The sensor is called passive because it gathers data through detection of radiation as opposed to transmitting a signal or electrons that bounce off the target. Infrared sensors are widely used in access control.
Passive infrared sensors contain two slots which are sensitive to infrared radiation. Rather than detecting an increase in temperature (the latter, for example, can happen because of the objects heating up in the sun), they record movement of a human being.
Normally two slots detect the same amount of IR which is emitted by the objects in the surrounding area. When a person or an animal passes by, they/it intercept(s) one half of the sensor. This makes the sensor detect a change in the level of infrared radiation in one slot. When a human body leaves the sensor area, the sensor generates a negative differential change. Thus, a sensor is activated by a moving warm spot and sends a signal to the alarm control panel.