Film Developers

The image on a film is formed by the action of light on materials coated on the film surface. That is not enough in itself to form a usable image, so the effect has to be amplified. There is a parallel with the electrical signals picked up by a radio receiver or from a tape or disc player. They are much too weak to provide sufficient power to drive a loudspeaker, so they have to be amplified. The electrical signals are amplified by electronic means. The effect of light on films is amplified chemically. The image is formed by a plastic strip coated with a light-sensitive material called the emulsion. It is actually a suspension of tiny grains of a silver compound in gelatin. The emulsion is microscopically thin but the silver halides within it are too small to be seen even by the ordinary microscope. Consequently they are suspended at various depths within the emulsion and can overlap. It is the silver halide grains that are light-sensitive and the gelatin is simply a binder. Light reflected from the subject is picked up by the camera lens and projected to form an image on the film. The image consists of various densities of light representing the darker and lighter parts of the subject. The silver halides are affected by the light according to its intensity. The whole of a grain or only part of it may be sufficiently affected to contribute to the image. Grains at various depths within the emulsion are affected according to how far the light penetrates. The stronger the light, the deeper it penetrates into the emulsion.