Heliochromy color separation

“Carbon printing was originally a monochrome process. It was invented and patented by Alphonse Poitevin (1819–82) in 1855 and was the first practical printing method with pigments. After Poitevin, the carbon process was patented extensively by many different people, and the same idea or modification of the process was claimed time and time again (Friedman 1944: 445). However, the first three-color carbon printing method, called Heliochromy, was invented by Louis Ducos du Hauron, who showed successful prints at the meeting of the Société française de photographie on May 7, 1869. Heliochromy combined the principles of Young’s three-color theory and those of carbon transfer printing introduced in 1860 by Adolphe Fargier and modified in 1864 by Sir Joseph Wilson Swan (1828–1914). In spite of this early invention, it was not until the turn of the twentieth century and the manufacture of panchromatic plates sensitive to all visible colors that tricolor pigment processes became practical and commercially available.”

(Pénichon, Sylvie (2013): Twentieth Century Colour Photographs. The Complete Guide to Processes, Identification & Preservation. London, Los Angeles: Thames & Hudson, on p. 82 and on p. 84.)