These pages are intended to give a brief description of the processing steps necessary to develop a photographic negative into a minilab considering each chemical function . The information and the links give a help to new minilabs operators to monitor the chemical process and to achieve a good photographic quality .
In this page we study the film processing steps into a film processor , the following stage is the paper process , in which is the captured image developed in the negative is projected on the paper and chemically processed until obtaining the picture .
To actually reproduce the colors of a photographed scene, three different complementary dyes or tints are used , through the subtractive process , to represent the original colors registered or captured in the three optically sensitized silver halide emulsion layers.
The three layers are produced in the film only in areas where the light sensitive compound or silver halide (AgX) has been exposed. The dyes form during the development process where couplers react with the oxidized development agent, CD4 or paraphenylendiamine , in the developer bath during processing .
It is crucial that each of the three couplers react properly with the same oxidized developer in all three layers to produce the correct amount of dye in each of these layers.
In order to produce a color print , first the image should be captured in a color negative . We will begin explaining what is the negative, the way it is structured , how the image is captured on it, and the form in that it is processed to reproduce the image . Dye-forming (chromogenic) development is the basis of the color process. It is a procedure which, during the development stage, generates a dye image consisting of the three subtractive colours ( subtractive colour mixture) in the individual emulsion layers at the same time as the silver image. The dyestuffs are insoluble in water and diffusion-resistant. They remain in the emulsion layer at the points where the silver image is located. After development the silver is bleached (converted into soluble silver salt) and dissolved out of the emulsion in the fixer.
What is left is a pure dye image. During colour development, the oxidation product of the developer substance reacts (" couples") with the colour couplers dispersed in the emulsion layers to form the emulsion dyes, yellow, magenta and cyan, due to the spectral sensitisation of the individual layers for the blue, green and red spectral components of visible light.
The chromogenic colour process, which is used for all modern colour films (and most printing materials), is based on the principle of subtractive colour mixture. At least three layers with integrated colour couplers are applied on top of each other, one sensitized for blue, one for green and one for red. The layer structure of colour films is, in reality, more complex, because there are other protective, separating and filtering layers in addition to the colour-sensitized emulsion layers containing the colour couplers ( layer structure, anti-halation).
During processing of these multi-layer films, the silver halide is reduced to metallic silver in the exposed areas of the photo-sensitive emulsion layers. The oxidation products of the colour developer resulting from this process react with the (colourless) colour couplers in the emulsion layers to give yellow, magenta and cyan image dyes. At the same time as the three black-and-white images in the individual layers, three dye images are thus created in the subtractive primary colours, which remain when the metallic silver is bleached out during subsequent processing. Depending on the type of film and process, the result is either a colour transparency or a colour negative.
In a slide, the colours of the subject are the right way round. When they are viewed through a projector, we see that part of the beam of light that passes through the transparency. The slide can also be printed onto special reversal copying material (or onto colour negative paper ). In a colour negative, on the other hand, complementary colours are formed. The positive image with the colours the right way round is obtained by printing onto colour negative printing material.
The first patents for chromogenic development (1910 -1913) stem from Rudolf Fischer.
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