PIR technology senses occupancy by detecting the difference between heat emitted from the human body in motion and the background space. Relying on a clear line-of-sight view, passive infrared sensors make 100 percent coverage cut off possible. These sensors utilize a unique fresnel lens which divides the coverage area into zones, enhancing detection of small movement. Using the Doppler principle, ultrasonic sensors work by bouncing ultrasonic sound waves off objects in an area and measuring the time it takes for the waves to return. Movement by a person in the area causes the sound waves to return at a changed frequency, resulting in a Doppler shift and occupancy detection.