A photocell is a technological application of the photoelectric effect. It is a device whose electrical properties are affected by light. It also some times called an electric eye. A photocell consists of a semi cylindrical photo sensitive metal plate c (emitter) and a wire loop A (collector) supported in an evacuated glass or quartz bulb. It is connected to the external circuit having a high tension battery B and micro ammeter as shown in the figure.
Sometime instead of the plate C, a thin layer of photosensitive material is pasted on the inside of the bulb. A part of the bulb is left clean for the light to enter it. When light of suitable wavelength falls on the emitter C, photo electrons are emitted. These photo electrons are drawn to the collector A. Photo current of the order of a few micro ampere can be normally obtained from a photo cell.
A photocell converts a change in intensity of illumination into a change in photo current. This current can be used to operate control systems and in light measuring devices. A photocell of lead sulphide sensitive to infrared radiation is used in electric ignition circuits.
In scientific work, photo cells are used whenever it is necessary to measure the the intensity of light. Light meters in photographic cameras make use of photo cells to measure the intensity of incident light. The photocells, inserted in the door light electric circuit, are used as automatic door opener. A person approaching a doorway may interrupt a light beam which is incident on a photocell. The abrupt change id photo current may be used to start a motor which opens the door or rings an alarm. They are used in the control of a counting devices which records every interruption of the light beam caused by a person or object passing across the beam. So photocells help count the person entering an auditorium, provided they enter the hall one by one. They are used for detection of traffic law defaulters: an alarm may be surrounded whenever a beam of (invisible) radiation is intercepted.
In burglar alarm, (invisible) ultraviolet light is continuously made to fall on a photocell installed at the door. A person entering the door interrupts the beam falling on the photocell. The abrupt change is photo current is used to start an electric bell ringing. In fire alarm, a number of photocells are installed at suitable places in a building. In the event of breaking out of fire, light radiations fall upon the photocell. This completes the electric circuit through an electric bell or a siren which starts operating as a warning signal.
Photocells are used in the reproduction of sound in motion pictures and in television camera for scanning and telecasting scenes. They are used in industries for detecting minor flaws or holes in metal sheets.
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