Photographic developer
In the processing of photographic films, plates and papers, a developer is a chemical solution that converts the latent image to a visible image. This may be a negative image, with the emulsion darkened by light, requiring a further imaging process to produce a print where highlights and shadows are not reversed, or a positive image which can be viewed or projected. In the gelatin silver process, the developer achieves this conversion by reducing the silver halides, which are pale-colored, into silver metal, which is black when in the form of fine particles.
Although the developer will react with all grains of silver halide, it acts much more quickly on those that have been exposed to light, which is how it produces an image: when left in developer, all the silver halides will eventually be reduced and turn black. The longer a developer is allowed to work, the darker the image. To produce a usable image, development must be stopped at the appropriate time.
The following sections first describe monochrome, black-and-white, development, then colour processes which add coloured dyes to the silver blackening process.