulsion-filter combinations to be used in obtaining the three records for color photography. Certain workers in the field felt that narrow bands of sensitivity in the red, green, and blue regions of the spectrum gave the most satisfactory results. Others felt that the sensitivity distributions of the three emulsions had to match the sensation curves of the eye. Others attempted to match the sensitivity-distribution curves of the emulsions with the absorption curves of the dyes or pigments being used in making subtractive prints. Although these earlier efforts did not lead to a resolution of these theoretical problems, enough practical experience was gained so that when improved sensitizers, emulsions, and techniques of making colored photographic images were developed it was possible to work out empirically methods of making quite satisfactory color photographs. Continued progress in the “techniques” of making colored images and superimposing them have brought color photography to the present state.
Finally, knowledge as to the theoretical requirements for “exact” photographic color reproduction followed close on the heels of better data describing the color-vision characteristics of the eye. Hardy and Wurzburg1 applied the principles of colorimetry and the characteristics of the human eye to the problem of establishing the theoretical requirements for the perfect additive three-color photographic process. They showed that the sensitivity distributions of the three emulsions used in obtaining the three images must correspond with the color-mixture curves determined with the three primaries which were used in showing the additive color picture. These distribution curves would be some linear transformation of the color-mixture curves obtained by using other primaries, including the standards selected by the ICI. Of course, for any primaries which could be used in practice, that is, real colors, even spectrum colors, these film sensitivities would require negative proportions of certain regions of the spectrum. Although a number of suggestions have been made as to how such negative sensitivities might be achieved, and some methods have been patented, to date no satisfactory practical solution has been found.
Later, Yule2 and MacAdam3 extended these principles to the problem of subtractive color photography. Although it was not possible to establish the so-called ideal dyes for use in subtractive photography, MacAdam showed that for one set of dyes actually being used in practice, it was possible to establish so-called additive primaries which would describe the behavior of subtractive mixtures of these dyes and was able to show that by the use of six masks it should be possible by photographic means to obtain a very close approximation to “exact” color reproduction. The basic principles were those developed by Hardy and Wurzburg, and the sensitivity requirements of the three emulsions in this process were the color-mixture curves derived from the primaries. These, of course, contained negative portions at certain regions in the spectrum.
This can be summarized by stating that the theoretical requirements which a subtractive color process must fulfill in order to give “exact” color reproduction have not been established completely. They do indicate a need for negative sensitivities and for the use of six masks. The first of these needs cannot be fulfilled at all and the second is entirely impractical. So, the color processes have to struggle along without fulfilling these requirements, and they do give satisfactory results.
However, even though present-day color processes do give satisfactory results, there are certain deficiencies which must exist because of the failure to meet the requirement that the film-sensitivity distribution be a linear transformation of the color-mixture data of the eye. For example, there are an infinite number of energy distributions of light which appear the same to the eye. A color film will not necessarily see such colors as being alike.
A pair of dye combinations which produce a very close visual match is shown in Fig. 5. These spectrophotometric measurements show the densities of the two combinations to light of the various wavelengths in the visible range. As is often the case, these colors which appear to be identical have very different absorption characteristics. When photographed with one of the commercially successful color films, the resulting photographs are also very different.
At first thought, one may say, “This doesn’t make too much difference because we shall never encounter two colors of this sort side by side.” However, the fact that the two colors do not match indicates that at least one of them is not properly reproduced. In fact, any color might be improperly reproduced by any of the present color processes which in normal practice give excellent results. Fortunately most of the colors which we normally encounter have more or less continuous light-absorption bands and are reproduced fairly accurately. It may appear that an undue amount of emphasis is being put on this type of problem. The important point is that in dealing with flowers, new types of fabrics, or with new color situations in general, it is wise to make a test with a given photographic process to see that it will reproduce adequately the specific colors which are important rather than to assume that the process is perfect and start shooting.
For most practical applications of color photography, the reproduction of colors need not be theoretically perfect. Even with very pleasing color pictures, an analysis of the individual colors will reveal considerable departure from the hue, saturation, and brightness of the original colors of the scene. However, when combined in a picture of familiar and pleasing composition, the color reproduction is plausible enough to give the impression of correct reproduction.
Let us now look at some of the requirements which can and must be fulfilled in obtaining satisfactory color photographs.
The first of these requirements is color balance. This is usually best observed in the accuracy with which grays of various brightnesses are reproduced. One might consider this the minimum requirement of a color process. However, the errors encountered in matching grays to the original subject are present in about the same degree in the reproduction of all colors. In the case of pastels and other colors of low saturation, this error in balance may become a serious distortion.
Color balance is measured by reading a scale of grays with a color densitometer and plotting the densities of the dyes against the logarithm of the exposure. By definition,4, 5 the equivalent neutral densities (END) of the dyes of a given color process are those which, in superposition, will appear gray under the viewing conditions for which the color film is designed. A correctly balanced color process would have a gray scale in which all three dye curves were superimposed. Slight deviations from this ideal are usually encountered at very low densities and also in the region of maximum density.
If, however, the color balance is uniformly high in any of the dyes, magenta in the case of Fig. 7, the picture through such a process will also show a decided shift to a magenta balance. This is noticed not only in the reproduction of grays but in a change in all the colors of the pictures. Thus, blues become more purple, yellows become more orange, and greens become darker. This type of distortion results also from any change in color temperature of the exposing light from that for which the color film has been balanced.
This uniform shift in color balance of a color film may at first appear to be very objectionable but when a picture is viewed by projection in a darkened room the distortion appears to become less objectionable with continued viewing. This accommodation of the visual process, or color adaptation, does tend to make an off-balance picture appear more nearly satisfactory by projection. If, however, as in motion picture projection, the color balance shifts from scene to scene the color change is very noticeable.
In Figs. 6 and 7 the gammas of all the dye scales were equal. If the gammas of the three dye images are not equal, Fig. 8, the color photograph varies in color balance from one density level to another and the distortion in color reproduction varies depending on the color and its brightness. In a picture through the process represented in Fig. 8, the light densities would be much too green and the darker portions of the pictures much too magenta. This is a very undesirable type of distortion, for the eye cannot become adapted to both errors. Such a picture continues to be objectionable, no matter how long we look at it. From the shapes of the curves it is easy to see why such a distortion has been termed a kink.
Assuming that the first requirement is fulfilled and that correct color balance and matched relative gammas of the three dye images can be achieved, and these are no small assumptions, we are still faced with a difficult decision: “What gamma or contrast level is most desirable for a specific color process?” In black-and-white photography, the question is completely answered by the requirements for pleasing tone reproduction. In color photography, the problem is complicated by the fact that color saturation varies with the gamma. At low gamma, we can have all the advantages of pleasing tone rendition and greater latitude but we must pay for these advantages by sacrificing color saturation. At high gamma, color saturation is satisfactory but latitude must be low.
The relationship between gamma and color saturation can be explained quite easily by expressing the amounts of the three image dyes present in a given area of a color photograph in terms of equivalent neutral density. For example, suppose that a given color is reproduced by a color process having a gamma of 1.0 by the amounts of the dyes: cyan, 0.8; magenta, 0.1; and yellow, 0.6. If the same process were operated at a gamma of 1.5, the reproduction densities would be: cyan, 1.2; magenta, 0.15; and yellow, 0.9. It is possible to determine the color densities (in terms of END) over and above the gray content (in terms of END) merely by subtracting the density of the dye occurring in the lowest amount. The results of this subtraction from the two groups of densities above are given in Table I.
The much greater density differences at the gamma of 1.5 represent a significant increase in color saturation.*
Experience has shown that the color saturation obtained at higher contrast is quite desirable so that in practice, processes are usually operated at a relatively high contrast. The desired tone reproduction must be obtained by much flatter lighting than normally would be used in black-and-white photography. It may be noted in passing that the opposite approach is not satisfactory, that is, a low-contrast process with contrasty lighting. The maximum color saturation possible is limited by the density range and gamma of the color process and is only slightly affected by lighting variations.
The decision between usable contrast and acceptable color saturation again arises in duplicating a color film. The process of duplicating a color film results in a loss in color saturation, providing the contrast is reproduced at the same level as in the original picture. This loss in saturation is a result of the properties of the dyes available for use in color photography. The only way this color-saturation loss can be improved is by increasing the contrast of the reproduction. This again results in a very definite compromise in the over-all quality of the reproduction. A more complete discussion of this problem is given in a separate paper by Miller.6
After considering some of the theoretical problems involved in obtaining more nearly perfect color reproduction and some of the more elementary variables in color processes, we should like to stress that what is necessary and what is desired in a color process depend very largely on the manner in which the process is to be used. The requirements for a good motion picture print in color are different in many respects from the requirements for a good reflection print process.
Evans7 has directed attention to the many psychological effects which complicate any orderly analysis of color vision and color photography. Brightness constancy, color and brightness adaptation, and simultaneous contrast have a profound influence in all color systems.
These phenomena result from the fact that the visual process does not function merely as a physical instrument for measuring the stimuli from different areas. On the contrary, the appearance of an object is always affected by the spatial relation of the object to other objects and to lighting conditions. The eye always sees things as the observer thinks they really are rather than as they happen to appear at the moment. A simple example is that of a white object in a shadow near a black object in full sunlight. Although the luminance of the white object may be much less than that of the black object under these conditions, the eye immediately recognizes the true brightness relationship of the two objects. This brightness-constancy effect may not be shown to the same degree in a photograph as in the original scene since the viewing conditions are in most cases entirely different.
The eye varies in sensitivity to light over a considerable range depending on the intensity level under which it is used. Something similar to this brightness adaptation causes the eye to become adapted to colored light so that it tends to accept that color as white. The maximum effect, of course, is realized when all of the light reaching the eye is of the same color.
Simultaneous contrast has been related to the adaptation of the eye to local areas of a picture. That may be simplified by stating that areas of complementary or contrasting colors appear to be increased in saturation by their proximity within the picture. In addition to selecting colors which produce a desired hue in the finished color photograph, simultaneous contrast can be used to striking advantage in obtaining pleasing color pictures.
These effects are distinctly beneficial in the projection of color transparencies in a darkened room and are therefore very important in the success of many motion picture processes. Furthermore, these psychological effects explain in part why the data obtained in an isolated field of a colorimeter and the mathematical derivations from such data may have very little correlation with the infinite variety of conditions which can be encountered in photography and in the presentation of the resulting pictures of everyday objects.
You will recall the emphasis on the word approximation, in comparing the color photographic process to the visual process. We think it is still safe to state that the perfect color process has not yet been realized. There are, however, many successful color processes which can give very pleasing results in spite of the many compromises which must be present in each system. This means that to obtain satisfactory results the user must learn quite a lot about the color process with which he is working. As he learns what a particular color process will, and equally important, what it will not do, his success with that process will become more consistent.
Close co-operation between the user of color photographic materials and the manufacturer of them has been and will continue to be very important in obtaining satisfactory results with what we now have and know. This sort of co-operation is also necessary for the introduction of improved color photographic materials and techniques which will give better results.
*This method of expressing colors in terms of END does not express a quantitative value for hue and saturation such as a Munsell notation or the like, but it is a useful technique in the field of color photography.
1 A. C. Hardy and F. L. Wurzburg, Jr., “The theory of three-color reproductions,” J. Opt. Soc. Amer., vol. 27, pp. 227–240; July, 1937.
2 J. A. C. Yule, “The theory of subtractive color photography,” J. Opt. Soc. Amer., vol. 30, pp. 322–331; August, 1940.
3 D. L. MacAdam, “Subtractive color mixture and color reproduction,” J. Opt. Soc. Amer., vol. 28, pp. 466–480; December, 1938.
4 R. M. Evans, “A color densitometer for subtractive processes,” J. Soc. Mot. Pict. Eng., vol. 31, pp. 194–202; August, 1938.
5 M. H. Sweet, “A precision direct-reading densitometer,” J. Soc. Mot. Pict. Eng., vol. 38, pp. 148–173; February, 1942.
6 T. H. Miller, “Masking: A technique for improving the quality of color reproductions,” J. Soc. Mot. Pict. Eng., this issue, pp. 133–155.
7 R. M. Evans, “Visual processes and color photography,” J. Opt. Soc. Amer., vol. 33, pp. 579–614; November, 1943.”
(Hanson, Jr., W. T.; Richey, F. A. (1949): Three-Color Subtractive Photography. In: Journal of the Society of Motion Picture and Television Engineers, 52,2, pp. 119–132, on pp. 120–132.)
In Fig. 2 are shown transmission curves of three so-called ”ideal subtractive dyes.” The cyan dye absorbs light only in the orange and red parts of the spectrum, the magenta dye only in the green part, and the yellow dye only in the blue part. By varying the concentrations of these dyes the amount of light in each of the three blocks of wavelengths (marked R, G, and B) is varied. If, then, the colour photograph consists, as is usual, of a cyan dye-image, a magenta dye-image, and a yellow dye-image superimposed, assuming that we are using these ideal dyes, the colour of the reproduction may be regarded as an additive mixture of light of the three blocks of wavelengths, R, G, and B. It is clear that the use of these blocks of wavelengths, rather than the monochromatic radiations used previously, will result in still further departures from correct colour reproduction. For the areas marked R, G, and B in Fig. 3 show the size of the signals to which the three blocks of wavelengths give rise respectively. It is seen that the red block gives rise not only to a ρ-signal but also to a considerable γ-signal. Similarly the green block gives rise to considerable ρ- and β-signals as well as the required γ-signal, and the blue block gives rise to considerable γ- and ρ-signals as well as the required β-signal. Hence, using the subtractive process, even with ideal dyes, there will be considerable desaturation of colours in the reproduction, and also errors of hue and lightness.
Of course, the ideal dyes of Fig. 2 are not available in practice, and the transmission curves of a set of dyes typical of those that have to be used are shown in Fig. 4. It is seen that they do not absorb uniformly over the blocks of wavelengths where they are supposed to absorb, and that they do not transmit 100 per cent. of the light in the other regions of the spectrum. These differences will, of course, add further to the already considerable errors in saturation, hue, and lightness inherent in the system.
It has been assumed all along that the three photographic layers have had the sensitivities β, γ, and ρ of Fig. 1, but in practice this condition is also not usually met. This is not because it cannot be met; by choosing suitable sensitizers and filter layers it can be well enough approximated to; but in view of the large number of errors inherent in the system it is generally considered advisable to use sensitivity curves more widely separated than those of Fig. 1, and a set typical of those used is shown in Fig. 5 together with the curves of Fig. 1 for comparison. The use of such a set results in some increases in the saturation of the colours, which helps to counteract the desaturating effect described above, but it may also result in additional errors in hue and lightness.”
(Hunt, R.W.G. (1951): Colour Cinematography and the Human Eye. In: British Kinematography, 19,6, pp. 173–180, on p. 175.)
For instance, when several copies of a colour film are needed, or when paper-prints are required, the most convenient method to use is that in which a colour-negative is made by the camera exposure, the required number of positive prints being printed subsequently on to films or paper, as the case may be. The colour negative is like an ordinary black-and-white negative in that whites are rendered as blacks, and blacks as whites, but in addition to the lightness of the colours being reversed the hues are also reversed; that is, they take on the hues of the complementary colours. These complementary colours are formed by means of three dyes, usually a cyan, a magenta, and a yellow dye of the type shown in Fig. 4. The colours in the final print are also formed by means of such a set of dyes, so that dyes are used twice in such a process. This means that the defects in the dyes take their toll on the colour reproduction twice, and the result often exceeds the tolerance. The particular faults which are most troublesome are the unwanted absorptions of the magenta dye in the blue region of the spectrum, and of the cyan dye in both the blue and the green regions. These unwanted absorptions cause large errors in lightness, saturation and hue, but the recent introduction of “coloured-couplers”7 can result in their being very effectively counteracted in the colour negative.
The principle on which coloured couplers work is shown diagrammatically in Fig. 6. Suppose that, in the colour negative, at its maximum concentration, m, the magenta dye, has red, green, and blue transmissions of 100%, 5%, and 50% respectively, as shown in Fig. 6 (a) by line A. It is assumed, for the sake of simplicity, that it is an ideal magenta dye except for a uniform unwanted absorption in the blue. The lines, B, C, D, E, and F, show what the absorptions would be at concentrations (3/4)m, (1/2)m, (1/4)m, (1/8)m, and zero, respectively. It will be supposed that this dye is formed by the colour-development of a magenta “coupler” in the green-sensitive layer of the emulsion. Let the concentration of the coupler before development be c. Then the concentrations of the coupler corresponding to the dye-concentrations A, B, C, D, E, and F will obviously be zero, (1/4)c, (1/2)c, (3/4)c, (7/8)c and c respectively.
Suppose, now, that the coupler, instead of being colourless, was yellow, having red, green, and blue transmissions (at concentration c) of 100%, 100%, and 50%, respectively. As it is colour-developed to form the magenta dye, its yellow colour in the layer gradually becomes less and less, and its transmission curves for the same levels A, B, C, D, E, and F discussed above would be as shown in Fig. 6 (b). The full transmission curve for the layer, of course, is given by combining the appropriate pairs of curves from Figs. 6 (a) and (b) and these are shown in Fig. 6 (c). It is seen that the transmission in the blue region remains constant. When there is no magenta dye, the coupler alone has a transmission of 50%; when all the coupler has been used it no longer absorbs at all, but the magenta dye has a transmission of 50%. At all intermediate stages the blue transmission of the coupler multiplied by the blue transmission of the magenta dye is also found to be equal to 50%.
Clearly, with this system, the effect of light on the green-sensitive layer results in variation in the green transmission of that layer, but has no effect on the values of the red and blue transmissions which are fixed at 100% and 50% respectively. The low value of the constant blue transmission can be easily compensated for, simply by increasing the blue content of the light used for printing by a factor of 2. Thus, the magenta-dye, with its yellow coupler, together form an arrangement by means of which only light in the green part of the spectrum is modulated. And hence, from the photographic point of view, the unwanted blue absorption of the magenta dye has been eliminated.
A pink-coupler, which forms a cyan-dye in the red sensitive layer, can similarly eliminate the effects of the unwanted green and blue absorptions of the dye. The way in which this takes place is shown in Fig. 7. In Fig. 7 (a), for the sake of simplicity, we have shown the transmission curves of a cyan dye which is ideal except for two uniform unwanted absorptions in the green and blue regions. The line A refers to the dye at maximum concentration, the red, green, and blue transmissions being 5%, 30%, and 40% respectively. The other lines are analogous with those of Fig. 6. Suppose that the coupler is of a pink colour, having, at maximum concentration, red, green, and blue transmissions of 100%, 30%, and 40% respectively, as shown in Fig. 7 (b); when this coupler is present with the cyan dye which it forms on colour development, the red sensitive layer will have the transmission curves shown in Fig. 7 (c) for the different concentrations. Again it is seen that, where there were varying unwanted absorptions, now they are constant. Hence, by increasing the green content of the printing light by a factor of 31/3 and that of the blue by a further factor of 21/2, the net result of the effect of light on the red-sensitive layer is merely to modulate the red-transmission of the layers.
Of course, when actual dyes and coloured couplers are used the transmissions shown as constant in Figs. 6 (c) and 7 (c) are only approximately constant, but this scarcely impairs the degree of improvement resulting. In fact, by allowing these transmissions to rise, by using couplers of deeper colours, the unwanted absorptions of the cyan and magenta dyes used in the print can also be, to some extent, compensated for.
Most of the effects described in the paper were demonstrated during the lecture, either by actual experiments or by means of colour transparencies.
7 J. Opt. Soc. Amer., 40, p. 166, 1950.”
(Hunt, R.W.G. (1951): Colour Cinematography and the Human Eye. In: British Kinematography, 19,6, pp. 173–180, on pp. 178–180.)
“THE CAUSES OF INCORRECT COLOUR REPRODUCTION
Our knowledge of the way in which the retina responds to light of different colours is in many ways incomplete. But it is generally agreed that there must be three different types of receptor: one type mainly sensitive to orange and red light, another mainly sensitive to green light, and a third mainly sensitive to blue light. Moreover, from various experiments on colour matching,1 the spectral sensitivity curves of these three types of receptors are known approximately and are as shown in Fig. 1, the three sensitivities, β, γ, and ρ being plotted against wavelength. When light of any colour falls on the retina, the different sensitivity curves of the three receptors will result in three signals, β, γ, and ρ being sent to the brain, and if, for instance, the ρ signal is larger than the other two signals, then a reddish sensation will be experienced; if the γ signal is the largest, then the sensation will be greenish, and so on.
If, now, the three layers of the colour film had sensitivity curves closely matching those of the eye, then it might be thought that correct colour reproduction must necessarily result. But three layers of photographic emulsion are only capable in themselves of producing black-and-white images; the colour has to be put into the system elsewhere. The simplest way, from the theoretical standpoint, of doing this is to strip the three black-and-white negative images apart,* print (with perfect tone reproduction) black-and-white positive transparencies from them, and insert them in three separate projectors, one of which is fitted with a blue filter, another with a green filter, and the third with a red filter. If the red filter were chosen so that the light transmitted by it affected only the ρ-receptors of the eye, the green filter only the γ-receptors, and the blue filter only the β-receptors, then on superimposing the three images on a screen exact colour reproduction would result. For instead of the colours of the original scene producing their appropriate ρ, γ, and β signals, they have produced areas of photographic density on the positives in the projectors such that at each point the transmission is proportional to the ρ, γ, or β signal, as the case may be. Hence light from the red lantern produces the correct ρ signal at each point in the picture, that from the green lantern the correct γ signal, and that from the blue lantern the correct β signal, and hence the eye, receives the same signals ρ, γ, and β from the reproduction as it would have done from the original.
In order for the above state of affairs to obtain, it is essential that:
(a) the three layers of our film should have the sensitivity curves, β, γ, and; ρ
(b) perfect tone reproduction be achieved in our positives, and
(c) the blue, green, and red filters in our three projectors be such that they stimulate respectively only the β-receptors, only the γ-receptors, and only the ρ-receptors of the eye.
Unfortunately, this last condition is impossible to achieve. If reference is again made to the curves of Fig. 1, the reason will be obvious. Even if light of monochromatic purity is used in our three lanterns, it is only possible for the red lantern to meet the requirement. It is seen that light of wavelength R, 6500A (or any longer wavelength), stimulates neither the γ- nor the β-receptors, but only the ρ-receptors. Light of wavelength B, 4400A, is seen to stimulate chiefly the β-receptors, but unfortunately the ρ- and γ-receptors are slightly stimulated in addition. When a search is made for a wavelength of light which stimulates only the γ-receptors, none can be found which does so even approximately. The best that can be done is to choose the wavelength G, 5050A, which stimulates the γ-receptors strongly, and the ρ- and β-receptors to almost the same extent.
If, then, using the sensitivity curves of Fig. 1 for the three layers of the film, the three transparencies are projected in lanterns filtered to give the monochromatic radiations, R, G and B, how would the reproduction differ from the original? Any portion of the picture which was before reproduced by only red light, would still be correct, of course, for the red projector meets the requirement. But wherever the green projector was shining, there would be an unwanted excess of ρ- and β-signals, and wherever the blue projector was shining there would be an unwanted excess of ρ- and γ-signals. Thus, in general, the three signals in any part of the picture will now be more nearly equal to one another than before, and this means that the colours will be less saturated than they should be, and also that changes in hue and brightness will occur.
But triple projection with monochromatic light is not a commercially practicable method of colour reproduction, and, while no additive method of colour kinematography has yet met with much commercial success, there has recently been a renewed interest in the lenticular method of projection.2 If the strips of red, green, and blue filter fitted to the projection lens transmitted only very narrow bands of light, the desaturation of colours would not be much worse than with the monochromatic radiations. But one of the principle difficulties with the lenticular process is that the filters over the projection lens result in a considerable loss of light, and, in order to minimise this, filters transmitting broad bands of light have to be used. This means, of course, that the unwanted ρ and β signals given rise to by the green filter will be larger, as will also the unwanted ρ and γ signals given rise to by the blue filter.
Furthermore, even the red filter will give rise to some γ signal, in addition to the required ρ signal. So the result will be that the desaturation will be worse than in the case of monochromatic radiations.
At the present time the majority of modern processes of colour photography do not use the additive method at all, but the more convenient subtractive method. Fundamentally, however, both methods depend on the same principle.
* It is interesting to note that a practicable method of doing this has recently been developed (see Reference 8).
Most of the effects described in the paper were demonstrated during the lecture, either by actual experiments or by means of colour transparencies.
1 See, for example, W. D. Wright, “Researches on Normal and Defective Colour Vision,” Kimpton, London, 1946.
2 Brit. J. Phot., 98, p. 456, 1951.
8 J. Soc. Mot. Pict. & Tel. Eng., 54, p. 445, 1950.”
(Hunt, R.W.G. (1951): Colour Cinematography and the Human Eye. In: British Kinematography, 19,6, pp. 173–180, on pp. 173–175.)
“HELMUT FRIESER und RUDOLF REUTHER. Vorgetragen von H. FRIESER
Farbumstimmung und Farbenfotographie*)
Nun kann man aber durch farbreizmetrische Betrachtungen allein keine Aussage über das Aussehen der Farbe, über die Farbempfindung machen. Nur unter ganz bestimmten Bedingungen ist es möglich, einem reizmetrisch festgelegten Farbreiz eine Farbempfindung zuzuordnen. Selbst die Charakterisierung der von Spektrallichtern ausgelösten Farbempfindung durch deren Wellenlänge genügt nicht, wenn die Betrachtungsbedingungen nicht genau angegeben sind.
Diese Notwendigkeit einer eindeutigen Festlegung liegt daran, daß der Apparat unseres Farbensehens mannigfachen Einflüssen unterliegt, was sich auch durch seine Anpassungsfähigkeit zeigt. Die wichtigste dabei beobachtete Erscheinung wird als Farbumstimmung bezeichnet.
Im folgenden soll ein Weg gezeigt werden, wie man die bei der Farbumstimmung beobachteten Erscheinungen quantitativ beherrschen kann, und zwar soll das geschehen, ohne von der Grundlage der Reizmetrik, dem Gleichheitskriterium, abzugehen, so daß die Behandlung der Farbumstimmung durch eine erweiterte Farbreizmetrik möglich ist.
Im gewöhnlichen Leben erkennen wir die Umstimmung in der Tatsache, daß die meisten Pigmente bei nicht zu starken Änderungen der Art der Beleuchtung ihre Farbe im wesentlichen beibehalten. Besonders deutlich ist das bei Weiß. Ein bei Tageslicht als weiß gesehenes Papier erscheint uns auch bei Lampenlicht weiß, obwohl uns die von dem Papier zurückgeworfene Strahlung der Lampe gelb erscheint, wenn wir auf Tageslicht gestimmt sind. Man erkennt aus dieser Tatsache, daß das Auge versucht, sich so umzustimmen, daß wieder die Gegenstände weiß gesehen werden, die bei Tageslicht weiß erscheinen.
Durch einige einfache Versuche kann man sich von dem Wesen der Umstimmung leicht überzeugen.
1. Man projiziert zunächst durch ein Graufilter und erhält ein neutralgrau erscheinendes Feld. Nun tauscht man das Graufilter gegen ein Rotfilter aus. Die ganze Projektionswand erscheint dann hellrot. Dabei beobachtet man, daß das zunächst stark gesättigte Feld bei längerer Betrachtung an Sättigung abnimmt und weißlich erscheint. Projiziert man nach etwa zwei Minuten wieder durch das Graufilter, so erhält man jetzt ein deutlich blaugrün, also in der Kompensationsfarbe zu Rot erscheinendes Feld. Durch die Rotbelichtung wird das rotempfindliche Aufnahmeorgan des Auges ermüdet, und es überwiegt die Reizung des blau- und grünempfindlichen. Das Auge ist umgestimmt. Diesen Versuch kann man mit anderen Umstimmungsfarben leicht wiederholen.
2. Noch deutlicher erkennt man die Verhältnisse, wenn man nur mit einem Auge betrachtet und nur ein Auge durch Rot umstimmt. Betrachtet man hierauf das Graufilterbild abwechselnd mit den beiden Augen, so sieht man dieses mit dem früher geschlossenen Auge grau, mit dem anderen bläulich. Die Umstimmung erfolgt also für beide Augen getrennt.
3. Benützt man als Umstimmungsfeld nicht die einheitlich beleuchtete Projektionsfläche, sondern teilt diese in mehrere verschieden gefärbte Felder auf und beobachtet möglichst einäugig mit genau fixiertem Auge, so sieht man bei Betrachtung des Graufilterbildes, daß dieses entsprechend dem Umstimmungsbild in verschiedene Flächen aufgeteilt erscheint, die etwa kompensativ zu den Umstimmungsfarben gefärbt erscheinen. Man erkennt hieraus, daß sich verschiedene Stellen der Netzhaut unabhängig voneinander umstimmen lassen.
Macht man den Vergleich sukzessiv, indem man zuerst mit nicht umgestimmtem Auge betrachtet, dann umstimmt und wieder betrachtet, so muß man sich auf das Erinnerungsvermögen für Farben verlassen, das bekanntlich im allgemeinen sehr schlecht ist. Nur für den Fall der Kardinalfarben ist das möglich, indem man versucht, das reinste Rot, Gelb, Grün und Blau bei verschiedener Umstimmung einzustellen.
Die Einstellung dieser Farbempfindungen ist mit erstaunlicher Genauigkeit möglich. Nach dieser Methode führten Tschemark und seine Schüler ihre Untersuchungen über die Umstimmung und das Neutrallicht aus, über die auf der ersten Farbentagung1 berichtet wurde. Eine eingehende farbmetrische Untersuchung ist dadurch aber nicht möglich.
Es gibt noch eine andere Methode der Untersuchung, die auf beliebige Farben angewendet werden kann und welche die Tatsache benutzt, daß die beiden Augen unabhängig voneinander umgestimmt werden. Bei diesem Verfahren werden Farbgleichungen zwischen zwei Lichtern eingestellt, von denen jedes mit einem anderen Auge gesehen wird. Diese Methode setzt natürlich voraus, daß tatsächlich sich die beiden Augen unabhängig voneinander umstimmen und daß auch der Grad der Umstimmung bei einäugigem Sehen derselbe ist wie bei beidäugigem. Dies wäre noch einwandfrei festzustellen. Jedenfalls wurde von Tschemark2 nur ein geringes “Mitgehen” des einen Auges bei Umstimmung des anderen beobachtet. Aber auch wenn diese Bedingungen nicht streng erfüllt sind, so kann diese Methode doch eine Reihe von wichtigen Tatsachen vermitteln.
Diese Methode wurde von uns verwendet. Eine ähnliche, jedoch in vielen Punkten abweichende, benutzte auch Wright3, wie wir nach Durchführung unserer Versuche feststellten.
Das linke Auge wurde bei unseren Versuchen zur Einstellung des Vergleichsfeldes mit “Normalstimmung” bzw. mit Neutralstimmung benutzt. Das von ihm gesehene linke Vergleichsfeld konnte durch einen Farbmischapparat in seiner Farbe meßbar eingestellt werden. Das rechte Auge sah das Meßfeld und wurde umgestimmt. Den prinzipiellen Aufbau der Versuchsanordnung zeigt Abbildung 1. Von der gemeinsamen Lichtquelle (Punktlichtlampe) gehen zwei getrennte Strahlenbündel über je zwei Spiegel nach dem Vergleichs- und dem Meßfeld, die zur Vermeidung eines stereoskopischen Effektes nicht unmittelbar nebeneinander angeordnet sind. Im linken Strahlengang befinden sich der Dreifarbenmischapparat und ein verstellbarer Graukeil, im rechten Strahlengang können Filter zur Abänderung der Einstellfarbe eingesetzt werden. Beide Strahlengänge gehen durch einen rotierenden Sektor, vor dem sich auch die beiden Augen befinden. Die Anordnung ist so getroffen, daß entweder linker Strahlengang und linkes Auge gleichzeitig eine Sektoröffnung vor sich haben, rechter Strahlengang und rechtes Auge in diesem Augenblick aber verdeckt sind oder umgekehrt. Das Umfeld wird für jedes Auge getrennt durch einen Projektor erzeugt, wobei durch eine zweite Sektorscheibe der eine Projektor das Umfeld dann beleuchtet, wenn das linke Auge, der zweite Projektor aber, wenn das rechte Auge eine Sektoröffnung vor sich hat. Vor dem zweiten Projektor kann das Umstimmungsfilter angebracht werden. Rotiert der Sektor so schnell, daß die Flimmergrenze überschritten wird, so sieht man gleichzeitig beide Vergleichsfelder, das linke jedoch nur mit dem linken Auge, das rechte Feld nur mit dem rechten Auge, dabei kann man dem rechten Auge ein anderes Umfeld geben als dem linken.
Dieser besagt, daß man die Koordinaten einer Farbe nach der Umstimmung (R’, G’, B’) erhält, wenn man die vor der Umstimmung (R, G, B) mit bestimmten Faktoren multipliziert:
Es entspricht die Umstimmung formal also einer Änderung der Größe der Eichreize. Dies gilt aber in der einfachen oben angeschriebenen Form nur, wenn man mit den Grundreizen als Eichreizen arbeitet. Bei allgemeinen Eichreizen K, L, M gilt der Koeffizientensatz in einer erweiterten Form
Aus dem Koeffizientensatz folgt unmittelbar die sogenannte “Persistenz” der Farbgleichungen, d. h. die Tatsache, daß Farbgleichungen auch bei Umstimmung bestehenbleiben.
Bei der Betrachtung der Umstimmung nehmen also die Grundreize eine bevorzugte Stellung ein, im Gegensatz zu anderen reizmetrischen Problemen (mit Ausnahme von Betrachtungen über normales Farbensehen). Es ergibt sich dadurch die Möglichkeit, die Versuche über die Umstimmung zur Festlegung der Grundreize zu verwenden, was ja auch v. Kries bereits vorgeschlagen hatte und auch von anderer Seite benutzt worden war, indem man ein Auge durch farbige Bestrahlung für einen Grundreiz unempfindlich machte5.
Der Einfachheit halber sollen die folgenden Betrachtungen mit den Grundreizen ausgeführt werden. Zunächst ist es aber nötig, einige Definitionen und Festsetzungen zu treffen. Gemäß dem Koeffizientensatz war die Umstimmung durch die Umstimmungsfaktoren 1/1 charakterisiert worden. Da deren absoluter Wert für Beurteilung einer Veränderung der Reizart unnötig ist, setzt man zweckmäßig als relative Umstimmungsfaktoren
1. Einstellung auf Gleichheit von Vergleichs- und Meßfeld, auf das die Umstimmungsfarbe projiziert wird.
a) Für beide Augen kein Umfeld.
b) Links kein Umfeld, rechts Umstimmungsfarbe auch als Umfeld.
Der Versuch wurde für verschiedene Umstimmungsfarben durchgeführt. Abb. 2 zeigt, daß durch die Umstimmung alle Farbpunkte der Umstimmungsfarbe gegen einen Punkt (Neutralpunkt) verschoben werden. Es entspricht dies der früher erwähnten Verweißlichung bzw. Entsättigung der umstimmenden Farbe.
2. Einstellung auf Gleichheit von Meß- und Vergleichsfeld.
a) Beide Augen gleich gestimmt (5000 ° K Farbtemp.)
b) Links Umfeld 5000 ° K, rechts Umfeld U oder U2.
Abb. 3 zeigt die Verschiebung der Punkte in der Farbtafel. Außerdem sind die Punkte für vollständige Umstimmung berechnet und für einige Farben eingetragen. Man sieht, daß die entsprechenden Punkte, wie zu erwarten, auf einer Linie liegen. Der Umstimmungsgrad ist ohne weiteres aus der Farbtafel zu entnehmen als Verhältnis der Abstände.
Bei diesem Versuch ist der Koeffizientensatz geprüft und als gültig befunden worden, wie die in Tabelle 1 gezeigte Konstanz der q-Werte zeigt.
3. Versuch 2 wurde für verschiedene Leuchtdichten des Umfeldes und einige Farben wiederholt. Das Ergebnis zeigt Abb. 4.
4. Durch Einstellung in verschiedenen Zeiten nach Wechsel des Umfeldes wurde die zeitliche Abhängigkeit von Um- und Rückstimmung untersucht. Wie Abb. 5 zeigt, erfolgen beide entsprechend einer e-Funktion, und zwar ist a = 100 (1–10 kt).
Bisher war nur die Veränderung von Farblichtern betrachtet worden, die sich bei Veränderung der Umstimmung physikalisch nicht änderten. Praktisch wichtig ist aber besonders die Frage nach der Veränderung von Körperfarben bei Änderung der Beleuchtung. In diesem Fall ändert sich ja bereits ohne die Umstimmung der Farbort der Farbe in der Farbtafel, da sich ja die Zusammensetzung des reflektierten Lichtes ändert. Im allgemeinen wird nun diese Veränderung bei Umstimmung auf die neue Beleuchtung wieder etwas rückgängig gemacht. Aus dem Farbort der Körperfarbe läßt sich natürlich nichts Sicheres aussagen, da die Verschiebung von der spektralen Remissionsverteilung abhängt. Bei einem einigermaßen normalen Verlauf ist also mit einem Rückgang in die Nähe des alten Platzes in der Farbtafel zu rechnen. Die Verhältnisse für volle Umstimmung – theoretisch ermittelt für Beleuchtungen von 5000 ° K und 2600 ° K Farbtemperatur – zeigt Abb. 6 nach Werten aus einer Arbeit von Ströble6.
Für die Farbenphotographie erhält man durch die Berechnung Unterlagen für die Berücksichtigung der Umstimmung. Diese wird vor allem beim Schmalfilm und beim Kleinbild eine Rolle spielen, wo meist bei Tageslicht aufgenommen, bei Lampenlicht jedoch projiziert wird. Hier braucht man nicht das Weiß des Tageslichts bei der Projektion nachzuahmen, sondern durch die Umstimmung wird auch bei abweichender spektraler Zusammensetzung der Eindruck Weiß wenigstens angenähert erzeugt. Wieweit dies erreicht wird, kann bei Beherrschung der Verhältnisse bei der Umstimmung ausgesagt werden. Daß keine vollständige Umstimmung vorliegt, sieht man schon daraus, daß eine starke Abhängigkeit der Wiedergabe von dem Projektionslicht gut zu beobachten ist. Diese Tatsache unterstreicht aber gerade die Wichtigkeit der vorgetragenen Untersuchungen und ihre Weiterführung. Erst wenn man die Umstimmung beherrscht, kann man an die anderen, nicht weniger wichtigen farbmetrischen Probleme der Farbbildprojektion herangehen.
Ein wichtiger Effekt der Umstimmung bei der Projektion von Farbenbildern ist der Ausgleich eines Farbstiches. Ein blaustichiges Bild z.B., dessen Farbstich im ersten Moment stark auffällt, wird nach kurzer Zeit der Betrachtung den Stich weitgehend verloren haben, besonders wenn die Umstimmung durch vorherige Projektion eines hellen blauen Feldes unterstützt wird (Versuch). Die Projektion eines farbigen Umfeldes, die verschiedentlich zur Erzeugung einer Umstimmung vorgeschlagen wurde, hat nur eine geringe Wirkung, wenn das Umfeld nicht sehr hell gemacht wird, wobei es dann aber bereits den Bildeindruck empfindlich stören wird. Auch das Erzeugen einer besonderen Augenstimmung im Publikum, die verschiedentlich vorgeschlagen wurde, hat keinen Zweck. Andere Verhältnisse als beim Projektionsbild liegen beim Papierbild vor, wo die Betrachtung im hellen Raum erfolgt und das Bild die Stimmung des Auges kaum beeinflußt. Hier wird keine Kompensation eines Stiches durch die Umstimmung eintreten, und das dürfte auch ein Grund sein, warum uns Aufsichtsbilder oft viel weniger befriedigen als Projektionsbilder.
Ein Aufeinanderfolgen von Szenen verschiedener Farbstimmung wird trotz der Umstimmung vermieden werden müssen, da diese erst nach einiger Zeit einen größeren Wert erreicht. Je höher die Leuchtdichte, um so stärker wird sich die Umstimmung bemerkbar machen. Sollen zwei Szenen mit stark verschiedener Stimmung aufeinander folgen, so könnte man daran denken, den Übergang allmählicher zu gestalten.
*) Aus dem Wiss.-Photogr. Institut der TH. Dresden. (Direktor: Prof. Dr. Ing. H. Frieser.)
1 v. Tschermak-Seysenegg, A., Dia Bedeutung der Neutralstimmung des Auges für Farbmessung und -musterung. Licht 11 (1941), S. 66–71.
2 v. Tschermak-Seysenegg, A., Über chromatische Mitverstimmung zwischen beiden Augen. Festschrift für Prof. Dr. Anathon Aall, Oslo 1937, S. 97–108. Siehe auch Graefes Arch. 139 (1938), S. 181–231.
3 Wright, W. D., The Measurement and Analysis of Colour Adaptation Phenomena, Proc. Roy. Soc. B 115 (1934), S. 49–87.
4 v. Kries, J., Die Gesichtsempfindungen. In: W. Nagel, Handb. d. Physiologie d. Menschen, Bd. 3/1. Braunschweig: Vieweg 1904. Dort S. 211 f.
5 Vergl. z. B. Fedorow, N.T., u. W. J. Fedorowa, Untersuchungen über das Farbensehen. Bull. verein. Inst. exper. Med. 1936, Nr. 5, S. 29–35 (Russ.).
6 Ströble, W., Die Wirkung von Lichtfiltern, insbesondere von neodymhaltigen Gläsern, auf das Farbensehen. Dissert. TH. Berlin 1937.”
(Frieser, Helmut; Reuther, Rudolf (1943): Farbumstimmung und Farbenphotographie. In: Film und Farbe. Vorträge auf der gemeinsamen Jahrestagung “Film und Farbe” in Dresden v. 1.–3. Okt. 1942, Berlin: Max Hesse, pp. 109–112.) (in German)
“La colorisation est comme on sait une vieille idée. Ce qui est récent, c’est la possibilité de la réaliser numériquement et de manière courante sans y passer des mois, ou sans des budgets exorbitants. Des manipulations chromatiques avaient déjà été faites dans le passé, aboutissant à re-colorier des films classiques (en couleur) pour les mettre à ce qu’on pensait être le goût du jour.
J’ai donc pu voir, en France ou dans d’autres pays, un certain nombre de films colorisés: des drames, des mélodrames, des comédies, des “films noirs” (gialli – “jaunes” – disent les Italiens), des séries western, comme Au nom de la loi, avec Steve McQueen, et des Laurel et Hardy. Le point commun qui s’en dégage est que la colorisation de ces films des années trente à cinquante les fait évoluer dans une sorte de no man’s land esthétique et historique. A l’époque en effet où on tournait et où on éclairait dans un certain style, on ne coloriait pas dans ce style. Il y a discordance entre l’image des films en couleur tournés en 1950, l’année d’Asphalt Jungle, et le même Asphalt Jungle, tourné alors en noir-et-blanc, et colorisé pour la télévision: ce film, à l’époque, ne pouvait être filmé en couleur, non pour des raisons de budget, mais à cause de son appartenance au film policier, et s’il l’avait été, il l’aurait été dans une gamme beaucoup plus claire.
La colorisation ne concerne pour le moment que la diffusion des films en vidéo-cassettes ou par les chaînes de télévision, donc sur le petit écran. Mais bientôt peut-être elle s’appliquera à la projection en salle, que celle-ci soit faite à partir d’une pellicule, d’une bande vidéo, ou d’une vidéo-retransmission. Signalons à ce propos une donnée capitale, souvent oubliée et déjà ancienne dans cette situation, qui est l’interaction entre le téléspectateur et l’image du film qu’il regarde, et la possibilité de régler cette dernière à sa convenance. A l’heure où l’on parle beaucoup d’”interactivité” pour désigner certaines possibilités comme si elles étaient nouvelles et exaltantes à la fois, il n’est pas inutile de rappeler que du point de vue de la couleur, de la luminosité et du contraste, le petit écran a toujours été éminemment interactif, et “à la carte” pour l’usager. Celui-ci a toujours eu, même avant la télécommande, le contrôle de la saturation des couleurs sur son appareil, pouvant aussi bien outrer les teintes qu’arriver à un parfait noir-et-blanc. Ce qui évidemment limite les possibilités expressives des œuvres destinées uniquement à la télévision, puisque offrir un tel pouvoir au téléspectateur, c’est comme permettre à l’auditeur de changer à son gré l’instrumentation ou la tonalité d’un morceau de musique.”
(Chion, Michel (1995): Colorisations. In: Jacques Aumont (ed.): La Couleur en cinéma. Milan: Mazzotta, pp. 63–69, on pp. 63–65.) (in French)
“1. BASIC PRINCIPLES OF COLOUR REPRODUCTION
Most processes of colour reproduction depend on the well-known visual phenomenon that colours can be matched by the additive mixture of three radiations; for various reasons these are generally chosen to be saturated red, green, and blue lights. This phenomenon is most easily understood on the assumption that in the retina there are light-sensitive receptors of three types, one type being most sensitive to light from the long wave end of the spectrum, the second type most sensitive to the medium waves, and the third type to the short waves. Details of the physiological processes underlying this conception are not fully understood, but as a working hypothesis it is quite satisfactory so far as colour photography is concerned.
From colour matching experiments it is possible to derive the spectral mixture curves which for any specified red, green and blue radiations, record the amounts of the three stimuli required to match the colours through the spectrum, and from which the amounts required to match any composite radiation can be calculated when the spectral composition of such a radiation is known. A typical set of curves is shown in Fig. 1, but when the qualities of the red, green and blue matching stimuli are changed, the same data will be represented by a different, but linearly related set of mixture curves.
Three-colour Reproduction Process
In a three-colour reproduction process, the colour at each point of the reproduction is determined by the relative amounts of three radiations R, G and B present at the point in question. The intensities of R, G and B are determined, in colour photography, by the exposure of three negative emulsions to the object being photographed. The image of the original scene is focused on red-sensitive, green-sensitive and blue-sensitive emulsions, the emulsions subsequently being processed and printed as positives which control the brightness of the red, green and blue radiations at each point of the picture.
It should be emphasised that this principle applies to both the additive and subtractive methods of reproduction, since the three absorbing layers in the subtractive process serve in effect to control the intensities of the R, G and B components, as the descriptive names often used for these layers, namely minus red, minus green and minus blue, would indicate.
The accuracy of colour reproduction depends on a number of factors, but if the photographic processing is such that a linear relation holds between the exposure of each of the separation negatives and the resulting intensities of R, G and B in the positive, then the spectral sensitivity curves of the separation negatives should theoretically correspond to the spectral mixture curves that would be obtained from visual colour matching experiments, using the R, G and B radiations of the positive reproduction process for the matching stimuli. Thus, once R, G and B have been chosen, the sensitivity requirements of the rest of the process are merely a matter of calculation on well established colorimetric principles. In practice, the application of these principles is extremely difficult; and considerable departures from the theoretically correct relations are possible without very serious distortion of the colour rendering.
Sources of Error
There is in any case a number of potential sources of error inherent in the photographic process, such as the non-linear characteristic curves, the difficulty of securing identical gammas for the three processes, and, in the subtractive processes, gross departures of the minus red, green and blue dyes from absorbing media having the theoretically ideal spectral absorption characteristics. There are two major defects from which actual dyes are liable to suffer, namely, the tendency to absorb light in those parts of the spectrum where they should in theory be perfectly transmitting, and also a tendency for the actual qualities of the R, G and B radiations controlled by the three layers to vary according to the densities of each layer. The former defect can be partially compensated by masking techniques, at the cost of further elaboration of the process, while the latter can in some circumstances lead to the reproduction of more saturated colours than the simple theory would predict as possible.
The phenomenon of three-colour matching not only provides the basis of colour photography, but it also leads to the trichromatic system of colour specification, in terms of which colours can be specified numerically, and on which, for example, the errors of colour reproduction can be recorded. In the international C.I.E. system, a colour is specified in terms of three reference stimuli X, Y and Z (corresponding to defined red, green and blue stimuli). The colour quality of a stimulus C can then be plotted on a chromaticity chart, Fig. 2, in which the coordinates (x, y, z) of C correspond to the proportions in which X, Y and Z must be mixed to match C. The lightness or darkness of C is specified separately by the brightness factor.
We may note the following points about Fig. 2. The spectrum locus shows the colour quality of the individual wavelengths through the spectrum: any composite stimulus will necessarily have a colour quality lying inside the spectrum boundary and nearer the white point S. An alternative specification of C is its dominant wavelength λ and excitation purity p, where λ is the wavelength of the monochromatic light, which, when mixed with a specified white light S in suitable proportions, will match C, while p expresses the proportion in which λ has to be mixed with S, as measured by the ratio of the distance CS to λS, that is,
This dominant wavelength-purity specification is frequently very convenient in descriptive work, since the dominant wavelength will give an approximate idea of the hue of C, according to whether λ is in the red, yellow, green, etc. part of the spectrum, while p indicates the saturation or vividness of C; thus, as p approaches unity, C is approaching the saturated spectral colours or purple, and as it tends to zero, C is approaching white or grey.
In discussing the performance of some actual photographic process, it may be practically convenient to use a chromaticity chart in terms of the actual reproduction stimuli R, G, B. But where discussion between different workers and laboratories is involved, the international C.I.E. system should be used, as is done in the remainder of this paper.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 1–4.)
“2. CHARACTERISTICS OF A COLOUR REPRODUCTION
If reproduction of a scene were perfect, then the quality of the picture in every respect – colour, brightness, form, definition, depth and so on – would be identical with that of the original. We need not be unduly disturbed that such perfection is unattainable, but once departures from true reproduction in any respect are admitted, then it no longer follows that the closest possible approach to perfect reproduction in regard to the other attributes will necessarily be the best compromise at which to aim. For example, in the absence of any true stereoscopic effect, a useful sense of depth can be created by deliberate distortion of the light and shade; again, it is quite impossible with the restricted range of brightnesses available ever to give a true objective representation of the brightness levels of all types of scene that may from time to time be portrayed. Distortion of both light and colour values may, however, lead to very effective compensation.
Thus colour is an additional variable superimposed on a reproduction which in other respects is far from perfect. In that event we have to examine the principles on which to decide the best compromise between perfection and distortion.
As a start we may confidently summarise the more important sources of error in the subtractive process.
(a) The range of colours which can be reproduced is limited. The area of the chromaticity chart which can be covered may be as shown in Fig. 3. Here, Y, M and C represent the colours of the yellow, magenta and cyan subtractive primary dyes used in the process, while R, G and B correspond to the red, green and blue stimuli whose intensities are controlled by these dyes. We should, however, note that this range of colours will be less for the lighter colours and that R, G and B vary to some extent for different densities of the dyes.
(b) The characteristic curve for each process may not be linear and, where linear, may not have a slope of unity. Where all the gammas are greater than unity, the colours tend to increase in saturation and vice versa. If the three processes have different gammas, greys will acquire a hue at certain lightness levels.
(c) The spectral sensitivity curves of the negative emulsions may be incorrect, generally in a sense to make the colours more saturated.
(d) The yellow, magenta and cyan dyes have non-ideal absorption characteristics which tend to make certain colours, more especially blues and greens, too dark, and which may produce some error of hue. Partial correction by masking is possible.
To what extent is the limited range of colour a handicap? Fig. 4 shows the position on the chromaticity chart of a selection of well-known artists’ pigments, from data published by N. F. Barnes1. We may conclude from this that while some colours will be difficult to reproduce, the potential range of colours is likely to be sufficient for most normal subjects2. It is interesting to note, too, that many paintings by old masters include only a very restricted range of colours, and it is worth considering that certain sequences might be admirably reproduced with quite a limited colour range. In principle, this should simplify the reproduction process by the choice of less widely spaced positions for R, G and B, which in turn should lead to smaller errors and greater refinement in the more subtle and consistent reproduction of fine colour differences. In practice, of course, the use of different dyes for different sequences might be an intolerable handicap.
1 N. F. Barnes. J. Opt. Soc. Am., Vol. 29, p. 208, 1939.
2 A Pope. Vol. 1. “The Painters’ Terms” (1929). Vol. 2. “The Painters’ Modes of Expression” (1931). W. D. Wright, Phot. J. Vol. 80, p. 25, 1940.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 4–5.)
In the first place, consider the reproduction of the folds of a dress as illustrated m Fig. 5 (a), when the material is either silk, velvet or wool. The colour of a material is determined partly by the spectral absorption of the light as it passes into and through the fibres of the material before being reflected, and partly by the top surface reflection which has the colour quality of the illuminant. The relative contributions of these two factors depend on the nature of the fibres and on how the surface is woven.
In the case of a silk dress, there are strongly marked highlights in which the whiteness of the illumination almost completely swamps the body colour of the silk itself. If we consider the changes in light reflection and purity along, say, the line AB m Fig. 5 (a), we shall find an effect of the type illustrated in Figs. 5 (b) and (c). There will be large and rapid changes in light reflection accompanied by very large changes in purity; there may also be minor changes in dominant wavelength, but these we can ignore in this example.
The corresponding curves for a velvet dress might be as shown in Figs. 5 (d) and (e). A difference between the highlights and the lowlights (they may not be shadows as we normally understand them) will be even more marked than for the silk and, owing to the special nature of the pile of the velvet material, the highlights will be located at different positions relative to the folds of the dress, a feature which is peculiarly characteristic of velvet. Moreover, the manner in which the light penetrates into the pile leads to a deep, rich colour in the lowlights, which is recorded colorimetrically by the high purity, so that Fig. 5 (e) shows an even greater purity range than Fig. 5 (c).
The absence of gloss with the rougher woollen dress leads to the quite shallow types of curve of Fig. 5 (f) and (g). There will be relatively small variations in top surface reflection across the folds of the material and, since the surface reflection is at no point very intense, the body colour of the material will always be fairly well in evidence. The purity curve of Fig. 5 (g), therefore, runs at a moderately high level of purity, although never at the highest level attained by the velvet.
The curves of Fig. 5 are characteristic of these three types of material, and we can assume that, in a good reproduction, the light reflection and purity should vary relatively to one another somewhat in the manner shown. It is evident that materials such as silk and velvet are likely to present a much easier problem to the kinematographer, since the gross changes in light and purity will demand little subtlety of reproduction, and errors in the hue and saturation will be of minor significance provided the general character of the surface reflections is brought out. Moreover, when the person wearing the dress walks across the stage, the folds in the dress change their position so that the areas of high light and the areas of high purity move rapidly from point to point of the dress. These rapid movements are also easily reproduced on the screen and any lack of constancy of the processing conditions of the film is quite innocuous, relative to the genuine fluctuations that are taking place.
With the woollen material, the surface characteristics are less interesting and incidentally of less assistance in suggesting the third dimension, although their adequate reproduction calls for greater refinement and greater constancy of quality than in the case of silk and velvet. Also, the importance of definition becomes relatively greater in giving a good rendering of the softness of the texture, and if the fibrous nature of the surface, more especially at the edges of the material, is below the resolving power of the photographic process, then it may be desirable to introduce a pattern, such as a check or a herring-bone, which the audience would automatically associate with a tweed or serge type of material, but not with silk or velvet. Although very simple, this illustration provides a useful illustration of how the relative importance of colour, lightness and definition may vary from one type of surface to another.
A second example we can consider is that of a daffodil plant. If we examine the leaf of a daffodil we find there is a continuous variation of lightness, hue and saturation from one end to the other. It is not that the actual colour specification of any part of the leaf matters very much, but rather the simultaneous variation of the brightness factor, dominant wavelength and purity which we associate with the daffodil leaf. The colour of the leaf is also, of course, linked in our minds with the yellow daffodil flower. It may well be the case that both the brightness and the saturation characteristics could be distorted without seriously affecting the appearance of the leaf and flower, but if one were distorted and the other correct then the reproduction might be unsatisfactory. This type of problem needs quantitative investigation.
As a third type of subject, we may consider the problem of flesh tints, particularly the face, which is perhaps the most important with which the photographer has to deal. While the colours of faces differ considerably from one to another, the variations in flesh tints over any given face are generally surprisingly small. Fig. 6, for example, shows the area of the chromaticity chart within which the colour of the cheeks, chin, forehead, nose, neck and ears of a typical face will lie, if the white highlights having the colour of the illuminant are ignored. The smooth texture and delicate colouring of the cheek, for instance, are appreciated largely through very small variations in hue and saturation, while the modelling of the face as a whole depends more on variations in light and shade than on changes in colour. A coloured face is no adequate substitute for a finely modelled face, especially if the colour is not stable and if the finer variations of hue and saturation cannot be recorded. In my opinion only in the case of a negro is the rendering of a face by colour kinematography generally superior to that obtained in black-and-white films. Too often with pale faces the lightness, hue and saturation variations are smoothed out.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 5–7.)
It is easy to show that the permissible errors of colour reproduction are affected by so many factors that it would be practically meaningless to lay down any tolerance values. At the same time it is not very helpful to ignore the problem, and we can at least consider some of the factors which do affect the noticeability of errors.
First, there is the familiarity with the true colour of the object being portrayed. Take the Union Jack, for example. If the hue of the red is in error by being too orange or too purple it will be objectionable; if the red is too dark or too desaturated, it will be less serious as this can correspond to a flag being dirty or faded, provided the blue and white are similarly affected. For this type of problem, the data recorded on the noticeability of colour differences across the chromaticity chart by MacAdam and by the author3 may be applied fairly directly so far as the relative noticeability of errors in different parts of the chart is concerned. However, we may note that the sensitivity to colour differences is greatest when two large areas of colour are being simultaneously-compared, with a sharp dividing line between them. If the areas are small and are widely separated either in space or time, then larger differences will pass unnoticed.
In most problems, though, we are less concerned with the accuracy of two or three clearly defined colours as in a flag, than with the realism of an object perceived as an entity although built up from a varying pattern of light, hue and saturation. In the case of the daffodil, previously discussed, it may not be important if the hue of both leaf and flower are in error, provided they are in error in the same direction; or an error of saturation of the greenest part of the leaf may not matter if the whole of the leaf is affected by an error of the same type. To specify tolerances in such a case from laboratory data on discrimination is manifestly impossible; instead, the effect of different errors on the reproduction of the whole object must be studied as a problem on its own. Obviously, this could lead to a vast programme of work, but it could no doubt be kept within reasonable limits if carried out systematically on an appropriate selection of objects.
With some types of object, it might be quite acceptable to use a process which yielded relatively large errors in the average colour, provided the fine variations of colour over the surface of the object were successfully reproduced. This would apply particularly to the reproduction of the texture of matt surfaces; in the case of the texture of skin, for instance, the chromaticity variations over the cheek may not amount to more than 0.005 or 0.01 of x or y in the C.I.E. These differences set an exacting target in the discriminating power and constancy of the photographic process.
3 D. L. MacAdam. J. Opt. Soc. Am., Vol. 32, p. 247, 1942.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 7–8.)
“4. ADAPTATION CONDITIONS
In addition to the local contrasts which exist between neighbouring areas of a scene or picture, and which have a most important effect on the appearance of each area, the general state of adaptation of the eye can affect the brightness, saturation and hue of the colours in the scene. Owing primarily to the photochemical reactions which take place in the retina when stimulated by light, the sensitivity of the eye is continually being adjusted to suit the prevailing quality and intensity of the illumination. At high illuminations the eye becomes light-adapted and has a low sensitivity; at very low illuminations the eye is dark-adapted and has a very high sensitivity; if the illumination is reddish, the sensitivity to red light is diminished, and so on.
In the kinema, the audience is in a partially dark-adapted state and sees an illuminated screen against a fairly dark surround. This is almost always a very different state of adaptation from that in which the audience would have been if it had been at the actual location of the scene being portrayed.
Apart from questions of visual discomfort which may be felt when looking at an illuminated area in a dark surround, the existence of the dark surround might be regarded as advantageous when brightly illuminated scenes are being portrayed, since the limited amount of light available on the screen will have an enhanced apparent brightness due to the contrast with the black surround. On the other hand, for the correct portrayal of darker scenes, an illuminated surround would provide the appropriate contrast to create an effect of greyness and blackness on the screen without the need to lower the illumination on the screen itself to such an extent that the picture would be unduly flattened by the stray light in the auditorium.
So far as colour adaptation is concerned, there is room for a good deal more experimental work. In investigations carried out by the author on the effects of adaptation on the apparent colour of a test object by a binocular matching method,4 the-test object was viewed by the right eye and matched against a comparison patch viewed in the dark-adapted left eye. The right eye was then subjected to various adaptations of different colour and intensity, and a new match made between test and comparison patches. The change in the match was then a measure of the change in sensitivity of the eye produced by the adaptation.
These researches were carried out primarily for their interest in the physiological working of the retina, but R. M. Evans5 has applied them to problems of colour reproduction and drawn a number of tentative conclusions of some interest. In some respects the adaptability of the eye is helpful in the sense that any general error of colouring of a scene will be partially compensated by an adjustment in the sensitivity of the eye. If full advantage is to be taken of this compensation, however, it is necessary that there should be no external clues by which to judge the quality of reproduction, e.g., no illuminated white surfaces visible to compare with the whites in the picture. This point would have to be borne in mind in the use of an illuminated surround.
Another important conclusion reached by Evans is that the colours in a picture should be consistent internally; that is to say, the lightness and saturation of the colours should be correctly related to the subject, and the rendering of some colours should not be better than others. Another point he makes is that the larger the picture, the more brilliant will the colours appear, and since desaturation and degradation is the general tendency produced by some of the reproduction errors, this is a factor of considerable interest and significance. Further experimental work on these lines is required, but it is satisfactory that visual data are becoming available from which an analysis of phenomena familiar enough to artists, can be attempted in scientific terms.
Changing Colour of Scenes
A feature in the kinema which could be disturbing is the sudden change from one scene to another every few seconds. This could have disastrous effects on colour rendering if the eye, after having become adapted to a scene in which one hue predominated, were presented with another scene of a very different hue. Binocular matching experiments show that the eye may require about a minute to become approximately adapted to a particular quality and intensity of light and about three minutes to recover from the main effects of the adaptation. Normally in the kinema, a given scene is only viewed for a few seconds, and for this short duration of exposure the eye becomes only partially adapted, recovery follows within a few seconds of the removal of the adapting light. Most of the possibly unfortunate effects of this adaptation process are largely unnoticed in the kinema owing to the variegated pattern of colour of which the majority of scenes are composed; this has a double effect, since there is less likelihood of any given area of the retina becoming strongly adapted to a particular colour, while if such local adaptation were to occur, its effects would tend to be swamped by the pattern of colour of the succeeding picture.
We can conclude that any effect which is produced will be most noticeable when the preceding scene is highly coloured and having one hue predominant, and the following scene is fairly uniform and not very highly coloured. Binocular matching experiments have shown further, that the most specific adaptation is produced by red light, so that it must be anticipated that red scenes are likely to be most disturbing on subsequent scenes.
We should realise that the change of colour with time provides the kinema with an additional dimension which has not been at the disposal of the artist, and there may be some novel methods of handling this transition from one coloured scene to another which have yet to be explored. It is to be hoped that scientific information about the underlying visual phenomena may help to speed up the assimilation of the possibilities by the kinematic artist.
4 W. D. Wright, “Researches on Normal and Defective Colour Vision” (Kimpton), 1946.
5 R. M. Evans. J. Opt. Soc. Am., Vol. 53, p. 579, 1943.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 8–10.)
“In a work of art, however, naturalistic representation may not figure at all prominently in the mind of the artist; on the contrary, the ideas he wishes to express may require deliberate and conscious distortions of the light and colour relations in a manner which may become characteristic of the artist rather than of a particular type of picture.
Of course, much of the artist’s technique cannot easily be described in colorimetric terms. For example, the extent to which the pigments are mixed either subtractively on the palette or additively by suitable handling of the brush, so that different streaks of pigment lie side by side on the canvas, will not only determine the colour range that can be obtained with a given set of pigments, but will also greatly affect the texture of the painting. The artist attaches great importance to texture, and it is obvious that he has a freedom to control the colour and texture of his painting that is denied to the photographer.
Nevertheless, unless a picture is pure anarchy, a broad relationship must exist between the colours of a painting which it should at least be possible to describe in general, and possibly in fairly specific terms. Such relationships seem to be acknowledged by artists as a necessary feature of a painting if any degree of unity and suavity is to be achieved, and although it is doubtful whether they have ever been described on the trichromatic system, we should note that writers such as Pope6 have attempted an analysis in terms of a colour solid built up of pigmented patterns.
The analysis may be carried to various degrees of refinement and in the elementary stage, a picture may be characterised by:
(1) Its lightness or darkness,
(2) Its hue range,
(3) Its saturation level.
This is so obvious that it need not be elaborated here except to emphasise once again how many pictures have a relatively limited hue range.
We can see already, however, that by combining (1), (2), and (3) in various ways, a great variety of effects becomes possible. This is especially true if it is regarded as legitimate for the lightness and saturation relations to vary from one hue to another.
If the analysis is pressed further, we can see that the saturation distortion between original and reproduction may not be linear. We are familiar with the characteristic curve of a black-and-white photographic process in which the logarithm of the exposure is plotted against the density of the developed emulsion. Only part of this curve is linear, and the accuracy of light and shade reproduction depends on the extent to which the exposures of the different parts of the scene occur within the linear portion of the curve, and also on the slope of this linear part.
In the same way, some similar relationship can be visualised for the saturation reproduction, which in colorimetric terms might be expressed by a “purity characteristic curve” and a “purity gamma.” For each hue some such curve as Fig. 7 must exist in a photographic process by which the distortions in purity can be judged. The amount of published photographic data is too meagre to judge whether or not this curve is likely in general to be linear, while in the case of the artist’s painting it is doubtful whether any quantitative data is available at all.
Nevertheless, the existence of such relations is recognised, as Pope has described and in terms of which he has to some extent classified the works of different painters. An extraordinarily complex situation can arise if the likelihood of the purity gammas being different for different hues is acknowledged, and if there are hue distortions as well. No doubt a hue gamma could be envisaged, and although at this stage such a conception may not be very helpful, we can see that it is this possibility of infinite variety which gives the artist his freedom without anarchy, and which photographers may envy.
6 A Pope. Vol. 1. “The Painters’ Terms” (1929). Vol. 2. “The Painters’ Modes of Expression” (1931). W. D. Wright, Phot. J. Vol. 80, p. 25, 1940.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 10–11.)
“I would submit the following list of features as among the more important colour attributes which should be observed in a film in an attempt to assess its merits:
(1) General lightness values in relation to subject matter.
(2) General saturation values in relation to lightness.
(3) Lightness gradations.
(4) Saturation gradations.
(5) Overall range of hues.
(6) Hue gradations.
(7) Lightness and saturation of different hues.
(8) Errors of hue.
(9) Rendering of small differences of hue and saturation in highlights.
(10) Stability of hue, saturation and lightness with time.
If these attributes can be correlated with colorimetric data supplied by the photographic manufacturer – brightness characteristic curve, purity characteristic curves for different hues, hue range at different brightness levels, hue and purity discrimination limits and so on – it should become easier to decide what types of scene can best be handled by the process being used and what different characteristics are required for improved rendering of other scenes.
It is possible to visualise a systematic programme of research in which deliberate distortions of the various characteristics are introduced in an attempt to find the optimum combination for each type of scene. Unfortunately, the carrying out of such a programme postulates the easy control of emulsion types and processing conditions, and colour, chemists may well have doubts about its practicability. Perhaps with the perfection of colour television we may be provided with a tool more readily adaptable to such a research programme.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 11–12.)
Mr. H. Walden: What is purity? It is not saturation, is it?
The Author: Purity is the objective correlative of saturation; saturation is the subjective estimation of the increase in colourfulness – purity is one way (there are other ways) of measuring that objectively. The amount of a spectrum colour which has to be mixed with white to match a colour is a measure of its purity.
Mr. Rudd: I am a little confused about the tie-up between hue and lightness.
The Author: Lightness is governed by the amount of light reflected from a surface. Colours may be of the same lightness, but differ in hue. Two colours can be recorded by the same point on the chromaticity chart, but one can be light and the other dark.
Mr. W. Buckstone: Can Dr. Wright give us some idea of the time involved in getting the eye adapted to a colour, and also the recovery time?
The Author: It is not easy to measure these adaptation effects. Quite a lot of work was done during the war on dark adaptation, but less on colour adaptation. One method I have used quite extensively is a method of binocular matching, in which you view a test colour with the right eye, and a comparison colour with the left eye. You then subject your right eye to varying degrees of adaptation, either of light or colour, and on removing the adapting light, you re-compare the test and comparison colours. By measuring the change produced by the adaptation you can determine by how much the sensitivity has changed, and you can plot the course of light adaptation and recovery. To become adapted to a particular light takes something about one or two minutes, although even after the three minutes are up there may still be some degree of adaptation going on. Within about two minutes after the adapting light has been removed most of the loss in sensitivity has been recovered.
Mr. Green: You mentioned that it might be a good idea to have a lighter band round the edge of the screen, when you had a dark scene. Could you expound that?
The Author: If you want to portray a bright scene, a dark surround, by contrast, throws it up. If you want to show a darkened scene, then if you make your film too dense the light from the auditorium tends to de-saturate the less intense colours, and you get a loss of contrast, without getting the sense of darkness that you want. If you had an illuminated surround it would be easier to get, by contrast with the illuminated surround, the appearance of darkness, without having to reduce the amount of light so much on the screen, and get de-saturation from the auditorium.
A Visitor: Presumably you can plot on the chromaticity diagram colour temperature as a range of degrees, until finally it will pass through to equivalent daylight. Is that the same as the white on the chromaticity diagram, about 5,000°K.?
The Author: The locus of illuminants of different colour temperatures can be plotted on the chromaticity diagram; it passes from orange through white, to blue-white.
Mr. R. H. Cricks: We have seen a number of two-colour films, and the average layman cannot tell the difference from three-colour. Would Dr. Wright care to comment on that?
The Author: Properly exploited I would say that some two-colour processes might produce highly satisfactory results.
Mr. R. H. Cricks: Green is of course the biggest problem, as confirmed by Fig. 3.
Mr. Harte: Could Dr. Wright give some indication of the cut-off of the three components of the retina of the eye? Are they square in form?
The Author: We do not know with certainty, but on the orthodox trichromatic theory, the sensitivy curves of the three types of receptor are quite broad. They are not square-topped, like transmission curves of an ideal filter. The maximum red sensitivity is probably at .58µ, and comes to zero about .47µ. The green has a maximum at .54µ and comes down so that at .65µ it is pretty nearly zero, and at the other end it drops fairly rapidly down to .48µ, then continues at a low value right to the end of the spectrum. The blue has its maximum at .46µ, and is the narrowest of the three, approaching zero at .53µ. If the colours did not overlap, we might see black bands in the spectrum.”
(Wright, W.D. (1948): Colour Vision and the Film Industry. In: Journal of British Kinematography, 13,1, Jul., pp. 1–13, on pp. 12–13.)
“La question de la colorisation est aussi une occasion de réfléchir sur le fait que celle-ci est comme latente dans ce qu’on appelle le noir-et-blanc, à travers notamment les dialogues et les titres.
“I remember every detail. The Germans were grey, and you were blue.” (Je me souviens de chaque détail, les Allemands étaient gris, et vous étiez en bleu.) Cette réplique magnifique de Rick (Humphrey Bogart) à Ilsa (Ingrid Bergman) quand il évoque devant elle, à la fois caustique et amoureux, leur dernière journée d’amour dans un Paris investi par l’occupant, est une des nombreuses répliques-fétiches d’un film qui en comporte tant. Casablanca. Un film si légendaire pour le “movie buff” américain, que, sans parler des parodies explicites des frères Marx et de Woody Allen, on en trouve des traces jusque dans Blade Runner de Ridley Scott (le traitement de l’exotisme, les chéchias, les ronds de lumière qui balaient le décor comme des poursuites de théâtre, le thème de la vile qu’on ne peut quitter, etc…).
Cette réplique de Bogart – qui fait surgir une tache de couleur magiquement sur le fond du noir-et-blanc – semble donc faire de Casablanca un film en noir-et-blanc absolument non colorisable. Mais il ne faut jurer de rien, et on a vu que les marchands de couleur ne respectent rien. En tout cas pour toute personne ayant vu le film en noir-et-blanc, elle a une magie particulière. Le beau dans cette phrase, évidemment, c’est qu’elle intègre la différence même entre couleur et noir-et-blanc. L’amour fait voir à Rick une Ilsa en couleurs sur le fond de la bande d’actualités où figurent les envahisseurs.
Il ne me paraît pas sacrilège de citer à côté de cela le Schindler’s List de Spielberg, et l’épisode controversé du massacre du ghetto de Varsovie, à propos duquel la critique française a fait preuve d’un bien étrange puritanisme. Il s’agit du plan où une petite fille destinée à la mort est colorisée en rouge, seule tache de couleur dans un enfer de noir-et-blanc. Quand on sait que c’est la scène dans le film qui fait basculer le héros et le détermine à sauver des Juifs, on imagine que celui-ci pourrait dire: “I remember every detail. The Nazis were grey, and the little jewish girl was red.” Peut-être y a-t-il un écho de Casablanca dans Schindler’s List, et on ne voit pas où serait le mal. Là encore, le problème de fond (faut-il tourner des films de fiction sur les camps d’extermination, et il y en a eu des dizaines avant le film de Spielberg) ne gagne rien à être ramené à une question de fétichisme visuel (“cachez cette tache de rouge”).
Si j’ai cité ces deux moments de films, l’un en noir-et-blanc d’époque et l’autre en noir-et-blanc d’aujourd’hui (c’est-à-dire adopté comme parti pris dramatique et chromatique, comme choix de couleurs) c’est parce qu’ils posent tous deux la question de la couleur comme tache – question que l’enjeu de la colorisation reprend de manière paradoxale, puisque la couleur, dans un film colorisé comme on les traite actuellement, est là, nous l’avons vu, pour ne pas se faire remarquer.
Seulement, dans ces deux exemples empruntés à Casablanca et à Schindler’s List, il y a une différence capitale, qu’on a remarquée: le rouge de la petite fille, on le voit de ses yeux, et le bleu de la robe d’Ilsa dans Casablanca, il surgit par le pouvoir colorisant du mot, et il reste au niveau de la couleur mentale – ou plutôt, de la couleur verbale.
Pour reprendre l’exemple de Casablanca, imagine-t-on “le” bleu quand on voit la robe d’Ilsa à Paris? On peut en douter. D’abord, le bleu n’existe pas, on le sait, il y a du bleu ciel, du bleu nuit, de l’indigo, etc… C’est le mot “bleu”, non la couleur, qui éveille l’imaginaire. Dans les scènes de flashbacks à Paris, où l’on voit Rick et Ilsa vivre leur idylle sous l’œil du bon Sam, on ne pense plus à se demander si elle est bien en bleu…
Au passage, demandons-nous pourquoi, puisqu’il y’a des gris, les mots nous manquent pour les différencier et les évaluer. Le gris n’est-il qu’une valeur quantitative (placé sur une échelle entre le blanc ou le noir), ou accède-t-il au pur qualitatif? En tout cas, dans un film en couleur, on ne se pose la question de la couleur, dirais-je, que si un personnage en parle, ou si le titre en pointe une. Prenons l’exemple de Blue Velvet, de David Lynch: les couleurs, qui jouent un rôle très important dans le film, sont désignées d’emblées par les paroles de la chanson de Bobby Vinton qu’on entend au début (“She wore blue velvet”), et ces paroles nous font voir d’un certain œil les fameuses images chromo du début, avec le ciel radieux, les tulipes, le camion rouge du pompier, etc. Les dialogues aussi renforcent cette perception. Le héros, Jeffrey, désigne ainsi l’un des individus patibulaires qu’il piste sous le sobriquet de “the yellow man”. L’homme porte en effet un costume jaune canari très criard, mais le fait que la couleur soit dite change tout. La confrontation d’une couleur qu’on voit réellement, et du mot plus ou moins vague dont on se sert pour la désigner est aussi génératrice d’effets singuliers, même s’ils sont moins magiques que lorsque le film est en noir-et-blanc.
Il me faut aviser le lecteur que, comme beaucoup plus de gens qu’on ne croit, je suis affligé d’un daltonisme léger qui me rend inapte, dans certaines conditions précises – notamment de lumière et de distance –, non seulement à remarquer la tache d’une fraise des bois au milieu de ses feuilles, ou d’un coquelicot dans un champ, mais aussi face à certaines couleurs à me prononcer sur leur nature exacte. Si on me dit que c’est du gris bleu, je “vois” du gris bleu, et si on me dit qu’il n’y a pas de bleu dans ce même gris, je n’en “vois” plus. Le daltonisme est un mystère pour ceux qui n’en sont pas affectés. Moi-même, et je suppose mes pareil(les), ne comprends strictement rien à ce processus qui dans certaines limites fait voir ce qu’on dit. En tout cas, cela fait réfléchir sur l’articulation entre le “perçu” et le “nommé”, pour reprendre le titre d’un passionnant article de Christian Metz.”
(Chion, Michel (1995): Colorisations. In: Jacques Aumont (ed.): La Couleur en cinéma. Milan: Mazzotta, pp. 63–69, on pp. 65–67.) (in French)
“Photographic Aspects of the Theory of Three-Color Reproduction*
David L. MacAdam
Eastman Kodak Research Laboratories, Rochester, New York
(Received August 24, 1938)
*Communication No. 680 from the Kodak Research Laboratories.
The colorimetric and photographic principles of additive color reproduction are reviewed. The significance of the concept of photographic spectral sensitivity is examined, and the desirability of emulsions having contrast independent of wave-length is emphasized. Although the desirability of partially negative spectral sensitivities, such as described by Hardy and Wurzburg, is acknowledged, the photographic methods proposed by those authors for the realization of such sensitivities are shown to be rendered useless by fundamental difficulties. Two masking methods are described, one of which utilizes unusual types of emulsions to realize completely the desired partially negative spectral sensitivities, and the other of which utilizes conventional materials and techniques to realize a close approximation to the desired sensitivities. The errors of additive color reproductions resulting from several different compromises in which negative portions of the spectral sensitivity curves are ignored or avoided are computed and compared. The simple omission of all sensitivity in the wave-length regions where the theory calls for negative sensitivity, utilizing only the positive portions of the theoretical spectral sensitivity curve, results in the smallest errors. Finally, the effect of increase of contrast is examined, with the conclusion that purity can be increased, compensating for the loss of purity resulting from the failure to realize partially negative spectral sensitivities, but that considerable errors in dominant wave-length as well as in brightness result from the excessive contrasts.
The goal acknowledged by all who work for improvements in color photography is the production by photographic means of images in which are reproduced not only the relative brightnesses, but also the dominant wave-lengths (hues) and purities (saturations or chromas) of all details in the scenes originally photographed. Since colorimetry is the science of color measurement and specification, and is therefore the court of final appeal by which equalities of dominant wave-length, purity, and brightness are deterged, the conditions imposed on processes of color photography by the principles of colorimetry are important, and cannot be ignored with impunity. These conditions will be investigated for the general case of color photography. The special case in which it is desired to reproduce by photographic means any three-color picture, whether transparency or opaque print, permits several simplifications of the conditions. This special case, and the accompanying simplifications, will not be discussed in the present paper. Since the general case includes this special case, the methods to be discussed will be adequate, although perhaps unnecessarily complicated, for the duplication of three-color originals.
An adequate discussion of the conditions necessary for accurate photographic color reproduction with subtractive processes requires:
(1) A study of the colorimetric and photographic principles underlying additive as well as subtractive methods of color reproduction;
(2) An investigation of the phenomena of subtractive color mixture; and
(3) A demonstration of the manner in which the fundamental colorimetric requirements can be satisfied in a practical subtractive process.
The theory of three-color additive photography has been discussed by many authors. The discussions by Maxwell,1 Ives,2 Schrödinger,3 Schaefer and Ackermann,4 Neugebauer,5 Hardy and Wurzburg,6 Harrison and Horner,7 and Frieser and Reuther,8 have made increasing use of the color mixture functions of the human eye.9
Hardy and Wurzburg have shown the general consequences of the application of the criteria of colorimetry to the design of additive processes of color reproduction. They discussed for the first time the possibility of attaining the theoretically desirable spectral sensitivities which exhibit negative values in some spectral regions. The present discussion will deal at some length with several photographic problems involved in the realization of partially negative spectral sensitivities, and will suggest a practical method for closely approximating the effects of such sensitivities.
Although a brief discussion of subtractive processes was appended to their report, the principles enunciated by Hardy and Wurzburg are not directly applicable to any existing subtractive process of color reproduction. The concept of unstable primaries introduced by Hardy and Wurzburg led only to the description of the ideal dyes and pigments which have been long sought,10 but never obtained for subtractive color reproduction. Frieser and Reuther8 gave an analytical proof of the desirability of such colorants. A subsequent report by the present author will demonstrate the extent to which automatic compensation can be made for the unstable primaries exhibited by existing subtractive processes. This method of compensation for the nonideal character of available dyes and pigments will make possible a practical extension of the colorimetric principles of additive reproduction to commercially available subtractive processes. The discussion to be included in the present report will apply directly to additive processes, and will be applicable to a practical subtractive process only when the compensations alluded to have rendered the subtractive process colorimetrically equivalent to an additive process.
B. DESCRIPTION OF SPECTRAL SENSITIVITIES REQUIRED FOR ACCURATE ADDITIVE THREE-COLOR REPRODUCTION
The application of colorimetric laws to additive color photography results in the conclusion that the effective spectral sensitivity curves of the photographic devices used for the recording of the trichromatic analysis of the original scene should be in quantitative agreement with curves which can be computed from data which specify the color-matching properties of normal human vision. These spectral sensitivities should be linear combinations of the color mixture functions.6
The symbols, r, g, and b, are used to represent the spectral sensitivities of the three photographic recording elements. The symbols, x̄, ȳ, and z̄, represent the color mixture functions, or distribution functions, adopted by the International Commission on Illumination (1931) as representing the characteristics of normal human color vision. The coefficients, a1, a2, …, a9, depend on the chromaticities (locations in the I.C.I. color mixture diagram) of the primaries used in the additive synthesis of the color reproduction. Consequently, the required photographic sensitivity curves, determined by these computations, depend on the chromaticities of the reproduction primaries.
Figure 1 shows the spectral sensitivities required for a typical additive process. In general, regardless of the choice of reproduction primaries within quite generous limits, the wave-lengths at which such sensitivity curves attain their maximum and half-maximum values will be within ten millimicrons of the values indicated by the curves in Fig. 1. In other words, the positive portions of the theoretical spectral sensitivity curves can vary only very slightly from the values shown in Fig. 1 even though quite a wide choice of reproduction primaries is available. On the other hand, the limits and magnitudes of the negative portions of these curves depend very critically on the choice of reproduction primaries. These negative portions correspond approximately in magnitude and spectral location to the areas and locations of the portions of the color mixture diagram lying within the spectrum locus but outside of the triangle whose apices represent the reproduction primaries. These facts are well illustrated by Figs. 1, 2, 4, and 5 shown on pages 231, 232, and 234 of the article by Hardy and Wurzburg.6
C. VAN KREVELD’S LAW AND THE CONCEPT OF SPECTRAL SENSITIVITY
The difficulties involved in realizing any specified spectral sensitivity do not arise only when negative spectral sensitivity is demanded. As early as 1905, Chapman Jones11 pointed out that the variation of contrast with wave-length is a serious barrier to the attainment of definite spectral sensitivities with photographic emulsions. This difficulty is quite distinct from the limitation of exposure latitude, which, as F. E. Ives2 declared in 1889, should be no more serious in color photography than in ordinary photography.
The following extension of Van Kreveld’s additivity law12 shows that there is no sense in which the spectral sensitivity of any existing photographic emulsion can be regarded as an invariant property of the emulsion. Stated in terms of possible consequences in practice, this means that two adjoining surfaces having different spectrophotometric characteristics can with one intensity of illumination produce identical densities on an emulsion, and yet with the same quality but different intensity of illumination these “photographically equivalent” surfaces produce nonidentical densities and can be distinguished by the same emulsion which failed to distinguish them initially. This phenomenon can occur despite the facts that all other variables, time of exposure, development, etc., are kept unchanged, and that all exposures are well within the latitude of “faithful tone reproduction” of the emulsion. This phenomenon has no counterpart in vision at daylight levels of illumination: A visual color and brightness “match” remains valid for all photopic levels provided only that the quality of illumination is kept unchanged, and regardless of the fact that the spectrophotometric characteristics of the matched surfaces are quite different. In contrast with this, even if an emulsion and filter combination could be designed to give identical responses at one level of illumination for visually equivalent colors, the photographic responses would, in general, differentiate between the colors at some other level of illumination for which the visual color match was still valid. An obvious consequence of this essential difference between visual and photographic responses is to qualify the common assumption that photographic responses can be made equivalent to visual responses by the use of properly designed filters.
It will be shown that the magnitude of the variation of spectral sensitivity with exposure depends entirely on the variation from constancy of the contrast (γ) characteristics with wave-length. If emulsions can be designed having contrast characteristics independent of wave-length, then it will be possible to design filters which will render the response of the emulsion equivalent to those of the eye, for all normal levels of illumination.
Van Kreveld’s additivity law for photographic exposures was originally enunciated for the combination of a finite number of different, but not necessarily spectrally pure, samples of radiant energy. If the amounts of each of these varieties of radiant energy necessary separately to produce a density, D, on an emulsion under standardized conditions of exposure and development are designated by E1, E2, E3, etc., and if an amount of energy, Em, consisting of the mixture of amounts, e1, e2, e3, etc., of the former varieties, also produces the density, D, under the same conditions, then the following relation holds:
If the fractions, f1, f2, f3, etc., of the components in the mixture are defined by the relations, f1 = e1∕Em, f2 = e2∕Em, f3 = e3∕Em, etc., then the relation can be rewritten in terms of the total energies and the fractional composition of the mixture:
If the reciprocals of the energies, E1s, E2s, E3s, … Ems, necessary to produce some standard density, D, are defined as the sensitivities, S1, S2, S3, Sm, then the relation can again be rewritten in terms of the sensitivities and the fractional composition of the mixture:
It is obvious that this additivity law can be extended to the case in which the spectral sensitivities, 1∕Eλs = Sλ, are known, when the emulsion is exposed to a heterochromatic mixture, eλdλ, totaling Em = ∫ eλdλ. Thus:
Again, defining fλdλ = eλdλ∕Em, and remembering that Eλ and Em are functions of the common density, D, produced by the various spectrally pure energies, Eλ, and the heterochromatic mixture, Em, the relation can be recast:
In terms of the spectral sensitivities, Sλ, defined as the reciprocals of the energies, Eλ, necessary to produce a certain standard density, Ds, under standard conditions, the relation assumes a familiar form, the simplicity of which is qualified by the fact that the relative spectral sensitivities depend on the density, Ds, for which they are evaluated:
The spectral sensitivities of emulsions are usually given on the basis of arbitrary scales for the exposures, Eλ, necessary to produce the standard density, Ds. The relative spectral sensitivity, sλ = Sλ∕S(λ1), where λ1 is any conveniently chosen wave-length, therefore exhibits most of the information given by spectral sensitivity data. The sensitivity, Sλ(D), for some other than the standard density, can be computed from the relation,
provided that Ds and D are both on the linear portion of the D vs. log Eλ characteristic. The relative spectral sensitivity for the density, D, can therefore be defined as
The relative sensitivity of the emulsion for the heterochromatic energy, Em, as compared with the monochromatic sensitivity for λ1 is defined in the same manner,
The integral relation, Eq. (3.3) connecting the absolute sensitivities can now be transformed by use of the definitions of the relative sensitivities:
The following relation between the slopes, γ, of the D vs. log E curves can be derived from Eq. (6).
where γms, is the value of γm for the density, Ds, at which the relative sensitivities, sλ, and sm, are evaluated. It is evident from the fact that the ratio, sλ∕sm, is, in general, a function of D that γm is also a function of D, even in the exceptional case when all of the values of γλ are independent of density, D.
The relative spectral sensitivity, sλ, of a certain emulsion and filter combination, measured for Ds = 0.6, is indicated by curve (b) of Fig. 2. The contrast characteristics of this emulsion are represented in Fig. 3. According to Eq. (7) the value of γm at D = 0.6, for any relative distribution of energy, fλ, is simply the ratio of the areas of the curves of the products, fλsλ and fλsλ(1∕γλ). For a sample of energy characterized by a uniform distribution, fλ, is a constant, and γm is found to be equal to 1.63 for this emulsion and filter combination. Consequently, since γ590 mμ = 1.63, λ1 has been taken equal to 590 mμ. The relative sensitivity for different densities is therefore the ratio of the absolute sensitivities,
where the following values can be substituted for the absolute sensitivities
The unknown constant, k, which is the ratio of the absolute to the relative sensitivities for the standard density, is eliminated when the ratio of absolute sensitivities for any other density is computed. Curves (a) and (c) of Fig. 2 show the values of sλ(D) for D = 0.2 and D = 1.0. The emulsion used in this example has a gamma-wave-length variation somewhat more serious than that typical of the best panchromatic emulsions. However, this case is not the most objectionable which can occur, and should be regarded as an indication that the gamma-wave-length variations of poorly chosen emulsions ran cause very serious errors. Faithful reproduction of different colors can result only from the use of emulsions having contrast characteristics that are nearly independent of wave-length.
All panchromatic emulsions have noticeably different contrast characteristics for red and blue light, and for this reason it is not advisable to try to realize the secondary maxima of the theoretical red and blue sensitivity curves. The ratio of primary to secondary maximum sensitivities obtained in practice will double or even triple from points in the negative where the density is 2.0 to points where the density is 0.3. This variation will introduce errors which will be greater than the errors resulting from the complete omission of the secondary maxima of the sensitivity curves.
The discussion in the succeeding sections of this paper will be simplified if it is assumed that emulsions exhibiting negligible variations of contrast with wave-length are used. Only on the basis of this assumption can Eq. (3.3) be converted into the form which was assumed without proof by Hardy and Wurzburg (Eq. (4) of their paper6). We shall use the symbol eλ to represent spectral energy distributions. The sensitivity, Sλ, will be defined as the reciprocal of the energy, iλ, corresponding to the intersection with the D = 0.0 axis of the extended straight-line portion of the density-versus-log-energy characteristic of the emulsion and filter combination, for each wave-length of the spectrum. Thus, within the latitude of the emulsion, an exposure to the energy eλΔλ having wave-lengths within the narrow range from λ–Δλ∕2 to λ+Δλ∕2 will produce a density,
Similarly, the sensitivity, Sm, to the heterochromatic energy, Em, having the spectral distribution eλ will be defined as the reciprocal of the energy, im, corresponding to the intersection with the D = 0.0 axis of the extended straight-line portion of the density-versus-log-Em characteristic of the emulsion and filter combination. Thus, within the latitude of the emulsion, an exposure to the heterochromatic energy, Em, will produce a density
The value for Sm given by Eq. (3.3) can be used in this expression if, and only if, the contrast of the emulsion is independent of wave-length. In this case, since Emfλ = eλ, therefore
provided that E is greater than the minimum and less than the maximum exposures which can be recorded on the straight-line portion of the D versus log E characteristic. The quantity,
will be called the photographic exposure. It will be noticed that these definitions of spectral sensitivity, Sλ, and of photographic exposure, E, eliminate the necessity for the explicit inclusion of inertia values in the expressions for the straight-line portions of the density characteristics. The statements that Sλ and E are measured in inertia units will be used in the future to indicate that these definitions are being employed.
D. SPECTRAL SENSITIVITY, GAMMA-WAVE-LENGTH VARIATION, AND TRICHROMATIC CORRECTION FILTERS FOR EASTMAN TYPE B PANCHROMATIC SENSITIZATION
Of all the photographic materials for which adequate spectral sensitivity data are available, Eastman panchromatic materials having the type B sensitization appear to be best adapted for the purposes of color reproduction.
The effective sensitivities of this emulsion when used with the Wratten Filters No. 21, No. 40A, and the combination of No. 49A with No. 2A, are shown in Fig. 4. These sensitivities resemble rather closely the positive portions of the sensitivity curves shown in Fig. 1. Some improvements could undoubtedly be made by changing these filters slightly. Such changes will probably be made when other more serious sources of errors in color reproduction have been eliminated to an extent which renders preponderant the errors due to the filters. When color photography has reached such a degree of perfection, panchromatic emulsions will probably be available having contrast practically independent of wave-length. The filters for three-color photography will then be subject to complete revision, and high precision in their design will then be justifiable.
The effective sensitivities of the type B emulsion when used with Wratten Filter No. 63, and with the combined Nos. 31+80+2A Wratten Filters, are shown in Fig. 5. These sensitivities resemble rather closely the negative portions of the red and green sensitivities shown in Fig. 1. Methods for securing the effects of subtraction of the exposures recorded through such filters from the exposures recorded with sensitivities such as those shown in Fig. 4 will be discussed in later sections of this paper.
E. PHOTOGRAPHIC METHODS FOR OBTAINING THE EFFECTS OF PARTIALLY NEGATIVE SPECTRAL SENSITIVITIES
The photographic attainment of partially negative spectral sensitivities is not so simple as the discussion published by Hardy and Wurzburg6 might seem to suggest. The following discussions of the two photographic methods suggested by these authors will emphasize that fundamental difficulties, aside from the acknowledged difficulties of adjustment, render these two methods quite impracticable.
Let the function, Sλ, represent any one of the desired theoretical spectral sensitivities, such as one of those shown in Fig. 1. If λ1 and λ2 are the wave-length values between which Sλ is negative, then the integral expression for the exposure can be written in two parts:
In this expression ∣Sλ∣ is the absolute value (that is, all negative values made positive) of the spectral sensitivity, Sλ. It can be seen that the exposure, E, is the difference between two exposures, one of which, E1, would be the effective exposure on an emulsion having the spectral sensitivity given by the positive portions of the function, Sλ, and the other of which, E2, would be the exposure on an emulsion having a spectral sensitivity given by the absolute values of the negative portions of the function, Sλ. Although it would be possible to select mixtures of energy, eλ, for which E2 would be greater than E1, it would be discovered invariably that such a mixture would be of such a great colorimetric purity that it could not be matched under any circumstances by the primaries of the additive process for which the spectral sensitivity, Sλ, was calculated. Such a color will be unattainable by any three-color process, either additive or subtractive, and, since hope of faithful reproduction of such highly saturated colors must be abandoned in any case, the photographic embarrassment is best avoided by devising recording processes which respond to negative values of E in the same way that they respond to very low, or zero values. This statement makes more explicit the familiar generalization that the saturations of spectrum and near-spectrum colors cannot be reproduced faithfully by any three-color process.
(1) Selective reversal
No emulsions are known at present which have characteristics even remotely resembling the characteristics required by the theory. Fig. 6(a) shows a family of density, D, versus logarithm of E1 curves which would be characteristic of any emulsion which would respond in the desired manner to the difference of the exposures, E1–E2, defined by Eq. (9). Each curve shown in Fig. 6(a) is characterized by a constant value of E2, the reversing exposure. The dotted portions of these curves need not be attained in practice if perfect functioning is not required when the value of E2 is greater than seventy percent of E1. Even if the characteristic curves resemble the curves usually encountered to the extent indicated by the dashed extensions of the full curves, the optical reversal characteristics are radically different from any that have been reported for any emulsion. It is evident that the desired reversal is greatest at low densities. All reversal phenomena which are recorded in the literature are greatest at high densities.
Hardy and Wurzburg6 have suggested that use might be made of the Herschel effect in which the latent photographic image produced by energy of one wave-length region is partially destroyed by simultaneous or successive exposure to energy of another wave-length region. Fig. 6(b) shows a typical family of characteristic curves for different intensities of the Herschel reversing exposure.13 The unsuitability of this phenomenon for the attainment of partially negative spectral sensitivity is obvious when Fig. 6(b) is compared with Fig. 6(a).
L. T. Troland14 has patented the use of the Herschel effect for the purpose of subtraction of exposures. Since Troland has exhibited the reversal characteristics in a different manner from that used in Figs. 6(a) and 6(b) of this paper, the data contained in Fig. 6(a) are replotted in Fig. 7(a) to show that Troland’s Figs. 1 and 4 (similar to Fig. 7(b), which shows the data of Trivelli13) do not represent characteristics which permit the subtraction of exposures. Since the curves, A, B, C, D, of Troland’s Fig. 1 are practically parallel straight lines, and are so described in the patents, it is evident that densities and not exposures are approximately subtracted by the phenomenon described by Troland. Consequently, according to Troland’s own figures, which are substantially in agreement with Trivelli’s experimental data, the Herschel effect cannot accomplish anything which cannot be accomplished more conveniently by the masking method which will be discussed later. The curves shown in Fig. 7(a), and required of any reversal effect which would subtract exposure, are radically different from the characteristics of any known selective reversal phenomenon. Most photochemical phenomena appear to follow a law of mass action, that the effect of any exposure is proportional to the amount of sensitized substance present. In the Herschel effect, the latent image formed by the primary exposure constitutes the sensitization for the reversal by the secondary exposure. Consequently, it is to be expected that the greatest reversal will take place where the primary exposure was the greatest, and the latent image the most pronounced. This is actually the manner in which the Herschel effect behaves, as shown in Figs. 6(b) and 7(b). It seems quite improbable that any photochemical reversal phenomenon could depart so far from this behavior as to result in the characteristics shown in Figs. 6(a) and 7(a), in which the density decreases are greatest for low values of E1. Consequently, it seems improbable that any selective reversal phenomenon will exhibit the characteristics necessary for the subtraction of exposures.
Since, for the reasons outlined above, no single emulsion having the theoretically required partially negative spectral sensitivity is likely to be devised in the near future, the practical attainment of the desired effect must be sought in some less obvious artifice. The discussion in Section D of this paper indicated that it is possible to design two filter and emulsion combinations which would separately record E1 and E2. The first of these combinations would have an effective spectral sensitivity proportional to the positive portions of the theoretically desired spectral sensitivity curve. The second combination, which would record E2, would have an effective sensitivity proportional to the absolute values of the negative portions of the theoretical curve. With these two combinations, the exposures, E1 and E2, corresponding to the original scene could be recorded photographically. If some means could be devised for photographically subtracting these exposures, resulting in the production of a positive transparency having a transmittance proportional to the difference, E1–E2, of these exposures, the problem of attaining the effect of partially negative spectral sensitivity would be completely solved.
(2) Double printing
The possibility of photographically subtracting exposures by use of toe-recording emulsions was discussed in some detail by Hardy and Wurzburg.6 It was pointed out that the transmittance of negatives prepared with some emulsions is a linear function of the exposure over a limited range of exposures corresponding to the toe of the density-versus-log-exposure curve. An analysis indicated that the use of such emulsions would permit the successful subtraction of the exposures, E2 from E1. The method consisted in:
(a) Recording E1 on such an emulsion which, in combination with a suitable filter, has the spectral sensitivity indicated by the positive portions of the curve in Fig. 1;
(b) Recording E2 on such a “toe-recording” emulsion and filter combination, having the effective spectral sensitivity indicated by the absolute values of the negative portions of the curve in Fig. 1;
(c) Printing the second of these records on a toe-recording emulsion (resulting in a positive transparency having a transmittance proportional to E2);
(d) Double printing in optical register from the negative record of E1 and from the positive record of E2 onto another toe-recording emulsion. The double printing mentioned is not a masking process. The light which prints each of the images is modulated only by the transparent record of that image. The unusual feature consists in the superposition of the exposures in register on a single toe-recording emulsion.
Several comments can be added profitably to this outline of the toe-recording double-printing method. First, it is unnecessary to use toe-recording emulsions in the second and third steps listed above. An ordinary positive transparency prepared from a conventional negative will serve the purpose more efficiently. The second comment amplifies the remarks made by Hardy and Wurzburg concerning the limited latitude of the linear-transmittance-versus-exposure characteristics of available materials. The underlying reason for this limitation will be evident if a linear-transmittance-versus-exposure curve is transformed to the more familiar density-versus-log-exposure diagram. Fig. 9 shows the corresponding density-versus-log-exposure curve, in which the density becomes infinite at a value of log exposure (Em) only 0.3 greater than that value of log exposure at which the density is only 0.3 higher than the density of the unexposed portions of the emulsion. The curve in Fig. 9 is consequently characterized by an infinite contrast. This same curve enables us to determine the lower limit of transmittance, T, for which the T–versus-E curve is linear, as a function of the contrast, γ, of any material for which the shape of the toe is in otherwise perfect agreement with Fig. 8. In such a case, T = 1–E∕m only for values of T greater than 1∕(1+γ). Even if all other conditions were fulfilled, a contrast (γ) of 9.0 would result in a linear-T–versus-E curve only for T greater than ten percent. The over-all latitude of T–versus-E-linearity obtained by the use of such a remarkable material in steps (a) and (d) of the double printing process could not be greater than 10. There seems to be practically no probability that materials can be made available which would approach the latitudes of 30 to 50 which are demanded by the most liberal trade tolerances. This fundamental limitation of the minimum transmittance on the linear-T–versus-E characteristics appears to be the most serious of the many difficulties which would be encountered in the practice of the double printing method.
J. A. C. Yule of these Laboratories has suggested a modification of the masking method which would avoid the difficulties of double printing, and which would fully attain the desired subtraction of exposures, whenever E1>E2. This method uses the fact that for some photographic materials the densities are proportional to exposure, rather than to the logarithm of the exposures, and that for other materials the density is almost equal to the logarithm of a constant minus the logarithm of the exposure. Such characteristics are shown in Fig. 9. These characteristics are more similar to those of available emulsions than are those shown in Fig. 9. Since Yule’s method of subtracting exposures uses unconventional materials, other masking corrections which call for the use of linear-density-versus-log-exposure characteristics cannot be introduced simultaneously.
If the exposures, E1 and E2, are recorded on negative materials of the first kind, described by Yule and shown by curve (a) in Fig. 9, and if the proportionality coefficients are c1 and c2, then the densities of the images will be:
If a mask is now made from the second of these negatives with a conventional material, using the contrast, γm = c1∕c2, and the maximum density, D0, then
The density of the combination of the first negative with the mask is a linear function of (E1–E2):
The final positive transparency should be made on a material of the second kind described by Yule, and shown by curve (b) in Fig. 9. The most favorable choice for the arbitrary constant is unity, resulting in the characteristic equation:
Since the exposure, Ep, results from the light passing through the masked negative, and if E0 is the exposure where D1 + D3 = 0, then,
If the exposure, E0, used in printing is such that log E0 = D0 + 1, then,
Consequently, the transmittance of the positive has the desired value:
(3) Masking as a method for introducing negative spectral sensitivity
The masking method is the simplest, and most familiar to photographic technicians, of all of the methods which have been proposed as means for correcting color-separation negatives corresponding to the negative portions of the sensitivity curves. The following outline of the masking process is to be considered schematic, and is included only for the purpose of introducing several terms which will facilitate the quantitative discussion of the nature of the correction secured by the use of the masking process.
In the masking process to be discussed here, a negative is prepared with an emulsion and filter combination having relative spectral sensitivity proportional, for instance, to the portion of curve (SR) lying above the axis of Fig. 1. This negative will be called the principal negative. It is exposed so as to utilize most effectively the full latitude of the emulsion, and it is developed for γ1 = 1.00. The exposure of a particular spot (having a developed density, D1) on the principal negative will be symbolized by E1. If, as explained at the end of Section C, the exposure, E1, is measured in inertia units, then
When the exposure is less than the inertia, D will be assumed to be zero, and in such cases (E ≦ 1.0), log E will be set equal to zero in all subsequent analysis. When the exposure is greater than the latitude, L, of the linear-D–versus-log-E characteristic of the emulsion, the analysis based on the relation, D = γ log E, breaks down. Consequently, all exposures will be assumed less than the latitude, L.
In addition to the principal negative, a correction negative is prepared by photographing the scene with an emulsion and filter combination having its spectral sensitivity proportional to the absolute values of the portions of the curve (SR) lying below the axis of Fig. 1. This correction negative is also developed to γ2 = 1.00, but is exposed so that the density of the image of a white (nonselective) object in white light is less than the corresponding density of the principal negative by an amount equal to the logarithm of the ratio of the areas above and below the axis of curve (SR), Fig. 1. Since this ratio is 4.8 for the example being discussed, the exposure, E1, of the image of a white object on the principal negative must be 4.8 times the exposure, E2, of the corresponding image in the correction negative. The ratio, E2∕1, for white light will be symbolized by φ. In the example being discussed φ = 0.21. The density of any spot on the correction negative will be
A correction positive transparency, or mask, is prepared from the correction negative. The printing exposure, EA (in inertia units) used in the preparation of this correction positive should be equal to the latitude of the desired correction. In the example, if we assume that the latitude, L, of the principal negative is 100, and that the maximum practical correction, E2∕1, will be 0.50, a printing exposure, EA, of 50 inertia units will be sufficient. The correction positive is to be developed to a contrast, γ3, to be determined later from an analysis of the conditions for best possible masking correction.
The principal negative and the correction positive are finally superimposed in register, to form what may be called a corrected separation negative. A corrected positive is prepared by printing through the separation negative formed by these superimposed transparencies. The printing exposure, EB, and contrast, γ4, are to be determined by the analysis which follows.
If the exposures for the pair of negatives corresponding to the positive and negative portions of the desired spectral sensitivity curve are E1 and E2, respectively, then the full effect of the negative portions will be secured by a process which can produce a corrected positive transparency of which the transmittance, T, is proportional to the difference of the exposures, E1–E2. The proportionality coefficient, K, must be independent of E1 and E2:
The masking method only partially satisfies this condition. In the masking process, the coefficient, K, depends on the ratio of E2∕1. This coefficient, K, can be assigned any convenient value, k, less than 1∕L, corresponding to any two values of the ratio, r = E2∕1, less than 1.00. The value of K will vary from this assigned value for all other values of the ratio of exposures, r. The two values of the ratio, r = E2∕1, for which the coefficient, K, is adjusted to the assigned value, must be chosen arbitrarily. For best color reproduction, these two values of the ratio should correspond to the two classes of colors for which perfect reproduction is most imperative. Usually, in color photography, the first and most strict requirement is that all white and gray objects must be reproduced as white and gray images having the correct relative brightnesses. Consequently, the value of E2∕1 = φ characteristic of exposures by white light should be one of the two values of the exposure ratio, r, for which K has the assigned value. The second value, r1, of r for which K takes the assigned value may be chosen in various ways, the choice depending on the class of colors, in addition to grays, for which best color reproduction is desired. In general, these colors should be those with which the critic is most familiar, such as flesh tints in portrait and dramatic photography, and sky and foliage in landscape photography. The analysis is greatly simplified if the derivative dK∕dr is first made zero for some suitable value, r0, of the exposure ratio, r. The value of r0 necessary to make K = k when r = r1 can be easily computed as shown below.
The following analysis will determine the transmittance, T, of the corrected positive in terms of the exposures, E1, E2, EA, EB, and the contrasts, γ3 and γ4. The exposure, E3, of the correction positive, mask, at a point in the image corresponding to the exposure, E2, and the density, D2, in the correction negative is:
The density resulting in the correction positive is therefore:
The combined density of the superimposed principal negative and mask is therefore:
The exposure of the corrected positive, produced by light passing through these superimposed densities of principal negative and mask, is therefore:
The corresponding density of the corrected positive is therefore:
As stated previously, this analysis holds only when both E1 and E2 are greater than 1.00, the inertia of the recording emulsion, and when they are less than the latitude, L, of the emulsion. The exposure, E2, frequently will be less than the inertia, and for such cases, log E2 should be assigned the value zero in Eq. (10.1). Consequently, when, E2 ≦ 1.00:
We have defined the coefficient, K, such that
We now impose the condition that K = k when r = φ, two relations will be established:
If we also impose the condition that dK∕dr = 0 when r = r0, then one additional relation is established, with the aid of Eqs. (10.3), (11.1), and (12.1), thus:
When evaluated at r = r0, this derivative is to equal zero, therefore:
If we eliminate γ4 from Eq. (12.3) by use of Eq. (12.1), we derive an expression for the value of γ3 necessary to satisfy the two conditions:
These values can be substituted in Eq. (11.1) with the result that:
The coefficient, K, is obviously equal to the assigned value, k, when r = φ, but it also equals k when
This expression determines r0 as a function of the second value, r1, of the exposure ratio, r, for which the coefficient, K, is to be assigned the value, k. For instance, if it is desired that K = k when r1 = φ∕ 10, then:
In conclusion, the transmittance, T, of a corrected positive prepared with a masking process, using the values of γ3, γ4, and EB, shown in Eqs. (13.1), (13.2), and (13.3), will be related to the exposures, E1 and E2, through the expression: T = K(E1–E2) provided that E1>E2>1.00. The value of the coefficient K is shown by Eq. (14.1) as a function of the ratio of exposures. This coefficient takes the assigned value, k, for r = φ and for r = r1. Curve (a) in Fig. 10 shows the variation of K with r for the example considered above. A process giving the full effect of the negative sensitivities shown in curve (SR) of Fig. 1 would be characterized by a constant value of K, equal to the assigned value, k.
The deviations of the curve for K from a horizontal straight line at K = k = 0.01 therefore indicate the direction and relative magnitudes of the irreducible errors of the masking process when used as a method for introducing partially negative spectral sensitivity. The very great deviations of K from the assigned value, k, which are indicated by curve (a) in Fig. 10 for high values of r, exaggerate to some extent the seriousness of the errors of the masking process. Actually, the value of E1–E2 is small compared to E1 when r is nearly 1.00, and the magnitude of the error in transmittance is not so serious as curve (a) in Fig. 10 might seem to indicate. The coefficient, K, becomes infinite for r = 1.00, but
This expression for T when E1 = E2 also gives approximately the value of T for all high values of r. Consequently, the transmittance, T, for values of r nearly equal to 1.00 is more conveniently represented as proportional to E1 rather than as proportional to E1–E2. In the example being discussed,
The value of the proportionality coefficient, K, shown by curve (a) in Fig. 10 also has a limit of applicability for low values of r, corresponding to the inertia of the emulsion on which the correction negative is recorded. In other words, the value of the proportionality coefficient, K, indicated by Fig. 10 is valid only when E1 is greater than 1∕r. For all smaller values of E1, the transmittance, T, is given by the expression:
In the example being discussed,
It is of some interest to compare the performance of the masking process with the results which will be obtained if no attempt is made to introduce the effects of the negative portions of the spectral sensitivity curve. A positive prepared by direct printing from the principal negative alone, with γ4 = 1.00, which is comparable with the conventional, unmasked, print from a color-separation record, has
In order to compare directly with the results of masking, this expression can be rewritten, by making the substitution:
Curve (b) in Fig. 10 shows the proportionality coefficient, K’∕(1–r), for the positive prepared without masking. The value k(1–φ) has been signed to K’ in order to make the curve have the value k when r = φ, as in curve (a) in Fig. 10. It is evident that masking permits the assignment of the value of the proportionality coefficient at two values of r, whereas the conventional method of printing permits the assignment of the value of this coefficient at only one value of r. The ideal process for introducing the effects of negative portions of the spectral sensitivity curve would, of course, exhibit a proportionality coefficient which would not depend upon the ratio of the exposures, E2∕1 = r.
It can be concluded from an examination of the curves in Fig. 10 that a properly controlled masking process furnishes a fair approximation to the effects of moderate amounts of negative spectral sensitivity. Such a process gives results which are superior to any which could be secured by the use of recording emulsions exhibiting the most favorable type of Herschel effect. They are also superior to the results given by any double printing method which is subject to the limitations of latitude which were explained above.
The color reproduction secured by an additive process employing the masking method described above will be compared with the results which could be obtained with a conventional process using only the positive portions of the spectral sensitivities shown in Fig. 1. For the purpose of this comparison, twelve fairly bright pigmented paper samples were considered as the originals, the colors of which were to be reproduced. The spectrophotometric curves of these samples were determined. The colors of these samples when illuminated by the standard I.C.I. Illuminant “C” are represented in Fig. 11 by the points (a) to (l). The I.C.I. tristimulus values for these samples are shown in columns 1, 2, and 3 of Table I.
The points R, G, and B in Fig. 11 represent the additive primaries of the reproduction process under consideration. Consequently, all of the samples having chromaticities represented within the triangle RGB would be reproduced perfectly by a process which satisfied completely the sensitivity requirements shown in Fig. 1. Since it is impossible to use negative amounts of any of the primaries in an additive process of color reproduction, net theoretical exposures less than zero are meaningless. Samples (h) and (i), lying outside of the triangle RGB in Fig. 11, cannot therefore be reproduced by this additive process. If all net exposures less than zero are recorded in the same manner as a zero exposure, then the samples (h) and (i), will be reproduced on the sides of the triangle RGB. The point representing the reproduction of any one of these colors will be the intersection of the side of the triangle and the line through the original chromaticity and through the primary which is eliminated from the synthesis by the zero exposure. If the net exposures for any sample color are zero or less for two of the primaries, then the sample will be reproduced with the same chromaticity as the remaining primary.
The net theoretical exposures can be computed from the I.C.I. tristimulus values of the colors of the original samples by the use of the transformation equations which were used to compute the curves in Fig. 1.
R = 2.450 X – 0.959 Y – 0.388 Z,
G = – 1.047 X + 1.907 Y + 0.091 Z, (15)
B = 0.074 X – 0.136 Y + 0.886 Z.
The R, G, and B values for the twelve samples are shown in columns 4, 5, and 6 of Table I.
The errors due to the neglect of all negative portions of the sensitivities shown in Fig. 1 can be determined by a selected ordinate computation from the spectrophotometric curves of the samples. The values of φ, computed from Fig. 1, are φR = 0.21, φG = 0.11, φB = 0.05. Consequently, these errors are given by:
The selected ordinates, FR, FG, and FB, for any one sample are the reflectances indicated by the spectrophotometric curve at the wave-lengths shown in Table II. These wave-lengths were computed from the negative portions of the functions shown in Fig. 1, using the method described on page 49 of the Handbook of Colorimetry.9 These computed errors are shown in columns 7, 8, and 9 of Table I.
The intensities of the primaries are always adjusted to give a perfect reproduction of white. The intensities of the primaries used in synthesizing each reproduction, when negative sensitivities are completely ignored, are therefore given by:
The values of R, G, B, ΔR ΔG, and ΔB, from Table I, and of φR, φG, and φB given above can be substituted in these equations. The resulting values of R’, G’, B’ are shown in columns 1, 2, and 3 of Table III. The corresponding I.C.I. tristimulus values shown in columns 4, 5, and 6 of Table III were computed from the reverse transformation:
The trichromatic coordinates shown in columns 7, 8, and 9 of Table III were computed in the usual manner from these tristimulus values. These coordinates represent the chromaticities of the reproductions secured by the additive process, using spectral sensitivities corresponding to the positive portions only of the curves shown in Fig. 1. These chromaticities are shown on Fig. 11 by the circled points.
The R, G, and B values of the reproduction resulting from the use of the masking method with this same additive process are given by:
The coefficients, KR, KG, KB, can be determined from Eq. (14.1) where k = 1.0 and φR, φG, and φB have the values given above. The corresponding values of r0 are 0.086, 0.043, and 0.02. The exposure ratios are, of course, given by
Eq. (14.3) should be used whenever the exposure ratio is greater than 0.7, and Eq. (14.4) should be used whenever ΔR, ΔG, or ΔB are less than 0.01. The value, r1 = φ∕ 10 has been assumed in all cases, and the resulting values of R”, G”, B”; X”, Y”, Z”; x” and y” are shown in Table IV. The chromaticities of the reproductions secured by the additive process, using this masking method, are shown by the crossed points in Fig. 11. A comparison of the three sets of points in Fig. 11 shows the degree of correction secured by the use of the masking method for introducing negative sensitivity. A comparison of the values of Y, Y’, and Y” in Tables I, III, and IV shows the faithfulness of brightness reproduction secured with the additive process, with and without masking corrections.
F. PROPOSED COMPROMISES IN WHICH NEGATIVE SPECTRAL SENSITIVITIES ARE ELIMINATED
Correction methods, such as double printing, photoelectric correction,6 and masking, are so complicated that their use will probably be confined for some years to relatively few applications in the graphic arts. It is desirable to survey the relative merits of several possible compromises in which the goal of perfect color reproduction is abandoned in order to eliminate the necessity for partially negative spectral sensitivities. One of these compromises has been investigated in the previous section. The errors in chromaticity and brightness resulting from the use of spectral sensitivities corresponding to only the positive portions of the theoretical spectral sensitivities were shown in Fig. 11 and Table III. The chromaticity errors shown in Fig. 11 are compounded of errors in dominant wave-length and of errors in purity.
The widespread opinion that errors of dominant wave-length (hue) are much more serious than errors of purity (saturation), although unsupported by reliable data, has led to at least two suggestions for compromises which would reproduce dominant wave-lengths perfectly at the expense of losses of saturation considerably more serious than result from the compromise considered above.
The oldest of these compromises was first practiced by Ives,2 and described by Abney.15 This compromise was independently suggested in April, 1937, by Yule, who appears to have been the first to state it unambiguously in the precise terminology of colorimetry. This compromise is to use, for the purpose of computing the spectral sensitivities, unrealizable primaries having the same dominant wave-lengths as the projection primaries but having purities sufficiently greater than those of the corresponding spectrum colors so that the spectrum locus lies entirely within the triangle formed by these primaries. The resulting spectral sensitivities will be positive everywhere, and the additive synthesis of the reproduction will consist of the perfect reproduction colors desaturated by the white light corresponding to the sum of the desaturations of the projection primaries as compared with the theoretical primaries. Consequently, the dominant wave-length of the original colors will be reproduced perfectly. The magnitudes of the purity errors of the reproduction will depend critically on the choice of the theoretical and actual projection primaries. These errors will be nearly minimized if the theoretical primaries are:
and if the actual reproduction primaries are:
The reproduction of a sample whose chromaticity is specified by the coordinates, x and y, will then be represented by
Fig. 12 shows the errors of the reproduction secured by this additive process. The errors in purity are much more serious than when the negative portions of the spectral sensitivity curves corresponding to the actual projection primaries were simply ignored.
Another compromise which is frequently vaguely proposed was clearly described by Schaefer and Ackermann.4 This consists in adding to each of the theoretical sensitivities, such as those shown in Fig. 1, a function just sufficient to cancel all negative values. This is equivalent to reproducing the spectrum colors with colors having the same dominant wave-lengths, but desaturated so as to be represented along the sides of the triangle RGB shown in Fig. 11. As in the compromise of Ives, Abney, and Yule, this compromise eliminates errors in dominant wave-length, but sacrifices purity. The tristimulus values of the reproduction, using the primaries shown in Fig. 11, will be
The brightness, W, of the desaturating white can be computed from the spectrophotometric curves of the samples to be reproduced. It is 0.46 times the average of the ordinates of the spectrophotometric curve at the ten wave-lengths given in Table V. As indicated above, the brightness of the reproduction is decreased in the ratio of 1 : 1.46, in order to make the values of the brightnesses, Y’, directly comparable with the brightnesses shown in Tables I, III, and IV.
The values of W for the twelve samples previously considered are shown in the first column of Table VI. The tristimulus values of the reproduction are shown in columns 2, 3, and 4 and the trichromatic coordinates are given in columns 5, 6, and 7. Fig. 13 shows the errors of chromaticity exhibited by an additive process designed according to this compromise of Schaefer and Ackermann. The purity errors are slightly greater than those shown in Fig. 12 for the compromise of Ives, Abney, and Yule. The purity errors shown in Figs. 12 and 13 are, however, much more serious than those shown in Fig. 11 caused by the simple neglect of all negative sensitivity requirements. More reliable data than are now available concerning the relative tolerances for dominant wave-length errors and purity errors will be necessary before a decision can be made between these compromises.
That increase of contrast will increase the purities (saturations) of the colors in a color photograph has long been recognized as an empirical fact. Many processes take advantage of this fact, with the result that colors of high purity are secured at the expense of excessive brightness contrasts. The desirability of perfect brightness reproduction has been assumed in all of the preceding comparisons of processes. The success with which this goal can be attained by the several processes is revealed by comparison of the values in the Y columns of the respective tables with the values of the brightnesses of the original colors, given in the Y column of Table I.
It is of interest to determine what increase of contrast is necessary to compensate approximately for the losses of purity in the reproductions represented in Figs. 11, 12, and 13. The reproductions specified by Table III will be used as the colors whose purities are to be increased by an increase of the brightness contrast of the reproduction. The values of R, G, B shown in Table VII are the corresponding values in Table III raised to the 1.2 power. These are the amounts of the primaries which synthesize the reproduction when the projection positives of the additive process are developed to a gamma (γ) of 1.2. Columns 4, 5, and 6 of Table VII show the resulting values of the tristimulus values X, Y, and Z of the reproduction. Columns 7 and 8 show the trichromatic coordinates of the resulting colors. These quantities have been computed for γ = 1.1, 1.2, 1.3, 1.4, and 1.5, and the resulting chromaticities are represented by the series of crosses shown in Fig. 14. These displacements from the chromaticities of the normal contrast (γ = 1.0) reproductions (shown by the open circles) compensate in some respects for the loss of purity caused by the neglect of negative sensitivities. The chromaticities corresponding to γ = 1.2 approach the original chromaticities (shown by the solid dots) more closely on the average than is the case for higher or lower values of γ. It will be seen, however, that increases in contrast alter dominant wave-lengths as well as purities. In some cases, these dominant wave-length changes compensate and in others aggravate the errors of dominant wave-length exhibited by the normal contrast reproduction. These alterations of dominant wave-lengths are so seriously objectionable that the usefulness of the method of contrast control of purity may be almost completely nullified. It is obvious that increase of contrast will alter the colors of the reproductions shown by the crosses in Fig. 11 and by the open circles in Figs. 12 and 13 in approximately the same manner as shown in Fig. 14. Consequently, it can be estimated that contrasts of approximately 1.5 will be necessary to compensate for the losses in purity caused by the methods of Ives, Abney, and Yule and of Schaefer and Ackermann. On the other hand, the less objectionable contrasts of 1.1 to 1.2 are sufficient to compensate for the losses of purity caused by the masking method and by the simple neglect of negative sensitivities. The errors of the brightness reproduction in the last-named method, when the contrast is increased to 1.2, can be determined by comparison of the values in the Y columns of Tables I and VII.
The theory of subtractive color reproduction differs from the theory of additive color reproduction primarily because of the differences in the behaviors of subtractive and additive color mixtures. The theory of subtractive reproduction must be based on the theory of additive reproduction supplemented by a law of subtractive color mixture. Therefore, an analytically convenient law of subtractive color mixtures is necessary before the principles of additive color reproduction can be applied to practical subtractive processes. Pending the formulation of the desired law of subtractive color mixture, a complete discussion of the theory of additive reproduction, and of proposed methods of satisfying the theoretical requirements, appears to constitute a valuable preparation for the theory of subtractive reproduction. This article is intended as a contribution to such a discussion.
The discussion of the colorimetric and photographic principles of additive color reproduction given in this paper has indicated the desirability of panchromatic photographic materials having contrasts independent of wave-length. The desirability of some correction for the negative portions of the theoretical spectral sensitivity curves has been acknowledged. Fundamental objections to the correction methods using selective reversal and double printing, proposed by Hardy and Wurzburg, have been discussed. The limitations of all photographic materials known at this date render these correction methods inferior to the masking method of correction which is proposed in this paper. This practical method for securing some degree of correction for the negative sensitivities required by an additive process is shown to be superior to the best conventional methods, in which uncorrected color-separation records are used.
Several suggested methods for improving the conventional color-separation records have been discussed. Computations indicate that little if any improvement can be secured over the process in which the spectral sensitivities are proportional to the positive portions only of the tristimulus values of the spectrum computed on the basis of the actual projection primaries. It appears that these conclusions remain valid for subtractive color reproduction, since the errors indicated in Figs. 11, 12, and 13, caused by incorrect spectral sensitivities, combine vectorially with the errors due to the failure to simulate the phenomena of additive mixtures by the use of subtractive mixtures.
1 J . C. Maxwell, Trans. Roy. Soc. Edinburgh, 21, 275 (1855).
2 F . E. Ives, J. Franklin Inst. 125, 345 (1888); 127, 54 (1889).
3 E. Schrödinger, Müller-Pouillets Lehrbuch der Physik, II Optik, Erste Teil (Fr. Veiweg, Braunschweig, 1926), p. 488 et seq.
4 Cl. Schaefer and K. Ackermann, Zeits. f. tech. Physik 8, 55–62 (1927).
5 M. E. J. Neugebauer, Zeits. f. wiss. Photographie, Photophysik u. Photochemie 36, 86 (1937); also Dissertation (Leipzig, December 1, 1934).
6 A. C. Hardy and F. L. Wurzburg, Jr., J. Opt. Soc. Am. 27, 227 (1937). H. D. Murray and D. A. Spencer, Phot. J. 78, 474–482 (1938). This is a review, criticism, and extension of the theory of Hardy and Wurzburg. Some aspects of the theory of Hardy and Wurzburg are misinterpreted in this review, especially the relation of masking to the theoretical requirements. Hardy and Wurzburg stated clearly that masking does not accomplish the effect of partially negative spectral sensitivities demanded by the theory.
7 G. B. Harrison and R. G. Horner, Phot. J. 77, 706–13 (1937).
8 H. Frieser and R. Reuther, Zeits. f. tech. Physik 19, 77–85 (1938).
9 Handbook of Colorimetry (Technology Press, Cambridge, Mass., 1936).
10 E. J. Wall, History of Three-Color Photography (American Photographic Publishing Company, Boston, 1925). In addition to a clear restatement of the concept of ideal printing inks for subtractive reproduction given on pages 27 and 28, Wall refers to many earlier discussions of this subject. The most accurate and original of these are the following: W. deW. Abney, Phot. J. 39, 192 (1899). R. S. Clay, Proc. Roy. Soc. 69, 26 (1901). F. E. Ives, J. Franklin Inst. 153, 43 (1902).
11 Chapman Jones, Brit. J. Phot. 52, 13 (1905).
12 A. van Kreveld, Zeits. f. wiss. Photographie, Photophysik u. Photochemie 32, 222 (1934), J. H. Webb. J. Opt. Soc. Am. 26, 12 (1936).
13 A. P. H. Trivelli, J. Franklin Inst. 207, 765–97 (1929), especially Table II and Figs. 8 and 13. A. P. H. Trivelli and V. C. Hall, J. Franklin Inst. 208, 483–506 (1929).
14 T. Troland, U. S. Patents 2,098,441 and 2,098,442 (November 9, 1937).
15 W. deW. Abney, Phot. J. 23, 192–98 (1899); 24, 121–31 (1900).”
(MacAdam, David L. (1938): Photographic Aspects of the Theory of Three-Color Reproduction. In: The Journal of the Optical Society of America, 28,11, Nov., pp. 399–418.)
“La couleur a fait intégralement partie du cinéma tout au long de la période du muet et était déjà utilisée au moment de l’émergence du cinématographe à la fin du XIXe siècle à travers divers procèdes d’applications de la couleur. Certains d’entre eux étaient employés sur des films depuis au moins 1895. Il s’agit des procédés suivants: la colorisation manuelle pour laquelle les éléments de chaque photogramme étaient coloriés à la main au moyen de fins pinceaux; le teintage qui permettait de colorer un morceau du film d’une couleur spécifique; le virage qui attaquait l’argent présent sur l’émulsion soit en le transformant en élément coloré soit en le blanchissant – l’émulsion était alors ensuite colorée grâce à une teinture qui se fixait aux endroits blanchis et, enfin, la technique du pochoir. Chaque couleur avait son pochoir, les différents pochoirs étaient ensuite placés successivement sur la copie du film et l’encre était étalée aux bons endroits. Le teintage était également souvent utilisé avec d’autres procédés, produisant ainsi une grande variété d’effets combinés: teintage et coloris manuel, teintage et virage, teintage et pochoir.
Selon un compte-rendu critique de la première séance du Cinématographe des frères Lumière le 28 décembre 1895 au Salon Indien du Grand Café à Paris paru dans le Radical, des films colorés furent projetés ce soir-là: “Quelle que soit la scène ainsi prise et si grand que soit le nombre de personnages ainsi surpris dans les actes de leur vie, vous les revoyez, en grandeur naturelle, avec les couleurs, la perspective, les ciels lointains, les maisons, les rues, avec toute l’illusion de la vie réelle”14. De la même façon, lors de la première projection publique des films Edison le 23 avril 1896, au Music Hall de Koster & Bial à New York City et selon les comptes-rendus parus dans la presse de l’époque, deux films étaient colorés à la main: Umbrella Dance des sœurs Leigh (1895), qui ouvrait la série de projections et une danse serpentine qui la clôturait15. Avant ces projections, les films de Francis Jenkins et Thomas Armat et presque certainement des films Edison conçus pour le Kinetoscope furent également colorés à la main. La couleur existait donc au cinéma dès son apparition.
La présence précoce de la couleur au cinéma n’est guère surprenante: depuis le milieu du XIXe siècle, date à laquelle les premières teintures synthétiques abordables furent développées, la couleur a permis de transformer les espaces de la modernité, faisant ressembler le monde à un rêve fantastique qui prend soudain vie16. À cette période, les plaques de lanterne magique, les éclairages scéniques colorés saturaient les espaces des divertissements populaires tandis que les avenues étaient inondées d’une mode très colorée, conçue à partir des tissus nouvellement colorés grâce à l’aniline. L’impression des documents éphémères (affiches, brochures, cartes et autres dépliants) fut aussi un lieu important de l’expansion de la couleur dans les espaces du quotidien du XIXe siècle. Les rues des villes furent bientôt placardées d’affiches publicitaires traitées en chromolithographie. Les papiers peints, les reproductions d’œuvres d’art, des photographies peintes à la main et des cartes postales au pochoir coloraient les murs des espaces domestiques. Enfin, l’impression couleur révolutionna l’espace de lecture avec ses illustrations multicolores – dans la presse féminine (illustrations destinées à être détachées pour décorer la maison), dans les livres pour enfants et sur les couvertures des “romans à dix cents” – et l’usage de la couleur était devenu de plus en plus populaire et répandu à la fin du siècle. Dans les années 1890, la couleur prit également racine dans les journaux. L’une de ses incarnations les plus célèbres fut sans doute le personnage de Yellow Kid, inventé par Richard Outcault, dont les aventures furent publiées pour la première fois en couleurs dans la bande dessinée d’Outcault intitulée Hogan’s Alley et publiée au sein du New York World en 1895 […].
C’est dans ce contexte large et intermédial des usages de la couleur qu’il convient de replacer l’émergence du cinématographe et, même si j’ai déjà pu écrire sur ces questions, je souhaite ici concentrer mon propos plus systématiquement sur les relations entre la culture de l’imprimé et l’espace de la couleur dans le cinéma des premiers temps17. Dans ses travaux sur la littérature enfantine comme sur le surréalisme, Benjamin attire l’attention sur la nature changeante du paratexte au XIXe et au début du XXe siècle: l’espace du livre devient alors de plus en plus visuel et tactile, tant par les illustrations (en couleurs) que par l’expérimentation graphique des romans populaires, revues et journaux, dans les publicités comme dans de nombreux travaux d’avant-garde d’artistes comme Mallarmé, Moholy-Nagy ou Lissitzky18. Comme Benjamin et, plus récemment, Ian Christie l’ont noté, le cinéma s’inscrit dans cette généalogie graphique. À propos de la relation qu’entretenait Georges Méliès avec l’imprimé, Christie écrit: “le développement de la production en masse de publications illustrées, depuis les journaux satiriques populaires le Charivari ou la Griffe (à laquelle Méliès contribua comme dessinateur en 1889-1890) jusqu’aux fictions populaires illustrées, a entrainé l’interpénétration entre le verbal et le visuel, interpénétration qui sera plus tard développée dans le cinéma des premiers temps”19. Poussant plus loin cette idée, Benjamin déclare que l’hybridité entre les registres textuel et iconographique qu’on trouve dans l’imprimé moderne et l’image en mouvement favorise également l’interpénétration et l’immersion du lecteur/spectateur/observateur, leur corps et l’image fusionnant dans le jeu20. De la culture de l’imprimé au cinéma des débuts, la couleur fut l’un des éléments partagés par ces médias et qui facilita ces complexes interactions.
Une telle convergence des usages de la couleur dans ces deux médias peut s’expliquer par des pratiques de travail communes. Des entreprises artisanales spécialisées dans le coloris des films sont apparues en Europe et aux États-Unis au début des années 1900. Aux États-Unis par exemple, une société basée à Philadelphie, Harbach and Company, lance en 1901 une campagne publicitaire offrant aux éditeurs de sous-traiter le coloris manuel21. De la même façon, Pathé offre des services et publicités similaires en Angleterre, expliquant que “nous prenons en charge le coloris des films pour un tarif qui reste à déterminer. Nous colorons également les films fournis par nos clients, même s’ils ne sont pas produits par notre société”22. Si l’on regarde comment ces sociétés ont organisé leur travail de mise en couleurs, il est clair qu’elles se sont adaptées à des pratiques industrielles déjà existantes. Plus spécifiquement encore, des illustrations en couleurs des imprimés à chromolithographie, en passant par les plaques de lanterne magique et les cartes postales, il existait une pratique commune dans les arts décoratifs d’employer des femmes et jeunes filles pour réaliser ce travail de mise en couleurs, dans une large part parce que leur travail répétitif pouvait être exploité à un moindre coût. Cette pratique s’est perpétuée dans l’industrie cinématographique et le coloris des films devint bientôt la première et principale porte d’entrée pour les femmes dans la production des films.
Non seulement les pratiques du coloris proviennent de l’édition papier mais beaucoup de coloristes travaillèrent pour divers médias. En raison de la rapide expansion des journaux illustrés dans les années 1890 et 1900, on trouvait assez facilement des femmes illustratrices. Comme Ruth Copans l’a noté: “L’illustration était devenue une carrière acceptable pour les femmes […] L’apprentissage du dessin faisant souvent partie de l’éducation d’une femme de classe moyenne, au même titre que la musique, la broderie et autres “arts féminins””23. Un certain nombre d’artisans expérimentés trouvèrent du travail dans l’industrie émergente du film. Germaine Berger, par exemple, apprit d’abord à dessiner comme illustratrice aux côtés de son père, dessinateur pour un fabricant de meubles à Paris, avant d’être engagée en 1911 – elle est alors âgée de 15 ans – sur ces compétences, pour découper des pochoirs chez Pathé à Vincennes24. Élisabeth Thuillier, célèbre pour les coloris manuels effectués pour Méliès, dirigeait une société de colorisation de films à Paris, à l’origine destinée à colorier les plaques de lanterne magique25. Aux États-Unis, Gladys R. Scott travailla à la fois comme coloriste de plaques pour chansons illustrées pour la société Scott and Van Altena, et comme coloriste pour le film dans les années 1910, pour des compagnies comme Universal26. Cette porosité entre médias explique le transfert de diverses techniques (brosses à poils de chameaux utilisées sous une loupe, découpe de pochoir) et colorants (teintures d’aniline que les émulsions photographiques pouvaient absorber). Ces techniques furent adaptées au médium cinématographique, créant des images hybrides, multicouches qui superposaient les techniques d’impression photographique aux modes artisanaux de coloris et d’animation.
Au-delà des pratiques de travail, on peut également considérer la manière dont l’imagerie visuelle de la culture de l’imprimé servit de source iconographique pour les usages de la couleur au cinéma. Le Yellow Kid de Richard Outcault fut par exemple adapté très tôt au cinéma. Dès 1897, Edison produisit en effet Leander Sisters, Yellow Kid Dance, qui fut filmé dans le complexe de piscine des Sutro Baths à San Francisco. Le film, constitué d’un seul plan, présente deux sœurs: l’une est en robe de danse de salon et l’autre, costumée en Yellow Kid, porte une redingote et un bonnet avec des oreilles géantes. Le temps d’un plan assez long, elles dansent ensemble, faisant face et s’adressant directement à la caméra tandis qu’au second plan, les spectateurs en tenue de bain regardent leur performance. Selon un catalogue de Maguire & Baucus de 1898, le film était disponible en couleurs. La danse y est décrite comme “pleine d’action et de mouvements gracieux […] les costumes montrent une combinaison de perle, de gris et de blanc avec d’excellents effets de couleurs”27. Les copies colorées du film n’existent plus, mais comme le catalogue le souligne, l’attraction du film repose sur le coloris des costumes des danseurs. D’une certaine manière, ceci rejoint le travail d’Outcault sur la couleur puisqu’il coloriait souvent de manière vibrante les costumes de ses personnages afin de les détacher du fond et les faire ressortir dans la page. D’après la description de Maguire & Baucus, un effet similaire est mis en œuvre dans le film, les couleurs ayant été appliquées principalement sur les habits des sœurs Leander, les faisant gracieusement scintiller au milieu de l’espace, entre une caméra invisible et les spectateurs en tenue de bain.
Ce coloris sélectif correspond partiellement au style de coloriage des films d’Edison de cette époque, la société utilisant souvent les coloris manuels afin de faire ressortir les personnages de fonds relativement neutres. C’est le cas dans les divers films de danse réalisés avec Annabelle Whitford dans le studio Black Maria d’Edison tel que Annabelle Serpentine Dance (1894), comme dans un film avec les sœurs Leigh, Umbrella Dance (1895). Dans ces deux films, les danseurs exécutent leur danse devant un fond noir tandis qu’un coloris manuel, très saturé, est localisé sur leurs costumes et leurs chevelures. J’ai décrit ailleurs la manière dont ces additions colorées et saturées ajoutent une “dimension projective” [projective dimensionality] à ces films, à savoir un effet stéréoscopique où les figures colorées se distinguent sur un fond noir28. En effet, de nombreuses descriptions des premiers films colorés attirent l’attention sur la dimension projective de la couleur dans l’image. Comme l’indique une brochure de Raff & Gammon à propos du style coloré des films d’Edison: “Avec leurs visages, mains, bras et autres membres peints des couleurs de la vie et leurs costumes et accessoires aux vifs coloris, les sujets semblent sortir de la toile tels des hommes et femmes vivant”29. Par ces effets, l’espace en deux dimensions semble donc acquérir une troisième dimension, les couleurs ressortant en relief de l’écran pour immerger l’espace de la salle et rendant ces nuances presque tactiles au fur et à mesure qu’elles atteignent et traversent l’espace de la salle. Cette adresse virtuelle de la part de la couleur complète celle, ouverte et directe des interprètes à la caméra qui caractérise, comme Tom Gunning l’a noté, le cinéma des attractions.
Doublant l’adresse directe des danseurs à la caméra et, au-delà, au public l’usage des couleurs fait dans Leander Sisters semble fonctionner au moins en partie sur le même principe. Cependant, la mise en scène en décor naturel de cette adaptation comique diffère quelque peu des films précédents tournés au studio Black-Maria. L’arrière-plan du film est ici constitué de la foule des spectateurs qui font face frontalement et reflètent en miroir la caméra et les spectateurs. Ils sont littéralement des spectateurs à l’intérieur d’une mise en scène et sont de fait à relier aux spectateurs diégétiques qu’on trouve dans d’autres films d’Edison tels que Trapeze Disrobing Act (1901) et Uncle Josh at the Moving Pictures (1902). C’est entre ces deux positions spectatorielles – à l’intérieur et à l’extérieur de la diégèse – que la couleur des costumes des danseurs miroite, créant une double adresse au spectateur, en l’extirpant du film par cette adresse projective et en le plongeant simultanément dans l’univers immersif du film, par l’invitation à rejoindre les spectateurs de la diégèse et les danseurs. Les couleurs sont tel un Janus à deux visages: tout à la fois exhibitionnistes dans le sens où elles s’adressent par la projection au spectateur, et s’inscrivant dans une logique de représentation d’un monde diégétique que le spectateur regarde dans un mode de représentation que le cinéma classique perfectionnerait jusqu’au voyeurisme.
Comme Benjamin le note à propos des effets de couleurs dans les livres illustrés pour enfants, la couleur peut faciliter la construction par l’imaginaire d’un monde fictionnel, en transportant le spectateur dans un espace coloré immersif. Dans le même ordre d’idées, Benjamin relie ce pouvoir immersif de la couleur au conte de fées et, plus particulièrement, à la relation ludique que le conte établit avec le spectateur. D’ailleurs, l’un des premiers genres fictionnels du cinéma des premiers temps fut la féérie qui devint aussi l’un des genres les plus souvent coloriés à la main au début des années 190030.
Adaptées principalement du genre scénique français et éponyme du XIXe siècle, les féeries ont souvent des racines intermédiales dans la culture de l’imprimé. Cependant, comme avec le chatoyant Voyage dans la lune de Georges Méliès (1902), de telles racines littéraires peuvent parfois sembler ténues: le propos du film de Méliès par exemple est seulement et de manière fragile relié au roman de Jules Verne, De la Terre à la Lune (1865) et celui d’Herbert George Wells, Les Premiers Hommes dans la lune (1901). Or, comme l’a montré avec détail Thierry Lefebvre, le film partage plus d’affinités visuelles avec la féerie de Jacques Offenbach de 1875 du même nom et même avec cyclorama, “A Trip to the Moon”, montré à l’Exposition Pan-américaine de Buffalo en 190131. On le voit, à travers ces références, Méliès a été influencé moins par les intrigues de ces diverses versions médiatiques du Voyage dans la lune que par un paratexte rassemblant des motifs visuels mis en jeu pendant la Belle Époque.
Une telle gamme d’influences visuelles intermédiales est récurrente dans d’autres féeries filmées, indirectement adaptées de sources textuelles. Prenons l’exemple du film de Pathé, datant de 1906, les Fleurs animées (mis en scène par Gaston Velle et photographié par Segundo de Chomón)32 […]. Tirant son titre et une part de son inspiration du fabuleux ouvrage botanique et féérique de Grandville de 1846-1847, gravé et coloré à la main, le film présente l’histoire d’un magicien chinois qui fait des propositions à une belle femme au moyen d’une fleur33. Refusant son affection, elle le gifle. En guise de représailles et après le départ de la femme, le magicien arrache les pétales de la fleur et ordonne à ses serviteurs de détruire le jardin au sein duquel la fleur a poussé. Après la sortie du magicien du champ, les fleurs piétinées reviennent à la vie avec des têtes de femmes au cœur de chaque corolle. Puis, les fleurs se transforment en femmes qui se vengent en mettant du pavot ou de l’opium dans la tasse de thé du magicien afin de l’endormir et le transformer en plante vivante qui germe et jaillit de manière incontrôlée de son pot, face à la caméra, ce plan amenant au tableau final. Les femmes-fleurs dansent autour du magicien pris au piège et redeviennent fleurs avec des têtes de femmes, posant pour la caméra dans une apothéose.
L’intrigue du film n’a que peu à voir avec l’ouvrage de Grandville qui est un condensé de contes fantastiques dans lesquels les fleurs se transforment en femmes et permettant la fusion entre le floral et le féminin […]. L’histoire du film ne renvoie à aucun des contes du livre, quoique conforme aux récits fantastiques et exotiques qu’on pourra trouver dans ce recueil tels que “La Sultane Tulipia” (histoire d’une femme-fleur hollandaise prise au piège dans un harem turc), “La Traite des fleurs” (qui compare le marché aux fleurs avec le marché aux esclaves) et “Les Fleurs changées en bêtes” (conte oriental à propos de la ressemblance de l’orchidée malaisienne Katong-ging avec un scorpion).
Au lieu d’adapter avec fidélité l’ouvrage de Grandville, le film s’inspire davantage du monde fantastique que le texte construit à travers ces contes et en particulier ses illustrations colorées. Le monde de Grandville imagine la vie secrète, saturée et féérique des fleurs, cachées au sein du monde moderne. Au même moment, les visions botaniques des fleurs étaient alors courantes dans les publications du XIXe siècle, incarnées par des revues comme Curtis’s Botanical Magazine, The Botanical Register et un grand nombre d’ouvrages – citons pour seuls exemples les sept volumes du British Flower Garden (1823-1829) le Ladies’ Botany (1865) de John Lindley –, travaux que l’œuvre photographique de Karl Blossfeldt dans les années 1920 poursuivra34. En dehors des photographies de Blossfeldt, la couleur était un élément constitutif de ces visions botaniques: les nuances attractives entrainent alors le lecteur dans le monde fantastique des textes illustrés, comme s’il entrait dans un rêve.
De la même façon, les nuances du pochoir utilisées dans le film de Velle sont les couleurs de rêves cinématographiques qui aident à la production d’une mise en scène immergeant le spectateur dans l’écran. Les couleurs sont majoritairement appliquées au second et premier plans de la mise en scène, sur les costumes des personnages et des éléments spécifiques du décor (fleurs, lampes chinoises, vases). Les couleurs sont toujours conformes au cinéma des attractions, créant une série de vues fantastiquement colorées et séduisantes pour le spectateur, et se détachant sur le fond noir du film. En lien avec ceci, les personnages s’adressent directement à la caméra, saluent dans sa direction, alors qu’ils exécutent des danses, des divers tours et tableaux organisés afin de révéler le monde secret des fleurs. Les couleurs appliquées aux costumes des personnages renforcent en quelque sorte la force projective de ces adresses directes, créant un sentiment de profondeur car émergeant du fond noir du film. Cependant, ces coloris sont relativement moins saturés que les films à trucs et les féeries antérieurs. Les nuances des pochoirs sont plutôt pastels – bleus légers, roses, verts et jaunes – qui réduisent tout autant la force projective au-delà de l’écran. Ainsi, ces couleurs commencent davantage à sculpter en bas-relief un espace diégétique qui entraine par le jeu le spectateur dans le monde fantaisiste à l’écran plutôt qu’à sortir violemment de l’écran pour toucher le spectateur.
Ce mouvement vers des pastels désaturés se poursuivit après le cinéma des attractions, dans la période transitionnelle qui suivit. Les films à trucs et les féeries s’épuisèrent et Pathé, comme d’autres compagnies, travailla à intégrer leurs couleurs fantastiques dans d’autres modes de faire du cinéma. Pathé tout particulièrement, redéploie son usage du pochoir vers des genres narratifs comme le film historique (comme par le Film d’Art) et vers des genres relevant de la non-fiction: l’impact dimensionnel des couleurs est incorporé dans ces genres aux modes de narration et d’exposition différents. Avec ce redéploiement, le niveau de désaturation des couleurs et leur évolution vers des tons pastels se poursuit et est alors conçu pour fonctionner non pas au niveau de l’espace liminal qui relie l’écran au spectateur mais de manière non intrusive, dans le fond architectural de l’image. Ce style crée un espace coloré fictionnel que le spectateur regarde et dans lequel il se trouve aussi immergé, comme l’enfant absorbé dans l’espace de jeu d’un livre d’images.
Entre le texte et l’écran
L’influence intermédiale des couleurs en usage dans les illustrations imprimées sur le cinéma des premiers temps ne signifie pas qu’elle fut monolithique – il s’agit d’une influence intermédiale parmi d’autres aux XIXe et début XXe siècles. Depuis les ouvrages illustrés jusqu’aux vues, en passant par les féeries scéniques, les fantaisies qui proliférèrent à cette époque dans ces médias utilisèrent la couleur à la fois pour atteindre par la projection le spectateur et intégrer l’observateur dans un monde diégétique vibrant de vie. Avec l’invention moderne des teintures synthétiques au XIXe siècle, la couleur devint dans les arts un espace de jeu pour l’imagination, jouant sur des plans narratif, tactile et mimétique. C’est encore le cas aujourd’hui dans les arts numériques et analogiques qui poursuivent l’interpénétration des arts reposant sur l’écrit ou sur l’image qui débute au XIXe siècle.
Pour en revenir à la notion de Spiel-Raum et voir les implications et le potentiel de ces couleurs immersives, textuelles et visuelles, je conclurai en citant le film expérimental et muet de David Gatten, Hardwood Processes (1996), qui mélange une imagerie abstraite richement colorée et des intertitres qui rappellent les entrées d’un journal ou d’un agenda. Le film met en valeur des différences de textures abîmées, traumatisées, à la fois provenant du monde diégétique (la main d’un personnage en contact avec le grain du bois) et issues du film lui-même (le film est impressionné sur une surface grattée, solarisée, imitant la sensation du bois). Nos yeux traversent de manière haptique ces surfaces et les couleurs de ces détails quotidiens deviennent sensibles. Les images sont entrecoupées par des cartons noirs sur lesquels on peut lire une phrase, écrite en blanc comme extraite d’un journal: “JOUR 156 – il était une fois”, “JOUR 123 – carnet de voyage (film muet)”, “DAY 163 – une première tentative de traduction”35 et ainsi de suite. Entre le texte et l’image, les couleurs du film inondent de manière ludique l’œil et l’imagination, semblant parfois émerger et se projeter hors de l’écran, immergeant le spectateur de manière haptique, tandis qu’il l’attire à d’autres moments à l’intérieur du monde matériel du film. A environ dix minutes et demie (le film dure treize minutes et demie), un intertitre apparaît: “JOUR 303 – chromo pharmakon”36.
La couleur a été, au moins depuis Aristote, associée aux drogues (pharmakon), tant médicinales (on parle alors de “chromothérapie”) qu’hallucinogènes (pensées psychédéliques, rêves et troubles délirants). Les couleurs des divers films qui ont été discutés ici, d’Edison à Velle et Gatten, reflètent cette dualité: ils extraient et invitent le spectateur à plonger dans un espace de jeu immersif où chacun imagine un nouvel univers composé de fragments du quotidien. La fleur de pavot si colorée et qui était utilisée comme opiacée pour droguer le magicien dans les Fleurs animées de Velle, s’exprime ainsi dans l’ouvrage de Grandville: “Dès que l’homme m’a approchée de ses lèvres, son âme prend des ailes; elle quitte la terre. Elle retourne vers le passé ou s’élève vers l’avenir. Elle plane sur le souvenir ou sur l’espérance”37. La couleur, comme la drogue, dérange les sens, étendant et contractant le temps et l’espace tout autour de nous. S’il peut être fantastique ou hallucinogène, ce processus peut aussi être productif, et même thérapeutique, au sens où la couleur, en tant qu’immersion de nous-mêmes dans ces fragments prismatiques du quotidien, peut recalibrer notre sens incarné du monde, étendant notre conscience de ses limites et de ses possibilités alors qu’elle nous occupe de manière ludique à travers ses mouvements, dans nos rêves comme dans la vie.
Traduit de l’anglais par Priska Morrissey
14 Anonyme, “le Cinématographe”, le Radical, 30 décembre 1895. C’est moi qui souligne.
15 Voir les comptes-rendus publiés dans la brochure Raff and Gammon, “The Vitascope” (mars 1896), dans Reese V. Jenkins, Charles Musser, Thomas E. Jeffrey et al. (dir.), Motion Picture Catalogs by American Producers and Distributors, 1894-1908: A Microfilm Edition, Frederick, Md., University Publications of America, 1984-1985, A-009-022.
16 Concernant l’usage grandissant de la couleur dans les médias au XIXe siècle, voir notamment Neil Harris, “Color and Media: Some Comparisons and Speculations” dans Cultural Excursions: Marketing Appetites and Cultural Tastes in Modern America, Chicago, University of Chicago Press, 1990, pp. 318–336; et l’ouvrage, superbement illustré, de Nicholson Baker and Margaret Brentano, The World on Sunday: Graphic Art in Joseph Pulitzer’s Newspaper (1898-1911), New York, Bulfinch Press, 2005; Philip Ball, Bright Earth: Art and the Invention of Color, Chicago, University of Chicago Press, 2003; Ellen Gruber Garvey, The Adman in the Parlor: Magazines and the Gendering ofi Consumer Culture, 1880s-1910s, New York, Oxford University Press, 1996, pp. 19–25; Heinz K. Henisch and Bridget A. Henisch, The Painted Photograph: 1839-1914, Pennsylvania, Pennsylvania State Press, 1996; Robert Jay, The Trade Card in Nineteenth-Century America, Colombia, University of Missouri Press, 1987, pp. 1, 99–102; William Leach, Land of Desire: Merchants, Power, and the Rise of a New American Culture, New York, Pantheon Books, 1993, pp. 9, pp. 44–45; et Peter Marzio, The Democratic Art: Pictures fior a Nineteenth Century America, Boston, Godine, 1979.
17 Voir Joshua Yumibe, Moving Color: Early Film, Mass Culture, Modernism, New Brunswick (N.J.), Rutgers University Press, 2012, pp. 17–36.
18 Miriam Bratu Hansen, Cinema and Experience: Siegfried Kracauer, Walter Benjamin, and Theodor W. Adorno, Berkeley, University of California Press, 2011, pp. 130–133 et Frederic Schwartz, Blind Spots: Critical Theory and the History of Art in Twentieth-Century Germany, New Haven (Conn.), Yale University Press, 2005, pp. 38–55.
19 Traduction de: “the development of mass-produced illustrated publications, from popular satirical papers such as le Charivari and la Griffe (to which Méliès contributed as cartoonist in 1889-1890) to illustrated popular fiction, also brought about an interpenetration between the verbal and visual, which would be further developed in early film” dans Ian Christie, “First-Footing on the Moon: Méliès’s Debt to Verne and Wells and His Influence in Great Britain”, dans Matthew Solomon (dir.), Fantastic Voyage of the Cinematic Imagination: Georges Méliès’s Trip to the Moon, Albany, State University of New York Press, 2011, pp. 65–79.
20 Voir Walter Benjamin, “Surrealism: The Last Snapshot of the European Intelligentsia”, traduit par Edmund Jephcott dans Selected Writings, vol. 2, sous la direction de Howard Eiland et Michael W. Jennings, Cambridge (Mass.), Harvard University Press, 2002, pp. 217–218. Voir également les différentes réflexions de Benjamin sur la culture de l’imprimé et le film dans “One-Way Street” (1923-1926/1928), traduit par Edmund Jephcott, dans Selected Writings, vol. 1, op. cit., pp. 444–488.
21 Voir la publicité Harbach dans New York Clipper, vol. 49, n° 26, 17 août 1901, p. 554.
22 Traduction de: “We undertake to color the films at a price to be agreed upon. We also color the films supplied by our customers, even if they are not of our own make”, dans le catalogue Pathé “Cinematographs-Films (May, 1903)”, dans British Film Institute (dir.), Early Rare Filmmakers’ Catalogues: 1896-1913, Londres, World Microfilms Publications, 1983, bobine 3, p. 11 de la brochure. Pathé continue de pratiquer ainsi tout au long des années 1900. Voir par exemple “Cinématographes: appareils et accessoires” (1909), reproduit dans Motion Picture Equipment Catalogues, CD-ROM, The Projection Box, 2003, p. 11, on peut lire: “Coloris de bandes fournies par nos Clients (Net). 1.50 [fr. par mètre]”.
23 Traduction de: “Illustration was now considered an acceptable career for women. […] Learning to sketch was often an integral part of a middle-class woman’s education, along with music, embroidery, and other ‘feminine arts’ “, dans Ruth Copans, “Dream Blocks: American Women Illustrators of the Golden Age, 1890-1920”, dans Catherine J. Golden (dir.), Book Illustrated: Text, Image, and Culture, 1770-1930, New Castle (Del.), Oak Knoll Press, 2000, p. 243.
24 Jorge Dana, “Couleurs au pochoir: entretien avec Germaine Berger, coloriste chez Pathé”, Positif, n° 375-376, mai 1992, p. 126.
25 Séverine Wemaere et Gilles Duval (dir.), la Couleur retrouvée du Voyage dans la lune, Paris, Capprici, 2011, p. 129.
26 Voir Joshua Yumibe, Moving Color, op. cit., p. 43.
27 Traduction de “full of action and graceful movements […] the costumes show a combination of pearl, gray and white, with excellent color effects”, catalogue de Maguire & Baucus – qui font partie des premiers distributeurs des films d’Edison –, cité dans Charles Musser, Edison Motion Pictures, 1890-1900: An Annotated Filmography, Washington, D.C., Smithsonian Institution Press, 1997, p. 332.
28 Voir Joshua Yumibe, op. cit., pp. 55–56.
29 Traduction de: “With the life tint upon face, hands, arms and other features, and with vivid coloring of costume and accessory, the subjects stand out from the canvas like living men and women”. Voir la brochure publicitaire Raff & Gammon pour Edison, “The Vitascope” (mars 1896), dans Motion Picture Catalogs, A-021.
30À propos des féeries, voir Frank Kessler, “In the Realm of the Fairies: Early Cinema between Attraction and Narration” Iconics (Japan Society of Image Arts and Sciences), n° 5, 2000, pp. 7–26.
31 Thierry Lefebvre, “le Voyage dans la lune, film composite”, dans Jacques Malthête, Laurent Mannoni (dir.), Méliès, magie et cinéma, Paris, Paris Musées/Fondation Électricité de France, 2002, pp. 171–192.
32 Le film est annoncé pour la première fois en mars 1906 dans le supplément du catalogue Pathé destiné aux professionnels, puis dans les journaux au cours du mois qui suivit. Voir par exemple l’Avenir forain, n° 52, 15 avril 1906. La première projection publique recensée date du 15 mai 1906, lors de la Fête de Phono-Ciné-Gazette (salle du Trocadéro, à Paris). Voir Henri Bousquet, Catalogue Pathé des années 1896 à 1914, vol. 1, Bures-sur-Yvette, Henri Bousquet (1993-1996), p. 925. Des copies colorisées du film existent dans un grand nombre de centres d’archives et collections dont la George Eastman House, la Filmarchiv Austria, la Corrick Collection à la National Film and Sound Archive (Australie).
33 Jean-Jacques Grandville, Les Fleurs animées, 2 vol., Paris, Gabriel de Gonet, Éditeur, 1847. La bibliothèque-médiathèque de Nancy possède les dessins préparatoires de l’ouvrage, au crayon parfois rehaussé de couleurs; il est possible d’en consulter un certain nombre sur le site de l’institution: http://www.grandville.nancy.fr/fleurs.php [date de la dernière consultation: novembre 2013].
34 Voir les propos de Benjamin sur les Fleurs animées de Jean-Jacques Grandville en lien avec Bossfeldt dans son texte “Du nouveau sur les fleurs”, traduit par Christophe Jouanlanne et publié dans Walter Benjamin, Sur l’art et la photographie, Carré, coll. “Arts et esthétique”, 1997, pp. 69–73.
35 Traduction de “DAY 156 – once upon a time”, “DAY 123 – travelogue (silent film)”, “DAY 163 – an initial attempt at translation”.
36 Traduction de “DAY 303 – chromo pharmakon”.
37 Jean-Jacques Grandville, les Fleurs animées, vol. 1, op. cit., p. 243.”
(Yumibe, Joshua (2013): L’espace de la couleur comme espace de jeu dans la littérature enfantine, le cinéma des premiers temps et les fééries. In: 1895. Revue d’Histoire du Cinéma, 71, pp. 33–46, on pp. 37–46.) (in French)
There is evidence by this time of an apathy, if not an antipathy, towards the coloured film. The trial began with subjects in Edison’s Kinetoscope, 1891, before moving pictures were projected. With the era of the Screen, an indulgence in cosmetics naturally continued; and conspicuous in 1904 was a painted version of Melies’ Trip to the Sun. Short or long, educational or dramatic, such chromoscopic imitations appeared in the programmes of all succeeding years.
You may reason that artificial colours are not a fair test. I reason that all the other processes are artificial too. “Natural colours” is a convenient misnomer; autocolours would be a better term. When results are compared the advantage often lies with that reproduction we call artificial. At least the stencilled film continues to be countenanced by patrons as much as its rival.
The trials of the natural-colour processes need to be more discriminately reviewed. Wanting space, I must take notice of nothing until the introduction of Technicolour in 1920. From then onward this system shared the scenes – hence also the circulation and publicity – of most of America’s world-boosted super films, apart from its all-coloured investments, remembering especially the prestige of Fairbanks’ The Black Pirate. Then Al Jolson, singing about a rainbow round his shoulders, started the talkie boom that was to incur a new load of rainbows for all our shoulders in the form of talkie coloured attachments.
Why, then, do we still have films in black-and-white? Practicability? That question fails, in face of practice. It may be added that, twenty years back, topical reels were recorded and exhibited in auto-colours without departure from the normal time-table. The question of expensiveness likewise fails. First parasitically fostering itself for twelve years or so, then subsidised by the talkie gold mine, colour cinematography has enjoyed an economic advantage. Even that old alibi about “awaiting the supervision of an artist” holds good no longer; and never had it much to do with the question of colour’s popularity.
So the mystery grows and grows. Neither Hollywood nor Critical Opinion appears competent to clear it up, save to look around for their old friend “the technical solution” to help them out. This, too, when Critical Opinion, even more than those commercially concerned, has assured us that natural-colour photoplays have been “wondrous,” “exquisite,” “like old masters,” “like Nature herself.” Go back to The Black Pirate (1926); to Friese-Greene (1924); to The Glorious Adventure (Prizma, 1922); to Gaumont’s luxurious three-colour Chronochrome (1913); to Kinemacolour (1911). Read up the testimonials. If Critical Opinion can go into raptures about photochromy on the screen, much readier should the indiscriminate fans be to extol it.
Besides, it is human nature to tolerate imperfections in everything, until something better is put before us. So, if colour reproduction were a vital condition in the moving picture, mechanical crudities could not seriously stand in its way. After all, Nature is not very exacting, apart from the particularity of human complexions. Impossible to hit upon a green, a brown, or a yellow that does not represent some true appearance of a leaf. Then nearly everything the popular eye demands from colour, all its artificialness, all its extravagance – the sensational and the sentimental – can be supplied by quite a clumsy process. A venture like Kinemacolour, by its very evidence of practicability, profitableness and merit, is evidence of limitations not concerned with those factors. To-day also a market could be exploited through the enormous interest and attraction in coloured subjects. But – there is enormous interest and attraction in the movie dramas too. The markets must either be separated or be reconciled.
Colour is not missing in the movies. The real peculiarity is that our intelligence demurs at the presence of colour in a life-like representation, but never at the absence of it. The achievement of black-and-white drawings, photographs, and moving pictures ought to be regarded as a positive achievement. The pure photograph exerts an emotional influence so moving and so instinctive as to be unreached by any other form of picture. It registers some indefinable quality of the subject. It evokes a vivid recognition. More: from the box-office point of view, it is human.
To colour it is to transform it into the dead product of the hand. If it is a portrait of a face intimately known – and a film star’s face is intimately known – individuality and personality disappear. Vulgar people take a naïve pride in a coloured enlargement of someone dear to them; but for emotional purposes they are careful to keep a copy in black-and-white.
In so far as this refractoriness of colour disturbs the illusion of realness, it nullifies what is vital in our medium. This is, in a way, the most important part of the problem, but since it leads into a theme far out of our present path, I must be content to assume it is agreed upon, this need of some peculiar realness in the Image Play that is active in prompting a mental reciprocation of real experience.
Why colours seem unreal may be less a psychological than a visual problem. Not so much a pigmentary problem, either, because any colour reproduction – even paintings, even the best printing processes – have something false about them; and even an actual spectacle can be made to appear artificial in its hues.
Colour seems to be an entity in itself. It trespasses on the field of vision. It provides an abstract spectacle imposed over the spectacle concrete. Monochrome allows the eye to come closer, as it were, to concentrate more, to absorb more, to digest more. As infra-red rays, X-rays, and all the other rays afford our vision a means of deeper penetration, so black-and-white photography pierces a fog of light sensations.
The painter gets at the nude in defiance of prude. The sculptor, I contend, goes one better by stripping off the colour as well as the clothes. Colour brings visual indigestion.
These effects add to the troubles of pictorial composition. The eye can have no softer bearing than the tones of light and shade. Colour seeks rather to imprison the vision; and when there are casual, accidental colourings the field is strewn with obstacles, until the eye finally lands in a bunker. Painters are satisfied that they have turned these disadvantages to their own advantage. Lowbrows, failing such satisfaction, prefer paintings to be over-coloured, so that at least they can extract the sensation of colour as colour.
Circumstances of the cinema – which I shall not apologise for – inevitably widen the argument. A canvas, usually, is comfortably within one’s field of vision, and the light around you relieves the sight. The cinema screen necessarily fills the whole field of vision and annihilates consciousness of all other existence. This stresses an unnatural condensation of colour, just as there is an unnatural condensation of sound within the talkie frame. Free of colour, the black-and-white shadows on the screen join swifter the shadows of the hall. We can make a colour amalgam around the picture, but experience shows that the mind makes a much better settlement if left confronted with the ultimatum of one uncompromising boundary of sight.
Colour enthusiasts refer gleefully to the “pull” of gorgeous posters and magazine covers. When, however, you consider that these draw the eye to themselves amid a world of competing lights and forms, you get an idea of the force we are playing with in the cinema confines. Signal-lights also ought to warn us in more senses than one.
The chromotechnics of the screen cast me into a dungeon, with darkness and suffocation. Lost is the very essence of cinema, its space, and freedom, and light. Nothing like it on earth; although in the weird world underseas Technicolour seems singularly apt. Ignore the limitations of colour photography. Remember that in certain circumstances of vision, colours will always tend to denaturalise themselves. Above all, daylight in real existence, apart from its local decompositions into hue, prevails imponderously around every visible thing.
Screw up your eyes at a stage scene, squeeze out all the light you can spare. The result can be uncannily like a Technicoloured interior, even to the queer complexions. If you try the same trick on a Technicolour picture, the effect is an improvement! Colours become lighter and more distinguished, the flesh tints especially appearing more life-like.
Artificially coloured films are superior in their preservation of light. There is not that effect of depression, not to say oppression, to be found in natural colour photography. Moreover, the simpleness of the tints produces at least an elementary harmony, and often a finer delicacy. Our autocoloured reproductions should be severely diluted, by fair means or foul.
As there are physical, so there are mental conditions in actual life to modify the impertinence as well as the exuberance of colour. The want of such protection makes itself evident in a coloured reproduction of any kind. What should be latent, what is irrelevant, is all forced without mercy into one’s perception. A man can go a life-time without learning the true colours of a friend (symbolicalists, please note). He can fall in love with a girl with only the haziest notion whether her eyes are blue or grey. In a picture, these colours merely advertise themselves as abstract patterns. Often it is asserted that we cannot dream in colour – a fallacy, mind you, but not without a foundation of truth.
To remark further the cinema’s distinctions. Animated, the subject moves as well as the eye, and two motions have to correspond. An idiot wearing a scarlet scarf may walk from the foreground and far into the distance, and drag the helpless eye in chains behind him. Colours of stationary subjects (composing the bulk of the scene) mislead in another manner. They tend to make everything inert. Even perched on a moving object, colour appears to lack the agility of form. At times the forms seem to be struggling to move themselves beneath the weight of the colours. The dragging tendency is aggravated, I consider, by the known inclination of colours to jump into discrepant focal planes.
Nature demonstrates everywhere that colour is static in suggestion. Each creature loses an apparent ability of motion as it increases its colours. A peacock could never look swift. Such inaptitude is, indeed, utilised ecologically.* If this is so, it causes a serious retardation, not only in locomotion but in the finer mobility of facial expression. Ultimately, there is a similar influence on the tempo.
Brief, too, our changing scenes; and varying their scales. In the long run the all-coloured film can be more monotonous than the monotone film. It reveals yet more that real life does not permit an observation of colours commensurate with that lavished on us by the photochromatic drama. Hundreds upon hundreds of coloured records are thrown at us without remission, and the eye has no source of escape. No matter how different the subjects, the hues tend to become a stream of abstract, satiating sensations. It is like music trying to dispense with silence.
Undoubtedly there are technical improvements to anticipate, especially if we bear in mind that even objections against elaborateness of equipment and operation in the theatre are hardly valid in face of the talkie manœuvre. Observe, though, that colour has not yet provided in the box-office that irresistible lever that sound provided to make such an upheaval possible.
And that, first and finally, is what I want to impress. Claude Friese-Greene did once say that picture-goers would have to be educated to an appreciation of colour films. This was corroboration from a source where I least expected to find it; for, of all the colourites, inventors who have devoted everything they have to a practical solution of the problem are the ones I can most excuse for assuming that the public is clamouring for a solution.
If it had not been generally assumed, without leave to question, that picture-goers would turn away from black-and-white movies to coloured movies as greedily as a schoolboy turns away from bread to chocolate, inventors and investors would have saved a vast amount of effort and money. They would have tackled the problem forwards instead of backwards. They would have recognised that, even in the case of an article definitely desired by the public, application rather than cheapness, ingenuity or publicity, is the factor that finally counts in the market. Mechanically and commercially, as well as dramatically, coloured film has been badly mishandled.
“Awaiting the technical solution”! That is the stalemate position our colour was in ten, say twenty, years ago. Our first task is to make colour wanted in the cinema. Solve that problem, and the inventors will have in their hands the only weapon they really need for their advancement. Technical perfection comes last, not first. The moving picture proved it. The gramophone proved it. The radio proved it. The talkies proved it. I cannot think of any scope in popular entertainment that has not proved it.
* With certain natural colour processes still more fatigue might be supposed to result from the separation of the complementary images into successive frames. On the other hand it is a fact, I believe, that with colour the eye allows more latitude in a synthesis of animation. Incidentally this may point to a persistency in the colour sensation that is objectionable in other ways.”
(Elliott, Eric (1934): Wither colour? In: Cinema Quarterly, 2,3, Spring, pp. 161–165.)
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