PIR (Passive Infra Red) - an electronic sensor used to detect motion.
Widely used in alarm systems, automatic lighting and ventilation systems, etc. Motion detection is based on a very precise temperature measurement. Any temperature change to a higher or lower level (e.g. cold air blowing into a heated room) is treated as an alarm. The main elements of the PIR sensor are the pyroelement (infrared detector) and the Fresnel lens or mirror.