A densitometer is an instrument for measuring the darkness, or light-stopping power, of exposed and processed photographic material – i.e. its density.
Densitometers measure light reflected from a surface such as a paper print, while
densitometers measure transparent materials such as motion picture film.
Most modern densitometers measure the density by means of a photoelectric device and use electronic intergrated circuitry to measure and display the density values directly.