Solid-state device with a photosensitive cathode that emits electrons when illuminated and an anode for collecting the emitted electrons. Illumination excites electrons, which are attracted to the anode, producing current proportional to the intensity of the illumination. In a photovoltaic cell, light is used to produce voltage. In a photoconductive cell, light is used to regulate the flow of current. Photocells are used in control systems, where interrupting a beam of light opens a circuit, actuating a relay that supplies power to a mechanism to bring about a desired operation, such as opening a door or setting off a burglar alarm. Photocells are also used in photometry and spectroscopy.