Densitometers measure the optical, photographic or area density of a material. They are used in a wide range of applications, from film processing to medical scans. Densitometers are composed of a light source aimed at a photoelectric cell and an analog meter or integrated circuitry to show the data. There are three basic types of densitometers: scanning densitometers, reflection densitometers, and transmission densitometers. A scanning densitometer often has separate, pre-set steps for mid-density (MD) and density difference (MD) measurements. A reflection densitometer measures light reflected back from a surface. A transmission densitometer is used to measure the optical density of transparent materials.