In the
processing of photographic films, plates or papers, the
photographic developer (or just
developer) is one or more chemicals that convert the
latent image to a visible image. Developing agents achieve this conversion by
reducing the
silver halides, which are pale-colored, into
silver metal, which is black (when a fine particle). The conversion occurs within the gelatine matrix. The special feature of photography is that the developer only acts on those particles of silver halides that have been exposed to light. Generally, the longer a developer is allowed to work, the darker the image.