C-41 process

C-41 is a chromogenic color print film developing process introduced by Kodak in 1972, superseding the C-22 process. C-41, also known as CN-16 by Fuji, CNK-4 by Konica, and AP-70 by AGFA, is the most popular film process in use, with most photofinishing labs devoting at least one machine to this development process.

Processed C-41 negatives, as with all color films, consist of an image formed of dye. Due to the long-term instability of dyes, C-41 negatives can fade or color-shift over time. This was a significant problem with early films; whether the newer films are archival or not is a subject of some debate.

C-41 film consists of an acetate or polyester film base, onto which multiple emulsions are coated. Each layer is only sensitive to a certain color of visible light. In the classic illustrative example, there are three emulsions: one is red sensitive, another is green sensitive, and the top is blue-sensitive. Beneath the blue layer is a yellow filter, composed of dyes or colloidal silver. All silver-based photographic emulsions have some sensitivity to blue light, regardless of what other colors they may be sensitized for. This filter layer serves to remove the blue light, which would expose the layers beneath it. Beneath the blue-sensitive layer and the yellow filter are the green and red sensitive layers.

The illustrative example outlined above differs from the design of actual film, in respect to the number of layers. Almost all C-41 films contain multiple layers sensitive to each color. Each of these layers have different speed and contrast characteristics, allowing the film to be correctly exposed over a wider range of lighting conditions.

In addition to multiple emulsion layers, real films have other layers that are not sensitive to light. Some films are top-coated with UV blocking layers or anti-scratch coatings. There also may be layers to space different emulsions, or additional filter layers.

Each emulsion layer, in addition to the light-sensitive components, contain chemicals called dye couplers. These couplers, located in the blue, green and red-sensitive layers, produce yellow, magenta and cyan dyes, respectively, when developed.

The C-41 process is the same for all C-41 films, although different manufacturers' processing chemistries vary slightly.

After exposure, the film is developed in a "color developer". The developing ingredient is a paraphenylene diamine-based chemical known as CD-4. The developer develops the silver in the emulsion layers. As the silver is developing, oxidized developer reacts with the dye couplers, resulting in formation of dyes.

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