With retro-reflective photoelectric sensors, or reflector photoelectric sensors, the transmitter and receiver are in a single housing, just as with diffuse proximity photoelectric sensors. However, the sensor always looks at a reflector which is placed opposite the sensor. The output of the sensor becomes active as soon as an object moves inbetween the sensor and the reflector. Unlike with normal reflective sensors, polarized retro-reflective photoelectric sensors use special polarized filters so that the receiver accepts only light reflected by the reflector and not by another reflective object, such as, for example, a metallic paint of a car. Thus, it is possible to detect translucent and reflective objects as well.