Photoelectric Cell

Photoelectric Cell, also phototube, electron tube in which the electrons initiating an electric current originate by photoelectric emission (see Photoelectric Effect). In its simplest form the phototube is composed of a cathode, coated with a photosensitive material, and an anode. Light falling upon the cathode liberates electrons, which are then attracted to the positively charged anode, resulting in a flow of current proportional to the intensity of the irradiation. Phototubes may be highly evacuated or may be filled with an inert gas at low pressure to achieve greater sensitivity. In a modification called the multiplier phototube, or the photomultiplier, a series of metal plates are so shaped and arranged that the photoelectric emission is amplified by secondary electron emission. The multiplier phototube is capable of detecting radiation of extremely low intensity, hence it is an essential tool for those working in the area of nuclear research.

The photoelectric cell, popularly known as the electric eye, is employed in operating burglar alarms, traffic-light controls, and automatic doors. A phototube and a beam of light (which may be infrared or invisible to the eye) form an essential part of such an electric circuit. The light produced by a bulb at one end of the circuit falls on the phototube located some distance away. Interrupting the beam of light breaks the circuit. This in turn causes a relay to close, which activates the burglar-alarm, or other circuit. Various types of phototubes are used in sound recording, television, and the scintillation counter (see Particle Detectors). They are also used in exposure meters (see Photographic Techniques).