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Principle of photographic chemistry.

Latent Image:
When film or paper is exposed to light, a latent image is formed within the emulsion. This image is contained in the silver halide crystal in the form of energy, caused by the absorption of photons. The image is invisible. Latent image can deteriorate rapidly with time. A reducing agent can be used to develop this image. Before the invention of developer, people have to expose the film for a very long time until the silver halide reduced to silver by light energy. Unfortunately, this image can fade away; because left over silver halide can also turn into silver later. Sodium thiosulphate was discovered to be used as fixer.

Film is made up of a mixture of gelatin and silver salt to be coated on a carrier. Early film is colour blind. It can sense blue colour only. By doping with dyes, film can be sensitized to green and red colour as well. Colour film or paper is made up of at least three layers of emulsion with one layer of light filter. By mixing these three primary colours, we can produce a full spectrum of colours.

Developer is a reducing agent, which can accelerate the conversion of salt to metal silver. Colour processing is similar to black and white processing. When a black and white image was formed, a colour image was also formed at the same instance, by utilizing the newborn silver as a donor to a pre-mixed colour coupler in the emulsion. Colour dye is pre-embedded in the film or paper in the form of coupler, which is not visible before developing. Developer does not contain dye.

Since the colour image is super imposed on a black and white image. A bleach process is required to remove the silver image. Metal silver is oxidized to silver halide again by bleach agent. Bleach is a reversal of developing.

Left over silver halide salt and those reversed by the bleach process are dissolved by the fixer. Fix process can make the image transparent rather than milky.

Gelatin is very favorable to bacteria. A stabilizing bath can prevent fungus in the future.

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