Photographic Development

the conversion of a latent photographic image, formed on the photosensitive layer of motion-picture and still photographic materials upon exposure to light or other radiation, into a visible image. During the development of the most common silver halide photosensitive materials, the silver ions are reduced to metallic silver under the action of a photographic developer.

Two types of photographic development for these materials are usually distinguished: chemical and physical. In chemical development, silver is reduced from the silver halides of emulsion crystals in the photosensitive layer. In physical development, the silver is reduced from a soluble silver salt, usually AgNO₃, contained in the developer. In chemical development using an organic developing agent, the development process may be approximately expressed (using hydroquinone development as an example) by the overall equation

The development of color photosensitive materials follows a slightly different process.

Photographic development involves some loss in a developing agent, sodium sulfite, and an alkali. At the same time, bromine ions are accumulated in the developer, thereby retarding the process. Photographic development is a selective process, whose rate is substantially higher in the exposed sections of the photosensitive layer than in the unexposed sections. This difference in rates may be explained by the presence of specks of metallic silver in the exposed emulsion grains of silver halide. The specks act as a heterogeneous catalyst for the development process. As a result, in the time that the photosensitive material remains in the developer, the emulsion grains containing these specks become fully visible, whereas the grains without specks of silver remain undeveloped.

In physical development, the metallic silver specks in the photosensitive layer also serve to catalyze the reduction of silver nitrate in the developer. In one type of physical development, the exposed photosensitive material is subjected to photographic fixing; only the specks of metallic silver then remain. After immersing the fixed material in the developer, the silver nitrate is reduced to metallic silver, which crystallizes on the specks of silver. This method makes it possible to obtain an image with an exceptionally fine grain structure. Physical development is chiefly used for technical purposes.