The light sensitive component of a film emulsion is a microscopic grain composed of a silver halide (the halide usually being bromine, or sometimes iodine or chlorine). When the silver halide absorbs sufficient energy (in the form of light), it separates into an atom of silver metal and a free atom of halide. The silver metal migrates through the grain to imperfections on the surface of the grain. The final density that is formed in the emulsion is therefore proportional to the total energy of the light and the number of molecules of silver halide that separate. This is the formation of the latent image. To make this image visible requires intensification by the chemical action of the developer.
Apart from the formation of the image there are other considerations with exposure. One is the movement of light within the film’s emulsion. Not all the light energy is immediately absorbed by the silver halide grains; some of it is reflected or refracted from grain to grain. This is referred to as irradiation (Fig 13.7) and the effect it causes is known as image spread. This reduces the film’s ability to resolve fine detail. It can be said that optimum exposure therefore lies in the narrow range that allows full recording of shadow detail to the loss of detail (usually in the highlight or highly exposed areas) due to image spread. On small formats this usually occurs before the shoulder is entered.
Another closely related effect caused by unwelcome light moving in an emulsion is halation. This is the reflection of light from the internal surfaces of the emulsion and film base. Depending upon the generation of the material (positive or negative) where this has occurred, it can show as either a halo of lighter density around a light object or of darker density around a dark object. Control of halation is achieved by dyes in the base or a layer incorporated in the manufacture of the film. This layer is removed during processing.