Photographic film includes one or more layers of a photosensitive material. When a picture is taken, the light from the scene interacts with the film's photosensitive material to produce a chemical change in the photosensitive material. This chemical change is in direct proportion to the intensity of the light. Black and white photographic film generally has a single layer of silver halide emulsion coated on a transparent film support. Colour photographic films such as colour negative films and colour reversal films include 2 or 3 red-sensitive, cyan-coupling silver halide emulsion layers, 2 or 3 green-sensitive, magenta-coupling silver halide emulsion layers and 2 or 3 blue-sensitive, yellow-coupling silver halide emulsion layers. Each silver halide layer in color photographic film is sensitive to a different portion of the visible spectrum.Commonly used photographic films include 135 films, 120 films, 110 films and APS films, among which 135 films are the most popular films. 120 films are mainly used for commercial purposes because of their high quality and larger sizes.
