photographic processing

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photographic processing,

set of procedures by which the latent, or invisible, image produced when a photographic film is exposed to light is made into a permanent visible image.

Creation of the Latent Image on the Film

An emulsion holding grains of photosensitive chemical compounds called silver halides is spread over a film or other material. Light coming through the camera lens from an object being photographed strikes certain areas of the film, rendering the silver halide grains in those areas unstable. This creates an invisible, or latent, image of the object on the film. The areas of the latent image that receive the most light contain the largest number of unstable grains. Upon development they become the darkest areas of the visible image. Conversely, areas that receive little light form the bright parts of the visible image.

Processing the Negative

Because of the reversal of dark and bright areas from the latent image to the visible image, the visible image is often called a negative. The most common method of making the image visible is to bathe it in a chemical developer that reduces the unstable silver halide grains to black metallic silver, which forms the image. In addition to the reducing agent, which is generally an organic compound such as a phenol or an amine, the typical developer contains additives that cause development to go on at a desired rate, prevent the reducing agent from being destroyed by the air, and keep unexposed silver halide from fogging the film. Each developer is generally designed to be used with particular film emulsions and to produce certain desired effects, such as fineness of grain in the finished image.

After development the negative must be stabilized, or fixed, so that it will no longer be sensitive to light. In fixing, the unexposed silver halide grains are removed by immersion in a water solution of sodium or aluminum thiosulfate. Between the developing and fixing processes the negative may be placed in an acid bath to neutralize excess alkali left by the developer. After fixing, the negative is washed and dried. Next the negative may be subjected either to intensification, a process in which additional silver is deposited in exposed areas to increase the contrast in the image, or to reduction, a process in which silver is removed to decrease the contrast. Toning is a process in which a photographic image is treated to change its color, as by changing the deposited silver to silver sulfide or causing a colored metal salt to form along with the silver.

Production of the Print

The negative may be used to produce a positive image, often called a print, or photograph, in which the light and dark areas of the object and the image correspond. The positive is produced by first projecting the negative onto a photosensitive paper. When this is done by direct contact, i.e., placing the negative and photosensitive paper together, the positive produced is the same size as the negative. When a system of lenses is interposed, the positive image may be enlarged or reduced. After this the latent image on the photosensitive paper is developed by a process similar to that used on the negative.

In most color films there are three layers of emulsions, each sensitive to a different color of light and each capable of forming a different color dye when developed. There are many development processes in use, but nearly all use paraphenylene-diamine derivatives. In one process the exposed film is made into a positive color transparency, in another a negative from which positive prints are produced. In both processes the finished product contains three layers, each one containing an image in a different color. The superposition of these images reproduces the colors of the photographed object.

Polaroid photography uses a more complex process, by which the image diffuses from the top layer, where it is originally captured, to lower layers where it activates appropriate dyes to recreate the recorded image (see Land, EdwinLand, Edwin Herbert,
1909–91, American inventor and photographic pioneer. While at Harvard, Land became interested in the properties and manipulation of polarized light.
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).

Bibliography

See A. Feininger, Darkroom Techniques (1974); M. J. Langford, Basic Photography (1977); H. J. Walls and G. S. Attridge, Basic Photo Science (1978); G. T. Eaton, Photographic Chemistry (1981).

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