Any of several devices that produce an electric signal in response to exposure to electromagnetic radiation. The original photocells utilized photoemission from a photosensitive cathode (photocathode). The electrons emitted are attracted to an anode. A positive potential on the anode enables a current to flow through an external circuit, the current being proportional to the intensity of the illumination on the cathode. The electrodes are enclosed in an evacuated glass tube (see also photomultiplier).
More modern light-sensitive devices utilize the photoconductive effect and the photovoltaic effect (see photoelectric effect; photodiode; phototransistor; solar cell).