Subtractive 3 color: Monopack, stripping, still photography
John H. Smith
“To offset the possible effects of poor contact between the various members of the tripack, J. H. Smith coated the emulsions directly one on top of the other, but with an insulating layer of collodion between them. In this manner there was obtained a very compact pack, since the collodion layers could be made as thin as desired. Filter dyes could be placed in this insulating layer, so that there was removed the possible interaction between filter dye and sensitizer. The presence of the intermediate dyed collodion layers made it possible to strip the component emulsions apart, and process them individually.
This scheme is disclosed in United States patents 781469 and 886883; German patents 165544 and 185888; and English patent 19940/04. The idea crops up again in a disclosure by S. Schapavoloff (Eng. P. 205807) and more
recently by W. T. Tarbin (U.S.P. 1871479).”
(Friedman, Joseph Solomon (1945): History of Color Photography. Boston: The American Photographic Publishing Company, p. 86.)
Original Technical Papers and Primary Sources
U.S.P. 781469, Sept. 13,1904 and 886883, May 15, 1905 , D.R.P. 165544 and 185888; and E.P. 19940/04
Friedman, Joseph Solomon (1945): History of Color Photography. Boston: The American Photographic Publishing Company, pp. 94-107.
The most modern procedure for the elimination of complicated camera devices to achieve color reproduction lies in the use of monopacks. These substances are multi-layered films in which each layer contains an emulsion sensitized for a single primary color. The different layers are not separable, but must be treated as a single unit. This means that a new technique must be used in order that the images in the different emulsion layers be separated from each other. The monopack film is merely an integral tripack.
The multiple coating of emulsion layers one on top of another has been known for a considerable length of time. H. Kuhn (Eng. P. 6921/91) disclosed such a system in 1891, although he made no attempt to utilize it for the purposes of color photography. A mount is made waterproof, and is then coated with an emulsion of barium sulphate in gelatin. On this is coated a series of sensitive gelatin or collodion silver bromide emulsions (print-out), mixed with aniline colors. One emulsion is coated on top of the other after the first has dried. Each layer is dyed a different color.
From this description it is not such a stroke of genius to make the system applicable to color photography. The dyes in each of the layers can be so chosen that they act as filters or color sensitizers. Therefore it is possible to coat one emulsion in its native state so that it will be blue-sensitive only. A yellow dye could be incorporated in this layer. The next emulsion layer is dyed with a green sensitizer. The yellow dye in the front layer can be used to prevent any blue light from reaching this or the third layer. This last is dyed with a red sensitizing dye. In 1891 the powerful carbocyanine and cyanine sensitizers were not known, so that the designation of aniline dyes may be taken to mean sensitizing as well as filter dyes. The specification does not disclose why the dyes were put into the emulsion, nor why the multilayered paper was needed. But it does disclose the preparation of a monopack.
The first specific disclosure of the use of a monopack for the purposes of color reproduction came from K. Schinzel.1 The plate is coated with three silver-bromide emulsions colored complementary to their spectral sensitivities. The individual layers must be dyed with colors that do not bleed, and which are insoluble in water. The top layer is colored yellow and is sensitive to the blue. The second layer, colored cyan, is sensitized for the red. The bottom layer is colored magenta and is sensitized for the green. This is as clear and as definite a disclosure of a multilayered monopack for color reproduction as it is possible to make.
The pack is exposed, developed, and fixed in the ordinary manner. In this condition each layer contains a silver image of a single primary, imbedded in a layer of a complementary-colored gelatin. Upon treatment with two per cent peroxide, it is claimed that the dye in the immediate vicinity of the silver image will be bleached, and in an amount directly proportional to the image density. Therefore there remains a dye image in each layer which is the negative of the silver image. We will leave a critical discussion of the processing technique to a later chapter where we shall discuss the dye-bleach process of color reproduction. Here it is sufficient to point out that in 1905 there was disclosed completely the use of a Kuhn monopack for color reproduction purposes, thus anticipating Mannes and Godowsky, Troland, Gaspar, and a host of other inventors, who have made this system the best answer so far (1943) to the problem of original exposure.
Needless to say neither the state of the photographic art, nor that of the dye industry was sufficiently advanced to allow the Schinzel disclosure to be put to practical use. It remained for Dr. Bela Gaspar to accomplish this result. But the interest which it aroused was immediate, and the discussion that followed amplified the procedure. Neuhauss2 pointed out that the use of peroxide was not advisable as this chemical would not discriminate very carefully between the dye lying adjacent to a silver deposit, and the dye that lay in non-image portions. Schinzel admitted the fallacy, and suggested the admixture of the emulsion with colorless substances which would afterward give rise to color formation.3
Another suggested use of a monopack for color reproduction came in 1910, from F. Sforza.4 In this disclosure, the three layers were to be dyed with the primary colors that were mordanted to the gelatin. The dyes should have the added property of being catalytically destroyed in the presence of the silver. Somewhat similar ideas were expressed by R. Luther.5
From this, it is very evident that monopacks were well established by 1924 when Mannes and Godowsky were issued their first patent (U.S.P. 1516824). In this early patent there is disclosed a general two-layered monopack wherein the film base was first coated with a fast red-sensitive emulsion, then topped with a slower orthochromatic emulsion that contained a yellow dye dispersed throughout its depth. This prevented the blue rays from affecting the red-sensitive layer. The green rays would act only upon the top emulsion layer, since the bottom layer was not sensitive to the green. A two-color separation was achieved. If the two emulsions were correctly balanced, a single exposure yielded two equivalent latent images, one in each of the layers. By development the latent images were converted into metallic silver. The pack was then fixed and washed. Treatment with ferricyanide converted the silver into reducible silver salts. A further treatment with a developer compounded to diffuse very slowly into the depth of the emulsion, made it possible to confine the developer action to the upper layer only. This achieved the separation of the two images, for one was now in the form of a metallic silver image that could be chemically or dye toned to any desired color, while the other was in the form of a silver salt which could be treated independently to form a different color. A slight variation of this was contained in a later disclosure by the same men (U.S.P. 1659148).
At about the same time a large number of other inventors entered the field of monopack photography. Since the multiple coating of emulsion layers offered very little chance for invention, most of the disclosures differed among themselves only by the procedures which were adopted to separate the images in the different layers. J. F. Leventhal (U.S.P. 1697194) treated the pack, after development and fixation, with a chemical that retarded the action of a bleach. In that manner he was able to bleach the image in one layer without affecting the other. This left the film with one silver and one silver-salt image, which could be differentially dye or chemically toned.
The next advance came in 1931 in a series of patents issued to Dr. L. T. Troland, then director of research of the Technicolor Motion Picture Corporation (U.S.P. 1808584, 1928709, and 1993576, reissue 18680; Eng. P. 370908 and 382320). These disclosed several methods for the preparation of the monopack, several uses for the material, and several methods whereby it became possible to separate the images. There is no novelty in the obvious coating methods for the formation of the monopack, so we shall discuss only the novel forms he disclosed. Dr. Troland recognized that it is very difficult to separate the images in a three-layered monopack, so he proposed to use a two-layered pack sensitized to yield the blue and the red separations. This monopack he then used as the front element of a bipack, the rear component of which was a green-sensitive emulsion. This is merely a generalization of the Ives bipack disclosure where a screen plate is used as the front element.
Two methods were described for the formation of the monopack, which forms the front element. One is the obvious method previously disclosed by Kuhn, Schinzel, Sforza, etc., which consisted in physically coating properly sensitized emulsions, one on top of the other. There is very little to discuss here except that Dr. Troland preferred to have a separate filter layer, dyed yellow, between the two emulsion layers. The second method was to treat a color-blind, blue-sensitive plate or film, with a sensitizing solution under the conditions of controlled diffusion. This can be done quite easily if the sensitizing bath be compounded with a high alcoholic content. Under these conditions the colloidally dispersed sensitizing dye (pinacyanol) does not penetrate deeply into the depth of the emulsion. After drying to achieve thorough sensitization, the plate is bathed in a water solution of a yellow dye. This penetrates more deeply than the sensitizing dye. By this method the emulsion thickness of an ordinary negative material is divided into a lower blue-sensitive zone, and an upper red-sensitive one. The yellow dye, dispersed much more deeply than the sensitizer, prevents the blue rays from penetrating into the zone of red sensitivity, hence there is no color contamination. This bathing method yields a product which is quite unstable, but which otherwise is altogether suitable.
Dr. Troland was not the first person to disclose that stratification can be achieved by bathing the emulsion with the colloidal sensitizers. Indeed it was a well known and established fact that unless extra precautions were taken when sensitizing an emulsion by bathing, only a superficial layer of the coating would be affected. But a very clear statement of this phenomenon, and a clear disclosure as to the effect that an increase in alcohol concentration would have, is contained in a sensitizing patent by G. Selle (Eng. P. 12516/99) who wrote: “To achieve my object I use a sensitizing bath, for instance, a solution of 0.002 per cent of cyanine blue and erythrosin in a fluid composed of 60 per cent water and 40 per cent alochol. By this means the red dye (erythrosin), which is more soluble in the water, is carried into the film, while the blue dye (cyanine blue), which is more soluble in alcohol, remains substantially on the surface of the plate. But other sensitizers having the same properties or results may be used.” Pinacyanol or pinacyanol blue are red-sensitizing dyes that have the solubility properties of cyanine blue. Troland, therefore, did not discover the art of surface sensitization or stratification. But he appears to have been the first one to realize that this stratified emulsion could replace the screen plate in an Ives’ bipack.
The Troland patent was issued in 1931, but it was applied for in 1921. For this reason Dr. Troland was given priority to Mannes and Godowsky. Since the Eastman Kodak Company controls both these disclosures, the question of priority is merely academic and economic. The Troland patent is valid to 1948, while the Mannes and Godowsky patent expired in 1941.
Independent of the above, Emil Wolff-Heide disclosed a similar scheme for surface stratification (Eng. P. 340278) . He based his work upon the well known fact that pinacyanol was a colloidally dispersed dye when dissolved in a water-alcohol mixture, and that gelatin is an excellent semi-permeable membrane for colloids. Von Hübl (“Die Orthochromatische Photographie,” (1920) p. 69) has the following to say in this respect: “Such solutions are colloidal and tend to flocculate; if one filters it, the dye remains on the filter paper and the solution comes through colorless. . . . (During sensitization) the dye separates out for the most part upon the surface of the film, and only a colorless solution, which does not sensitize well, penetrates into the innermost parts of the film.” Mr. Wolff-Heide found it sufficient to bathe a film in a pinacyanol solution, together with filter yellow, rose bengal, and pinachrome. This is but slightly different from the Troland disclosure. Since his patent was issued several years prior to that of Dr. Troland, he could not have been aware of the information contained in the latter. H. von Fraunhofer patented essentially the same (U.S.P. 2030903 and 2030904). In the second patent it is suggested that the sensitizing solution be placed upon the subbed film-base prior to the coating. The sensitization under those conditions would take place at the bottom of the layer, so that the zone of red sensitivity would be that part of the emulsion layer that lies adjacent to the base. The two silver images were separated by treatment with a uranium toning solution. The heavy uranium ion is ideally suited for use in controlled diffusion processes. The time of treatment is so arranged that only the upper silver image becomes toned. The lower image could be converted into a blue by any other method that would not affect the uranium-toned image.
A more detailed description of a sensitizing technique, which will yield a stratified emulsion, is contained in United States patent 2047022 issued to J. S. Friedman, and assigned to the Omnichrome Corporation. The sensitizing bath is concocted as follows:
This bath contains 40 per cent alcohol. The time of sensitization is approximately two minutes at a temperature that is as close to 40 to 45 F as is possible. It is also permissible to pre-wet the emulsion by immersion in a water solution made slightly alkaline by the addition of borax. This will swell the gelatin to a considerable extent and it will therefore give a greater degree of control to the diffusion of the sensitizing agents into the emulsion. A somewhat more effective reaction between silver halide and dye is also obtained when the emulsion is thus pre-wetted.
Another method whereby a stratified emulsion is obtained, is disclosed in a patent issued to J. S. Friedman and A. Bruck (U.S.P. 2175836) and assigned to Color Processes Inc. Here advantage is taken of the fact that an alcoholic solution of a dye will not penetrate into the depth of the emulsion, but will leave a layer of dye deposited uniformly over the surface. Ives utilized this phenomenon to deposit a filter layer of dye upon the surface of an emulsion. In 1917 Koenig6 recommended this as a fool-proof method for sensitization. But he did not recognize that with a slight modification, the procedure could be used to give color separations.
A blind emulsion is first bathed in an alcoholic solution of the red sensitizer containing 0.030 gram of dye to the liter of alcohol. Time or temperature plays no part whatsoever in this step, since all that happens is that a thin layer of the red sensitizer is deposited upon the surface. Only a sufficient quantity of dye is deposited to react with the upper half of the emulsion layer.
After drying, the plate with the surface coating of dye is immersed in a solution that contains 20 per cent alcohol and 0.015 per cent auramine, previously purified by recrystallization from alcohol. As the water-alcohol mixture becomes absorbed by the gelatin, it dissolves the sensitizing dye, and carries it to the silver halide grains. Sensitization takes place until all the dye has been used up. Since only sufficient dye is deposited to react with the upper half of the emulsion layer, stratification of the emulsion results. The yellow dye prevents the blue rays from reaching the zone of red sensitivity if the exposure is made through the base. The Friedman monopacks were designed to be used as the front elements of a bipack. The rear component of the combination, being an orthochromatic emulsion, yields an image of the green primary.
In order to process these monopacks, due regard must be given to the fact that the developer will start its action first upon the latent image present in the upper portion of the emulsion, and that by the time the developer reaches the lower layer, it is considerably weaker. It is desired, if the two images are to be developed in a single solution, to compound a developer that diffuses very rapidly into the gelatin. This can be done if the carbonate content or the alkalinity is very high. But this has the drawback that considerable fog will be developed. Dr. Troland recommended a developer of this type, compounded by adding 10 parts of carbonate to one part of hydroquinone. Another drawback is that such a developer is very contrasty, and therefore hardly suitable for accurate photographic reproduction. Approximately the same result, but with much more accurate control of the contrast scale, could be obtained if the development were carried out in two stages. First the pack could be bathed for ten or fifteen minutes in a solution that contains all the ingredients of the developer, with the exception of carbonate, thus:
In this solution the development action will just start in approximately ten or fifteen minutes. This indicates that this length of time will be sufficient to bring about complete saturation of the emulsion with developer solution, and, more important, to allow adsorption equilibrium to be established between latent image and developing ions. From this solution, and with no intermediate wash, the pack is treated with a 10 per cent solution of sodium carbonate, containing one gram of bromide per liter. Development will be complete in approximately three to five minutes. Because the carbonate is so concentrated, there will be no appreciable diminution in its strength on its passage through the upper image layer.
It is possible to take advantage of the fact that two separate layers are to be developed. Here, again, a two-stage development is in order, but this time each stage utilizes a solution capable of yielding a developed image. The first stage will use what is known as a surface developer, one of the fine-grain low-potential developing solutions that is achieving so much current popularity. One such developer could be concocted as follows:
The high sulphite and low alkalinity retards the diffusion of the developer to the lower levels. This can be further improved by the addition of 200 parts of desiccated sodium sulphate, or 15 parts of chrome alum. If the alum is used, it would be necessary to add from five to fifteen parts of neutral sodium citrate to prevent the precipitation of chromium hydroxide.
After the plate has been developed sufficiently in this solution, usually about five or ten minutes, it is washed for ten or twenty minutes in running water to remove all developer and developer oxidation products. It is then developed in a developer with a high appearance time (low Watkins factor), a high diffusion constant, and potassium iodide. One such developer would be
This is a typical hydroquinone developer which has a very high appearance time. During this period the iodide reacts with the silver bromide in the upper layer, and converts it to undevelopable silver iodide. The main reaction is thus forced to take place below in the bottom layer. It may be advisable to interpose an alcoholic iodide bath between the two development stages. This will convert the silver halides in the upper strata to silver iodide. A thorough wash removes the excess iodide, after which the lower layer can be developed at will in a normally compounded developer.
The Friedman disclosures do not utilize a fixation reaction whereby the unreduced silver halides become dissolved out by the action of hypo. Instead, after the development is complete, the pack is again washed thoroughly, then subjected to the action of one per cent solution of potassium iodide, for a period of fifteen to thirty minutes. After a thorough wash, the pack is dried. In this condition the pack contains two black-and-white silver images imbedded in a yellowish-white, opaque, silver-iodide layer. When viewed from the emulsion side, the only one of the two images that will be seen is the image of the red densities. The same is true if the pack be viewed from the base side, but this time it will be the blue densities that will be seen. To separate the two images, it is sufficient to copy each side of the pack upon a different plate or film. In making these copies or “take-offs” it is desirable to use an orthochromatic process emulsion, and a K3 filter to offset the yellow stain of the silver iodide.
The reflection copies of the images on the pack will be positives. In some processes positives are required from which to print on to the coloring material. But in most cases it is desirable to have the separations in the form of negatives. It is possible, of course, to reverse the copies. This can be done quite easily. After the copy has been exposed it is developed in a developer to which is added ten or fifteen grams of hypo per liter. A thorough wash removes excess developer. Treatment with acid dichromate or acid permanganate etches out the silver image. By this treatment the silver is oxidized to ionic state, in which condition it has a great affinity for gelatin. The silver-gelatin complex must be destroyed and the silver ions removed before the next step is taken, otherwise this will be reduced to metallic silver again in the second development. A rinse in weak ammonia, or a bath of one per cent sodium sulphite usually accomplishes this. The washed plate is next exposed to light, or treated with thiourea or methylene-blue solutions which act exactly like light. It is then developed in a second developer that does not contain hypo. Or, the plate after the treatment with sulphite can be treated with sodium sulphide, sodium hydrosulphite or any other reagent which will convert it to a black.
The original plate can be processed so that it yields a positive rather than a negative. To this end it is possible, after the development of the two latent images is completed, to treat the plate with a strong sodium sulphide solution. This will convert the entire plate into a deep black, in which there is imbedded two silver images. The oxidation of silver to an insoluble salt proceeds much more rapidly, and at a considerably lower potential, than the oxidation of silver sulphide. It is therefore possible to find a mild oxidizing agent which will attack the silver and convert it into a white insoluble salt, and which will leave the silver sulphide intact. Mercuric chloride, cupric chloride, and ferricyanide solutions are substances that have this property. By this treatment there will be formed a white negative image imbedded in a black background, and this is identical with the presence of a black positive image imbedded in a white background.
Another possible routine would take advantage of the mordant power of silver iodide for basic dyes. After the monopack has been completely processed so that it contains two silver images imbedded in a background of silver iodide, the plate can be treated with a black basic dye, or with a mixture of methylene blue, rhodamine B, and auramine. The silver iodide will absorb the dyes deeply, especially if the iodide content of the final bath be increased to five or ten per cent. This treatment is to be followed by conversion of the silver to a white insoluble salt by treatment with mercuric, cupric, or ferricyanide ions, in the presence of halides or other cations whose silver salts are white and insoluble.
A further possibility would lie in the fact that soft gelatin is a strong absorber of certain dyes. After development of the monopack it can be fixed in alum-free hypo, thoroughly washed, and then treated with a bromoil bleach. This will harden the gelatin immediately surrounding the image, in which condition it will no longer absorb certain dyes like platinum black, or the pinatype dyes, while the rest of the gelatin will be very receptive of them. A uranium toning solution will affect the image gelatin in a similar fashion. Here again there will result a white negative image imbedded in a black background, or a black positive imbedded in a white background.
Dr. Bela Gaspar has disclosed a bipack which yields three color separations. The front element (Eng. P. 448161 and 450685) of this bipack was a monopack formed by coating two emulsion layers one on top of the other. What makes this pack differ from the one disclosed by Troland is that the layers contain dyes that are complementary to their sensitivity, and which by treatment with special chemicals become decolorized in direct proportion to the intensity of silver deposit. The preparation and chemistry of these solutions will be discussed in a later chapter, devoted to the Gaspar disclosures. The I.G. company also disclosed a bipack (Fr. P. 836173). The front element is a two-layered monopack that contains substances that react with the oxidation products of the developer to form insoluble dyes. These substances are called “couplers,” and the process of development in which they are used is called “color-development.” Because of the importance of this technique it will be discussed in a later chapter devoted exclusively to it. The tendency has been to utilize color-development almost exclusively with monopacks, so much so that the two appear to be bound in holy wedlock. This is a marriage of convenience, and it is not at all essential.
As was indicated above, monopacks are probably the most popular of all systems of photography at the present time, and this field is receiving considerable attention from all serious workers. Almost every film manufacturer has taken out patent upon patent to protect some supposedly novel feature in assembly or processing technique. But, truth to tell, there is very little real novelty in most of the disclosures. Gaspar (Eng. P. 421534; Fr. P. 753061) follows the lead of the I.G. company and adds substances to the emulsion that can be converted into colors. The duPont company would process the monopack film with color developers (U.S.P. 2133937 and 2140540; Eng. P. 497463 and 497698). In United States patent 2166617 Sease and Weber, of that company, disclose a scheme to treat the upper layer of the pack with concentrated solution of hypo (50 to 90 per cent). In English patent 505861, assigned to the duPont company, there is disclosed the following technique: A record is made of the combined images in the pack. These are then bleached to silver salts, and only the upper image is redeveloped. From these, the other image can be obtained by a system of masking.
J. H. Reindorp (U.S.P. 2153698; Eng. P. 465090, 467005 and 467380) discloses another method for processing each of the two layers of a monopack, individually. The method utilizes a controlled diffusion of an iodide solution to convert the upper emulsion layer or stratum into silver iodide. It is a known fact that when a silver chloride grain is converted into bromide or iodide, it does not lose its latent image. Therefore, the latent image existing in the upper stratum is not destroyed, although it will not be developable by means of the ordinary developer, but will require solutions with extremely high potentials. And upon this fact depends the entire procedure.
After exposure, the monopack is bathed in Renwick’s solution, which is as follows:
The time of treatment is adjusted so that the action is limited to the upper stratum only. The plate is washed thoroughly, then developed in
To this is added a half gram of para-nitro-benzyl-cyanide dissolved in 50 cc alcohol. In this solution the latent image in the non-iodized layer (the lower one) will be converted into silver. But at the same time there will be deposited in situ with the silver, and in an amount directly proportional to it, a magenta dye which is insoluble in alkaline media, but very unstable in acid.
The upper iodized latent image can now be developed by means of alkaline amidol, or by the following:
The unreduced silver halides are removed by treatment with hypo. The upper silver image is bleached with 2 per cent ferricyanide, 2 per cent ammonia, and sufficient alcohol and glycerin to prevent the penetration of the bleach to the lower depths of the emulsion. The newly formed silver salt is next developed with a solution that deposits a yellow dye beside the silver image. At this point the monopack film contains two silver and two dye images, the magenta lying in the bottom, and the yellow in the top layer. The silver is removed by the action of Farmer’s reducer made slightly alkaline to prevent any action on the dyes. This leaves only the pure dye images. These can be separated by filter action, since the magenta will absorb green light, and the yellow will absorb blue light. The yellow developer is formed by adding solution B to A, then adding 10 cc of a 2 1/2 per cent solution of alpha naphthol in alcohol, to every 100 cc of the mixture
This patent has been assigned to the True Colour Film Co., an organization which has done considerable work in the field of monopacks and color development. Other patents issued to them which are concerned with monopacks and their processing are United States patents 2137785 and 2163325; English patents 453674, 465765, 480251, 480287, 480291, 481274, 483020, 483035, 498749, 498762 and 505099.
Cinécolor is another firm actively engaged in monopack procedure. A film containing two images is treated with a mordanting solution whose action is limited to the upper image only (U.S.P. 2009689; Eng. P. 447412 and 459234). The action of the bleach is arrested by treatment with bisulphite. In this manner the two images are separated, for one is now in the form of a mordant image which can be dyed to any desired color, while the other (lower) is still in the form of a silver image which can be toned to yield the complementary color independently of the upper one. In another disclosure (Eng. P. 473993) provision is specifically made for the reflection printing of the two images. To this end there is interposed between the two emulsions a layer containing a yellow dye and some other chemicals that can later be treated to form an opaque, but highly reflective, substance. This intermediate layer can contain zinc hydroxide. After development, fixation, and washing, the film is treated with a solution containing sodium sulphide. This will convert the zinc hydroxide into zinc sulphide, a white substance that is a good reflector. Or the intermediate layer can be made to contain silver iodide. This will be sufficiently opaque to act as a reflector.
Comstock (U.S.P. 1956274) would process the monopack with a developer that yields a reflective image. Such a developer can be concocted as follows:
The developed silver image is white. After fixation, the two images can be printed by reflection from the opposite side of the film.
Following the acquisition by the Eastman Kodak Company of the Mannes and Godowsky disclosures, a whole series of patents dealing with monopacks and methods of processing them, were issued various members of the staff. Seymour (U.S.P. 1897866) would sensitize the different layers with dyes that are not destroyed by the action of acid dichromate. After exposure and development, the silver is removed by the action of acid dichromate. The sensitivity of the remaining silver halide is restored by a bath in 0.5 per cent sodium sulphite. At this point the film is exposed to red light. This will affect only the grains that are sensitive to the red. Development is accomplished by means of a solution that deposits a dye together with the silver. The remaining silver salts are then exposed to white light and developed in a solution that yields a color different from the first.
In another disclosure (U.S.P. 1900870) the monopack containing two layers, one of which is sensitized to the red with a stable dye, is exposed and then developed for seven minutes in D-16, which is concocted as follows:
After a ten-minute wash to remove the excess developer, the plate is exposed to red light and developed in the following:
In this bath the exposed silver halide grains will be reduced to silver and at the same time there will be deposited in situ and in exact proportion to the silver, a cyan dye. Hence there will be formed in the layer containing the red-density record, a cyan positive image of this record.
The other layer is then exposed to white light, and developed in a magenta dye-coupling developer made by adding solution B to A, and diluting to one liter
The silver can be removed by treatment with Farmer’s reducer, leaving pure dye images that can easily be separated by niters.
Capstaff (U.S.P. 1954346) simplified this treatment. The monopack, one of whose layers is sensitized by means of a dye that is stable to acid dichromate, is exposed, developed, washed, and then treated with acid dichromate to remove the silver images. It is next exposed to red light (if the stable dye is the red sensitizer) and developed with a cyan dye-coupling developer, such as the one given above. After this development, there is present a silver-plus-cyan dye image in one layer, and a silver halide image in the other. The silver halide image can be treated in a number of ways to convert it to a color different from that of the first, and in that way achieve an optical separation of the two images.
Burwell (U.S.P. 1966330) varied the technique somewhat. The monopack film is developed after exposure, in a cyan color-developer such as the one disclosed in the Seymour patent. Fixation in neutral hypo, is followed by a thorough wash. At this stage the monopack contains two silver-plus-cyan dye images in the two layers. The image in the upper layer is destroyed by treatment with acid-ferricyanide-bromide, which converts the silver to silver bromide, and destroys the dye. It is then redeveloped in a magenta color-developer, or toned red by means of a uranium toner.
These ideas bring to mind another possible method for separating the. Two images in a two-layered monopack. After exposure and development, the excess developer could be washed out very carefully, and the pack then bathed in a neutral solution of one per cent auramine. An exposure with blue light will not penetrate very far into the depth of the dyed emulsion, so that it will be possible to expose but one layer at any one time. If the first exposure will be upon the side carrying the red-sensitive layer, the development can be made with a cyan developer. Now again we have a cyan plus metallic silver positive image in one layer, and a bromide image in the other that can be converted into a magenta by any one of many methods.
It is not even necessary to use color developers at this point, for after a single layer has been exposed, there exist already two distinct and differentiable images, one a latent and developable silver halide image, and the other, an undevelopable silver halide image. The development can proceed in a normal manner so that after this treatment there will be formed two images: one a silver positive image, the other a silver halide positive image. The last can be converted, by treatment with strong iodide or thiourea solution (cf. chapter on Dye Toning), into a mordant capable of absorbing basic dyes. The silver image can then be converted into an iron blue or copper, uranium, or nickel-dimethyl-glyoxime red.
Murray and Spencer in a series of disclosures also describe a monopack and a system for its processing to yield differentiable images. These disclosures (U.S.P. 2140847 and 2163325; Eng. P. 440422, 470074 and 489299) are not sufficiently novel to merit critical discussion.
A novel type of monopack is disclosed in a series of patents issued to D. K. Allison and L. M. Dieterich and assigned to Detracolor (U.S.P. 2005790, 2014606, 2034220, 2034230, 2036994, 2151065 and 2161735; Eng. P. 462232). The different layers of the monopack film contain the leuco bases of the more stable diphenyl methane dyes. These bases are unaffected by treatment with developers and hypo solutions, and they have no effect upon the sensitivity of the emulsions. The exposed, developed, and fixed monopack is finally treated with a bleach such as
This oxidizes the leuco bases to the dyes and at the same time converts the silver image to a silver-copper complex that acts as a mordant. Bromine or chlorine water can also be used. It appears that the chemistry involved in the bleach solution may be a little weak. Unless a solution which contains both copper and ferricyanide ions is heavily loaded with citrate, oxalate or other organic polycarboxy or polyhydroxy acids, a precipitate of cupric ferricyanide is formed. But this is a minor defect in the disclosure, since it is possible to remedy it by the inclusion of the proper stabilizing ingredients.
1 Schinzel, K.: Photographisches Wochenblatt, 1905; British Journal of Photography, Vol. 52 (1905) p. 608; Aust. P. 42478/08.
2 Neuhauss: Photographische Rundschau, Vol. 19 (1905), p. 239.
3 Schinzel, K.: Chemiker-Zeitung, Vol. 32 (1908) p. 665; British Journal of Photography., Vol. 55 (1908) Col. Supp., Vol. 2, p. 61.
4 Sforza, F.: La Photographic des Couleurs, Vol. 5 (1910) p. 209; British Journal of Photography, Vol. 57 (1910), Col. Supp., Vol. 3, p. 83.
5 Luther, R: Photographische Rundschau, Vol. 25 (1911) p. 1.
6 Koenig: Photographische Korrespondenz, Vol. 55 (1918), p. 22; Photographische Rundschau, Vol. 54 (1917), p. 257.”
(Friedman, Joseph Solomon (1945): History of Color Photography. Boston: The American Photographic Publishing Company, pp. 94–107.)