A photoswitch is a kind of sensor that detects the presence of light, generally in a certain wavelength. Photoswitches are used for numerous scientific and manufacturing applications. They are also commonly used in residential areas to turn lights on and off based on the time of day. It is a photo-electric cell connected to a circuit that measures how much electricity the cell produces and according the setting of minimum and maximum lux level, the circuit decides and gives the output.
In popular culture (popular culture) Photoswitch shows itself in many places, not least of which is Star Trek. In Episode 207, entitled "Rules of Acquisition", the Ferengi character Quark receives a large load of "Photoswitch" in the early part of the show. From the episode: