A New Subtractive Colour Film

Abstract

THE Eastman Kodak Company has introduced a new 16 mm. cinematograph colour film. This is known as the Kodachrome film. It differs from many other colour films now available in that the colour effects are produced ‘subtractively’. The super imposition of ‘minus’ colours is used to yield the final colour. For example, a blue is produced by placing a minus green (magenta) on a blue-green (minus red), green by minus blue (yellow) on a minus red and so on. This is, of course, the ordinary principle of three-colour printing, but to apply it to this process has involved the use of a film coated in three layers. Each of these layers records one of the primary colours only. The top layer is sensitive to blue and transmits only green and red; the second layer records green and transmits red, while the last layer of all records only red. The film is exposed in the ordinary way and has a sensitivity which is said to be about half that of the normal film used for black-and-white cinematography. Processing is divided into a number of separate processes: (1) Development to negative. (2) Bleaching and clearing to get rid of the developed silver. (3) Exposure and redevelopment to positive. This redevelopment is carried out with a ‘dye-coupling developer’ which attaches a blue-green dye to the silver produced by development. All three layers are treated indiscriminately up to this point. Then follow further stages. (4) Bleaching the two upper layers; silver chloride is formed in place of the silver in these layers and the blue-green dye in them is destroyed. (5) Exposure and redevelopment of the two upper layers with a magenta coupling developer. (6) Bleaching the top layer and destroying the magenta dye. (7) Exposure and development of the top layer with a yellow coupling developer. The pictures so formed are continuous in tone.