A lighting sensor (PIR) is a small, inexpensive device which detects whether a human has moved into or out of the sensor’s range. They are commonly found in numerous appliances used in homes or businesses.
Everything emits low level radiation. The sensor will detect motion and not average infrared levels as it is made up of two halves. The two halves are wired in such a way that if one half sees more or less infrared radiation than the other, the output will move from high or low or vice versa.