Beam Detector uses invisible light to scan a defined area. The transmitteremits a beam of light into the scan zone. The light, which is reflected by the background returns to the receiver, which constantly monitors the scan zone. When a person or object enters the zone the infrared light is interrupted. It then sends a signal to the internal relay, which is wired into the door controls. One variation of this operating mode is called "background suppression". This is when the receiver only detects a change in the reflected light when a person or object enters the scan zone thus causing a reflectance variation of the light, sending a signal to the internal relay.