Bi-pack color

In bipack color photography for motion pictures, two strips of black-and-white 35 mm film, running through the camera emulsion to emulsion, are used to record two regions of the color spectrum, for the purpose of ultimately printing the images, in complementary colors, superimposed on one strip of film. The result is a multicolored projection print that reproduces a useful but limited range of color by the subtractive color method. Bipack processes became commercially practical in the early 1910s when Kodak introduced duplitized film print stock, which facilitated making two-color prints.

Bipack photography was, from about 1935 to 1950, the most economical means of 35 mm natural color cinematography available, used when color was wanted but the budget could not bear the much higher cost of three-strip Technicolor or the less well-known alternative three-color processes sometimes available outside the US. After 1950, when economical "monopack" color negative and print stocks such as Eastmancolor and Ansco Color were introduced, the use of bipack photography and printing rapidly declined. By 1955 all two-color motion picture processes were commercially extinct in the US.

Bipack and three-element tripack sandwiches of plates and films were used in some early color processes for still photography, the field in which the concept originated.

Bipack color refers to the type of camera load that is used for the effect. Bipack photography refers to two strips running through the camera at once, for the purpose of recording two different spectra of light, generally.

Color photography begins with any standard camera. Special magazines or adapters must be provided to accommodate two separate rolls of film. Two films are loaded, passing through the photographing aperture with the emulsions towards each other. The front film is orthochromatic, to record the blue-green portion of the picture. On the surface of its emulsion is a red-dye layer equivalent to a Wratten 23A filter. The rear film is panchromatic, and being photographed through the red coating of the front film, records only the red-orange components of the picture. No filtering is necessary either for exterior or interior photography, as all necessary color corrections are made by adjusting the development of the two negatives during printing.

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Wikipedia