Photographic Film

Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion holding microscopically small light sensitive silver halide crystals, used to take photographs, with the size and character of the crystals setting the film's sensitivity, contrast and resolution. A brief exposure through a camera lens produces a slight, proportional chemical change in the crystals, creating an invisible latent image that is later developed into a visible photograph. Because unmodified silver halide crystals respond only to the blue part of the visible spectrum, sensitizing dyes were developed to make film respond to green and, eventually, all visible colors, yielding orthochromatic and then panchromatic film, and similar techniques extended some films' sensitivity into the infrared. Black and white film normally carries one layer of silver halide crystals that becomes metallic silver on development, while color film carries at least three differently sensitized layers whose development byproducts combine with color couplers to form the dyes making up the final color image.