High Quality Content by WIKIPEDIA articles! In the processing of photographic films, plates or papers, the photographic developer (or just developer) is a chemical that makes the latent image on the film or print visible. It does this by reducing the silver halides that have been exposed to light to elemental silver in the gelatine matrix. As a generalisation, the longer a developer is allowed to work, the greater the degree of reduction of the silver halide crystals to silver and therefore the darker the image.