Motion sensors for security.
Active Infrared Sensors
Active sensors depend on transmissions and feedback to pick up changes in the area of coverage. By sending out a constant stream of Infra-red, these sensors then measure and compare changes from previous readings. Due to this perpetual back and forth activity, active sensors consume a larger amount of energy compared to passive alternatives.
Passive Infrared Detectors
An infrared sensor, otherwise known as passive infrared (PIR), is the only option available in this category. These detectors work on a simple premise. Instead of sending out emissions and comparing previous readings, the devices simply wait for a heat signature. The human body, along with most other animals, is a source of heat. As this heat is released from the body, it can be viewed as an infrared light.