The RLK31 range of retro-reflective photoelectric sensors from Pepperl + Fuchs offers a compact sensor suitable for factory automation applications within industrial environments. Retro- reflective sensors work through emitting a beam of light which is then mirrored from a partnered reflector, the reflected light is then detected by the receiver unit. The sensors are triggered when the light beam between the sensor and reflector is broken by an object. Retro reflective sensors are used in part presence detection applications.