|
|
|
|
|
Photographic DeveloperIn film developing, photographic developer (or just developer) is a chemical that makes the image on the film or print visible. Formulae For black and white photography, the developer is often a mixture of metol and hydroquinone. These are made up in aqueous solution with sodium carbonate to create the appropriately high pH and sodium sulphite to provide strong reducing conditions to delay oxidation of the developing agents by atmospheric oxygen. Most developers also contain small amounts of potassium bromide to modify and restrain the action of the developer to enable penetration of the developer throughout the emulsion before over-development occurs on the surface - chemical fogging. This also changes the colour of the silver image from sepia (low bromide) through black to blue (high bromide). Developers for high contrast work have higher concerntrations of Metol and lower concentrations of Hydroquinone and tend to use strong alkalis such as Potassium carbonate to push the pH up to around pH14. Because Metol is difficult to dissolve in solutions of sodium sulphite it is important to dissolve a very small measure of sodium sulphite in water together with the sodium carbonate before adding the Metol and only add the remaining sodium sulphite once all the Metol has been dissolved. Other modern constituents include small amounts of wetting agents and small amounts of alcohol, often propanol In the early days of photography, a wide range of developing agents were used includings ferrous oxalate, hydroxylamine, lactate of iron, ferrous citrate, Eikonogen, atchecin, resorcin, antipyrin , acetanilid and Amidol {which unusually required midly acidic conditions) Other constituents of developers in the past have included ammonia, glycerine sulphuric acid , sulphurous acid , potassium ferricyanide, potasium oxalate , citric acid and methanol. Development The developer reduces the silver halides in the latent image in the exposed photograph into reduced, opaque, black silver metal. The image is then fixed using photographic fixer. Colour development In colour and chromogenic black and white photography, a similar development process is used except that the reducing silver catalyses the production of dye-stuffs in the emulsion. There are three distinct processes used here. The C41 process is used for almost all colour negative films and in this process dye couplers in the emulsion react with dye formers in the developer to generate the visible dyes. An almost identical process is then used to produce colour prints from films. The developing agents used are however identical to those used in black and white films. Reversal film development In many colour transparencies, the film is first processed in a sophisticated black and white developer and is then treated with a 'reversal chemical which stops the initial development and converts the previously unexposed silver into the new latent image. The film is developed again using a process similar to C41 before final fixing and washing. The most common processing chemistry for such films is E6, derived from a long line of developers produced for the Ektachrome range of films. Proprietary methods The Kodachrome process is a proprietary process in which all the dyes are added to the emulsion during development. In colour print development, the Cibachrome process also uses a print material with the dye-stuffs present and which are bleached out in a appropriate places during developing. The chemistry involved here is wholly different from C41 chemistry.
|
 |
|
| Copyright 2005-2009 OnPedia.com. All Rights Reserved |
|
|