Retroreflective Sensors, Reflector

Size of Reflector

Another thing to be aware of with retroreflective mode sensing is the size of the reflector. It in effect becomes the receiver lens, because if the target is not bigger than the reflector, it cannot block all the light being bounced back from the reflector to the sensor.

Effective Beam and Retroreflective Sensors

The receiver lens in a retroreflective sensing application becomes the reflector itself. The target must be big enough not to break the "effective beam", but in fact it must be large enough to break the reflector's light as well. In other words, if your receiver lens is 3/8", that would normally be your concern. If your target was greater than 3/8", it should break the effective beam (the size of the receiver lens). But not so with retroreflective sensing. If the reflector is 3", then the target needs to be greater than 3".
The example at above left demonstrates what happens when the target is smaller than the reflector. Light from the reflector wraps around the sides of the small target, therefore the sensor is always seeing light. No output occurs, because the beam is never fully broken. With retroreflective sensing, you've got a beam from the sensor, and light coming back from the reflector.