Photographic film is a strip or sheet of transparent plastic film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast and resolution of the film. The emulsion will gradually darken if simply left exposed to light, but that spontaneous visible change is too slow and incomplete to be of any practical use. Instead, a very short exposure to the image formed by a camera lens is normally used to produce only a very slight chemical change, proportional to the amount of light absorbed by each crystal. This creates an invisible latent image in the emulsion which can be chemically developed into a fully visible photograph. In addition to visible light, all films are sensitive to X-rays and high-energy particles. Many are at least slightly sensitive to invisible ultraviolet light. Some special-purpose films are sensitive into the infrared region of the spectrum. In black-and-white photographic film there is usually one layer of silver salts. When the exposed grains are developed, the silver salts are converted to metallic silver, which blocks light and appears as the black part of the film negative. Wikipedia [ - ]

Photographic film is a strip or sheet of transparent plastic film base coated on one side with a... [ + ]