Photoelectric eyes consist of a transmitter, which sends out a beam of light, and a receiver which "sees" this beam. If someone walks through the beam the receiver will detect this break and can sound the alarm etc.

These devices require a straight line of sight between the transmitter and the receiver. Contrary to many movies, the light source used in toady's PE's can not be seen with the human eye. The receiver detect the infrared energy given off by all objects and will trip if it senses a movement of this energy above a pre-set limit. Photoelectric transmitters emit pulsed infrared beam that is invisible to the naked eye. Some units have a range over 1000 feet and can be used outdoor for perimeter protection. However for outdoor applications, one needs to keep in mind that birds and cats could also cut the beams and trigger false alarms.