Film processing is a procedure whereby a latent image on a film is processed to become a visible radiograph. It consists of four major steps namely: development, fixing, washing, and drying.
Process
Development
Developer solution (e.g. hydroquinone or phenidone) is used to reduce more silver ions into silver atoms while the developer solution itself is oxidised. The process will usually begin at the sensitivity speck. Unexposed silver grains will also be developed much later, given that the development time is long enough. Thus, development should be stopped when the maximum amount of exposed grains is developed while unexposed grains are left untouched.
Fogging is the development of unexposed silver halide crystals.
In high-volume film processing centers, hydrogen and bromide ions accumulate in the developer solution, making the developer solution more acidic. Thus, the developer solution requires regular replenishment every two to three months. In low-volume settings, oxidation reaction predominates, thus making the developer solution more alkaline instead of acidic, and no bromide ions are produced. Thus, a more acidic developer solution with higher bromide ions and sulfite is used as replenishment and to reduce the rate of oxidation 1.
Fixing
Sodium thiosulfate or ammonium thiosulfate is used as fixing agent. The purpose is to remove the remaining unreacted silver halide crystals. Silver halide crystals are only slightly soluble in water. The number of silver ions and halide ions dissolved in water is a constant as given by the formula below 1:
number of silver ions x number of halide ions = constant
As the number of silver ions decreased, the number of halide ions have to increase for the product of the two to remain constant. Thus, thiosulfate agent binds to silver ions tightly, removing silver ions from the solution. Therefore, more silver halide crystals have to dissolve to make up for the silver ions loss 1.
Fixing agent also contains chromium or aluminum compounds to harden the gelatin, making it tougher and more resistant to abrasion 1.
An incompletely fixed film has a "milky" or "cloudy" appearance due to dispersion of transmitted light by small silver iodobromide crystals 1.
Washing
Washing is a process whereby the remaining fixed agent are removed. Incomplete washing will cause the film to turn brown after prolonged storage. This is because the remaining fixing agent will react with silver atoms to form silver sulfide which is brown in color 1.
Drying
This process is to remove excessive water from the film.
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