THE COLOUR NEGATIVE/POSITIVE PROCESS
Integral tripack colour films are the only photographic colour system used today for cinematography, and exactly parallel the black and white process except that the final image is comprised of three subtractive dyes, Cyan, Magenta and Yellow, in three separate and discrete superimposed layers.
Each layer has an effective sensitivity to Red, Green and Blue wavelengths and the Cyan, Magenta and Yellow dyes are produced in these respective layers by the development process.
The process of dye development principally relies on one group of developing agents, the para-phenylenediamine derivatives. Three insoluble, or non-diffusing, couplers, chemicals that produce dyes by reaction with oxidised para-phenylenediamine, are coated in the emulsion layers. The process of development produces the silver image and the oxidised developing agent produced reacts with the adjacent coupler to produce an insoluble dye. The silver can then be removed. The essential stages are:
Colour developers contain a colour-developing agent, which reacts with chemicals in the film called dye couplers during development. The couplers and developing agent produce dye in proportion to the amount of silver developed.
The colour film has three light sensitive layers that are sensitive to red, green and blue light. In fact, all layers are sensitive to blue light as well as their other sensitivity. In colour negative films, the blue sensitive layer is on top; there is a yellow filter layer next to stop blue light reaching the red and green sensitive layers. The filter layer is usually a dispersion of very fine particles of silver known as colloidal silver. The film manufacturers use silver because it is easily removed later in processing.
Colour positive films do not employ a filter layer; the red and green layers have their colour sensitivities adjusted so that the blue sensitivity is much less than the red or green sensitivity. It does mean that the speed of the film to red and green light is very low and they require a high exposure.
However, because we expose the films in a printer where we can adjust the exposures the lack of speed does not matter. This system is not used in camera films because the speed of the film would be too low.
After development, we bleach the film to remove the silver formed in development as well as the filter layer if present.
We fix the film to make any unused silver halide water-soluble followed by a wash and a stabiliser bath to reduce the amount the dyes will fade.
When we process colour positive with a sound track an additional stage is required. After bleaching but before fixing we coat the sound track area with a viscous black and white developer by means of a wheel called an applicator wheel. The developer turns the silver halide into silver so that the fixer does not remove it.
Colour reversal processes follow the same pattern as black and white reversal processes with the substitution of colour developer for the second developer. The first developer in a colour reversal process is a black and white developer.
The specifications for colour processes should be found from the stock manufacturers processing manual.