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Concept
Photographic processing
Summary
Photographic processing or photographic development is the chemical means by which photographic film or paper is treated after photographic exposure to produce a negative or positive . Photographic processing transforms the into a visible image, makes this permanent and renders it insensitive to light.
All processes based upon the gelatin silver process are similar, regardless of the film or paper's manufacturer. Exceptional variations include instant films such as those made by Polaroid and thermally developed films. Kodachrome required Kodak's proprietary K-14 process. Kodachrome film production ceased in 2009, and K-14 processing is no longer available as of December 30, 2010. Ilfochrome materials use the dye destruction process. Deliberately using the wrong process for a film is known as cross processing.
Alternative process
All photographic processing use a series of chemical baths. Processing, especially the development stages, requires very close control of temperature, agitation and time.
The film may be soaked in water to swell the gelatin layer, facilitating the action of the subsequent chemical treatments.
The developer converts the latent image to macroscopic particles of metallic silver.
A stop bath, typically a dilute solution of acetic acid or citric acid, halts the action of the developer. A rinse with clean water may be substituted.
The fixer makes the image permanent and light-resistant by dissolving remaining silver halide. A common fixer is hypo, specifically ammonium thiosulfate.
Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading.
The washing time can be reduced and the fixer more completely removed if a hypo clearing agent is used after the fixer.
Film may be rinsed in a dilute solution of a non-ionic wetting agent to assist uniform drying, which eliminates drying marks caused by hard water. (In very hard water areas, a pre-rinse in distilled water may be required – otherwise the final rinse wetting agent can cause residual ionic calcium on the film to drop out of solution, causing spotting on the negative.
Official source
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