device that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Such devices are generally transducers that convert an electric current into a mechanical indication—e.g., a pointer moving across a dial. The source of the current may be a selenium cell, which generates a current when light falls on it, or it may be a permanent source, such as a battery, in which case the current passes through a cadmium sulfide cell whose resistance varies with the amount of light falling on it.

Photometers are made in many forms for various applications. They may be used directly, as in photography or in measuring glare, colour variation, reflectance, or other qualities, or they may be incorporated into such devices as densitometers, spectrographs, and telescopes.