color film

Color film is photographic film that records the hues of a scene, not only its brightness. Human color vision is trichromatic, so a full-color image can be reconstructed from three records — the blue, green, and red content of the light. Color processes differ chiefly in how they capture those three records and how they recombine them into a viewable image.

Additive and subtractive color

The earliest practical approaches were additive: colored images were built by adding beams of red, green, and blue light. Additive systems recorded the three components through colored filters and projected them back through the same filters, either in rapid succession or through a fine mosaic of colored elements on the film. They were simple in chemistry but wasteful of light, since each filter absorbed most of the spectrum, and mosaic screens limited resolution.

Modern color film is instead subtractive. A subtractive image begins as white light and removes portions of the spectrum using superimposed layers of cyan, magenta, and yellow dye — each dye subtracting its complementary primary (red, green, or blue). Because the dye layers are transparent and stacked, a subtractive print transmits far more light than an additive one and needs no special projector.

The integral tripack

The dominant form of color film is the integral tripack, a single strip carrying three emulsion layers coated one atop another. The top layer is sensitive to blue light; beneath a yellow filter layer that blocks blue lie the green-sensitive and red-sensitive layers. During development, chemical couplers react with the byproducts of silver development to form a dye in each layer — yellow, magenta, and cyan respectively — in proportion to the exposure that layer received. The developed silver is then bleached and fixed away, leaving three superimposed dye images that together render the full range of color. In some processes the couplers are built into the emulsion; in others they are supplied by the developing solutions.

Landmark processes

Technicolor, in its mature three-strip form (introduced 1932), exposed three separate black-and-white negatives through a beam-splitting camera and combined them by dye transfer, or imbibition, onto a single print. Kodachrome (1935) was an early integral tripack whose dyes were formed by couplers introduced during a complex development sequence, yielding fine grain and stable dyes. Eastmancolor (1950) placed the couplers within the emulsion itself, enabling a single, relatively simple development process; this “monopack” approach made color practical for routine production and became the basis for most subsequent color negative and print stocks.

Color balance and lighting

A color film’s emulsion layers are balanced for a particular color temperature of illumination — typically daylight (around 5500 K) or tungsten (around 3200 K). Light that departs from the film’s balance shifts the recorded colors, so lighting is either matched to the stock or corrected with filters on the camera or the light sources. This dependence on the spectral quality of illumination distinguishes color film from black-and-white film, which is largely indifferent to the color of the light it records.