A latent image is an invisible image produced by the exposure to light of a photosensitive material such as photographic film. When photographic film is developed, the area that was exposed darkens and forms a visible image. In the early days of photography, the nature of the invisible change in the silver halide crystals of the film's emulsion coating was unknown, so the image was said to be "latent" until the film was treated with photographic developer. In more physical terms, a latent image is a small cluster of metallic silver atoms formed in or on a silver halide crystal due to reduction of interstitial silver ions by photoelectrons (a photolytic silver cluster). If intense exposure continues, such photolytic silver clusters grow to visible sizes. This is called printing out the image. On the other hand, the formation of a visible image by the action of photographic developer is called developing out the image. The size of a silver cluster in the latent image can be as small as a few silver atoms. However, in order to act as an effective latent image center, at least four silver atoms are necessary. On the other hand, a developed silver grain can have billions of silver atoms. Therefore, photographic developer acting on the latent image is a chemical amplifier with a gain factor up to several billion. The development system was the most important technology that increased the photographic sensitivity in the history of photography. Mechanism of formation The action of the light on the silver halide grains within the emulsion forms sites of metallic silver in the grains. The basic mechanism by which this happens was first proposed by R W Gurney and N F Mott in 1938. The incoming photon liberates an electron, called a photoelectron, from a silver halide crystal. Photoelectrons migrate to a shallow electron trap site (a sensitivity site), where the electrons reduce silver ions to form a metallic silver speck. A positive hole must also be generated but it is largely ignored. Subsequent work has slightly modified this picture, so that 'hole' trapping is also considered (Mitchell, 1957). Since then, understanding of the mechanism of sensitivity and latent image formation has been greatly improved. A latent image is formed when light changes the charge atoms in the molecule. Taking bromine as a halide for this example, when light hits a silver halide molecule, the halide is