In most cases an optical triangulation sensor is mounted near the top of the swinging door frame, inside a long extruded aluminum tube with an optical window facing the floor. An 880nm LED light source emits a collimated light beam that is emitted from the sensor , bounces off the floor and is received by a photodiode positioned adjacent to the LED source. A second photodiode (or a linear array of photodiodes) are positioned farther along the length of the sensor. When the emitted beam bounces off the floor, the reflected energy is almost exclusively concentrated on the first adjacent photodiode. When a human or object moves into the optical path, the reflected beam bounces back from the object. Since the beam is no longer traveling the full optical path length to the floor then receives the optical energy and the sensor responds by sending a signal to the automatic door controller. At that point the door stops, slows or reverses, hence averting a collision and providing safety.
Triangulation sensors are found on a number of door brands and manufactured by several sensor manufacturers. One example is the SuperScan-II from BEA. Other products from other manufacturers are also available.