The earliest process of motion picture colour photography was an additive process brought out under the name of "Kinemacolor" and developed by G. A. Smith and Charles Urban in 1906. It is now obsolete. In this process film was exposed alternately through rotating red and green filters so that one picture was taken through the red filter and the next through the green. A positive print was made and projected through a machine fitted with a similar rotating colour shutter. The eye integrated the alternating red and green colours by persistence of vision, and a picture in natural colours was seen on the screen. The objec tions to the process were the use of special projectors; the loss of light owing to absorption by the filters; the alternation of the colours which produced a flickering effect unpleasant to the eye; and a "striping" of red and green in the case of rapidly moving objects owing to the red and green objects being taken when in different positions. Blues were lacking owing to the use of two colours only.
The pictures through the filters may be taken simultaneously by the use of several lenses. Since these lenses must be in slightly different positions, each lens will see the object from a slightly different point of view, and if the scene taken has considerable depth, it will be impossible to register near and far objects simul taneously, an effect known as "parallax." Such a process was used by Gaumont who employed three lenses. The pictures were taken simultaneously and projected simultaneously by means of a triple lens projector. The results were excellent, but great difficulty was experienced in keeping register on the screen, variations in the film shrinkage producing differences in register which had to be corrected while the picture was being projected.
The several pictures may be taken simultaneously through one lens, the beam of light being subdivided by systems of prisms or reflectors. Such a camera is used in making pictures by the "Technicolor" process. It obviates both the striping defect and the parallax difficulty. In another system for making motion pictures in two colours, the separation into red and green records is made without an optical device by using a "bipack" of two films in contact. The front one gives the green record and the rear one the red record through a red dyed layer coated on its surface.
All the natural colour pictures which are available in theatres at the present time are made by the "subtractive" processes of colour photography. In the "Technicolor" process, three colour separation negatives are made in a camera having a prism "beam splitter," two films being used as a bipack and the third is sep arate. From the three negatives three relief images in gelatin are prepared on other films. These are dyed in three appropriate col ours, and the dyes are transferred in succession and in register to a final film which carries a faint silver image to aid registration and improve the definition. Thus, a film carrying the full colour
picture on one face results.
In 1935, another subtractive process, the new Kodachrome process, was introduced. Any ordinary camera can be used, no special optical device or filter being necessary. The three colour separation images are recorded in three sensitive layers coated one over the other. and made so that of themselves they separate the blue, green, and red records into the three layers. In developing, the images in the three layers are converted into direct positive dye images which are complementary in colour to the colours which produced them. The separate layers are so thin that their com bined thickness does not exceed that of an ordinary film, and the dye images act as if they were all in one layer. Films made by subtractive processes such as Technicolor and Kodachrome can be projected in any ordinary projector with no filters or other attachments.
A screen plate process, known as Dufaycolor, was introduced for amateur motion picture work in 1935. An additive process, somewhat related to the screen-plate processes, was put on the market in 1928 for amateur cinematography under the name of Kodacolor. It was based on an invention of Berthon (see PHO TOGRAPHY) In this process rolls of film, coated with panchromatic emulsion and embossed on the back with cylindrical lenses, are used in con junction with a lens of high aperture, f/2 or over, and provided with a special diaphragm containing a compound filter which takes the place of the ordinary hood. After exposure. the film is proc essed by a reversal process, by which it is converted into a posi tive ready for projection. The projector is fitted with a lens hav ing a filter corresponding to the camera filter and on projection natural colour pictures are obtained on the screen, the chief limi tation being that the loss of light caused by the absorption of the filters involves the use of a screen smaller than that used for black and white.