Photocell is a technological application of the photoelectric effect. It is a device whose electrical properties are affected by light. It is also sometimes called an electric eye. A Photocell consists of a semi-cylindrical photosensitive 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 (µA). Sometimes, 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 inter it. When light of suitable wavelength falls on the emitter C, photoelectrons are emitted. These photoelectrons are drawn on the collector A. Photocurrent of the order of a few microampere can be normally obtained from a photocell. A photocell converts a change in intensity of illumination into a change in photocurrent. This current can be used to operate control systems and in light measuring devices. A photocell of lead sulphide sensitive to infrared radiations is used in electronic ignition circuits. In scientific work, photocells are used whenever it is necessary to measure the intensity of light. Light meters in photographic cameras make use of photocells to measure the intensity of incident light. The photocells, inserted in the door light electric circuit, are used as automatic door opener.