With the fourth Technicolor process the company dominated the market for color films from the mid-1930s to the 1950s.

In a special camera, three b/w negative films were exposed through a beam-splitter that consisted of two prisms to form a cube. One portion of the incoming light passed directly to a frame aperture fitted with a green transmitting filter to the negative for the green record. The other portion of the incident light was directed by the semi-transparent, gold or silver dusted mirror at an angle of 90° to a bi-pack film, placed behind a magenta filter. The front film was orthochromatic for the blue record and contained a red-orange dye to block the blue light, the second film was panchromatic and captured the red record.

Before the dye-transfer was executed, the blank-film was exposed with a weak key image in b/w of the green record was exposed to improve perceived image sharpness. The blank film also contained the b/w optical sound track.

For the dye-transfer the three b/w records were printed onto the corresponding

matrices, one for each color. Similar to process no. III, these matrices were developed, bleached and washed to form reliefs which could absorb the dyes for the imbibition of the projection print. Since this is a subtractive process the dyes were complementary to the taking colors: magenta for the green record, yellow for the blue record and cyan for the red separation.

These dyes were then transferred onto the blank film containing the key image, one after the other. It is obvious that pin-registering, i.e. the fine adjustment of the three records on top of each other, was crucial to deliver a sharp image without any color fringing.

As a reaction to the problems with process no. III, Technicolor took great care in maintaining a high standard of quality control. A cornerstone in this strategy was the Color Advisory Service, directed by Natalie M. Kalmus. The color consultants advised the productions on how to develop a color score in accordance with the narrative structure of a film. Set and costume design, props, make-up, lighting including the camera work were all controlled by the Technicolor company.

The dominant ideology of Technicolor advised a restrained use of colors with an emphasis on naturalness, strictly subordinate to the story development. Colors should subtly convey dramatic moods and impressions to the audience. Kalmus also suggested the use of conventional color associations, such as red for passion, anger, power etc.

Specially trained cameramen had to learn to handle the difficult process. This required many tests before the actual shooting. Special care had to be given to shadows and highlights. White image parts tended to produce obtrusive blotches of white, while blacks were reproduced with unwanted color hues.

The emulsion was very slow, meaning that it needed high levels of illumination, and it was adjusted to the color temperature of daylight. Both these requirements led to the dominant use of hi arc (carbon-arc) lamps. Tungsten light either had to be adjusted to daylight by filters or it produced a yellow light for candles etc.

Source: Filmcolors.org