Every person radiates a certain amount of heat from their body. Infrared sensors will detect these fluctuations and the sudden rise in voltage will cause the occupancy sensor to switch the light "On." Some of these pyro-systems are designed to sense movement as well. In order to detect movement, the system divides its segment lens into active and passive zones. When heat radiations pass through the senor zones, a signal is emitted, and the light will switch "On."