A photoelectric sensor, or photoeye, is a evice used to detect the presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. There are three functional types. An opposed arrangement consists of a receiver located within the line-of-sight of the transmitter. In this mode, an object is detected when the light beam is blocked from getting to the receiver from the transmitter. A retroreflective arrangement places the transmitter and receiver at the same location and uses a reflector to bounce the light beam back from the transmitter to the receiver. An object is sensed when the beam is interrupted and fails to reach the receiver. A proximity-sensing arrangement is one in which the transmitted radiation must reflect off of the object in order to reach the receiver. In this mode, an object is detected when the receiver sees the transmitted source rather than when it fails to see it.