Photo eyes are invisible beams that stretch across the vehicle lane. If the beam is broken, the photo eye output activates. Photo eyes come in 2 different flavors: reflective and a transmitter/receiver pair. The reflective photo eye uses a reflector to bounce the invisible beam back to the photo eye. The receiver/transmitter pair photo eyes have a transmitter photo eye on one end and a receiver photo eye on the other end. The transmitter/receiver photo eye can achieve higher distances. Photo eyes are typically used with swing and slide gates.