also phototube, electron tube in which the electrons initiating an electric
current originate by photoelectric emission. In its simplest form the
phototube is composed of a cathode, coated with a photosensitive material,
and an anode. Light falling upon the cathode causes the liberation of
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 door openers. 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 energizes 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 Photography: Lenses: Light Metering).