The beam bounces off the floor and is received by a photodiode positioned adjacent to the LED source.
When the emitted beam bounces off the floor, the reflected energy is almost exclusively concentrated on the first adjacent photodiode.
Since the beam is no longer traveling the full optical path length to the floor, its reflected angle changes.
One of the adjacent photodiodes 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.