Chromogenic

Chromogenic refers to photographic processes that work by forming a conventional silver image and then replacing it with a dye image. Most films and papers used for color photography today are chromogenic.

Chromogenic film or paper contains one or many layers of silver halide (AgX) emulsion, along with dye couplers that, in combination with processing chemistry, form visible dyes. In processing, the silver image of each layer is first developed. In concert with the dye couplers in each layer, the process subsequently forms dyes only in those areas where silver is present.

In full-color chromogenic materials, multiple layers of emulsion are sensitized to different wavelengths of light. Three layers are usually present, generally sensitive to red, green, and blue colored light. Cyan-colored dye is formed on the red-sensitive layer, magenta-colored dye is formed on the green-sensitive layer, and yellow-colored dye is formed on the blue-sensitive layer, following generally the CMY color model.

Some chromogenic black-and-white negative films also exist, mainly to exploit the wide availability of C-41 processing. These films have softer grain and less contrast than traditional silver halide films. In these films, a single emulsion layer has panchromatic sensitivity. The dye image is typically slightly blue because of the choice of dye couplers, and this tends to produce a dark sepia tone when printed on full-color chromogenic paper. Examples of black-and-white chromogenic negative films are Ilford XP2 Super and Kodak BW400CN.

It is also possible to develop a standard black and white silver gelatine film or print using specially formulated chromogenic developers so that a colored dye image only remains after processing, the silver image being totally or partially bleached away during processing.

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