Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
SUBTRACTIVE COLOUR PROCESS
The subtractive principle, which was first described by du Hauron and Cros in 1869, is used almost exclusively today in the photographic industry for making transparencies and reflection prints, and in the printing industry as well.
Du Hauron’s original process involved the preparation of separation negatives which were then exposed onto three sheets of gelatin containing ‘red, yellow, and blue’ colouring. After processing, the relief images of the gelatin tissues were superimposed onto each other, thus producing a colour transparency.
During the next 60 years numerous modifications in the chemistry, materials, and techniques of the du Hauron method culminated in the Trichrome Carbro system, which is still considered one of the best methods for producing high quality reflection colour prints. The process requires a high degree of skill, and since the rate of production is very low, it no longer competes with more recently developed systems.
The obsolete wash-off-relief process and the current Kodak Dye Transfer Process are somewhat similar to Carbro.
The Dye Transfer system involves the exposure of separation negatives, or a colour negative, onto matrix films. The latter are developed in a tanning developer which hardens the exposed portions of the film. Washing in hot water removes the soluble gelatin from the unexposed areas and leaves a positive relief image or matrix. The matrices are soaked in cyan, magenta, and yellow dyes, and successively registered in contact with a mordanted sheet of white paper. Transfer of the dyes from the matrices to the paper produces a colour print.
The subtractive colour system did not become really practical until manufacturing technology was advanced sufficiently to permit films and papers to be manufactured with three sensitive layers coated on a single sheet. Not only was it necessary that the layers be uniformly applied, buy chemistry for the processing of these materials had to be worked out so that the resulting dye images had the proper hue and saturation, and produced a good neutral gray scale from white to black.
The two general methods for producing full colour transparencies on multilayer films are the dye bleaching process and the dye forming process. The former can be illustrated by the Gaspar Process, in which selected dyes are incorporated in the film.
During the development of the exposed film, the amount of silver formed is in proportion to the amount of light which reached the film, and the dyes are subsequently removed or bleached near each grain of silver.
Thus, where the greatest amount of exposure occurred, there is a maximum of silver and a minimum of dye, and where no exposure occurred, there is only dye. Removal of the silver, which formed the negative image, leaves only the positive dye image.
In part five of our series, we will explore more types of the subtractive colour process.
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