photocell

Device for measuring or detecting light or other electromagnetic radiation, since its electrical state is altered by the effect of light. In a photoemissive cell, the radiation causes electrons to be emitted and a current to flow (photoelectric effect); a photovoltaic cell causes an electromotive force to be generated in the presence of light across the boundary of two substances. A photoconductive cell, which contains a semiconductor, increases its conductivity when exposed to electromagnetic radiation.

Photocells are used for photographers' exposure meters, burglar and fire alarms, automatic doors, and in solar energy arrays.