The Baumer RR30.RAH5-GXPB.9VF radar retro-reflective sensor operates similar to a light barrier.
Via a special teach-in-algorithm, the sensor learns the reflector position. After teaching, the sensor monitors the reflector position.
If an object breaks the line of sight between the sensor and reflector, the sensor output switches. The advantage of this configuration is that it is unnecessary for the object to reflect a signal back to the sensor.
Therefore even randomly-shaped objects or radar absorbing objects can be reliably detected.