There are a couple of problems to be aware of with retroreflective
sensing. Most often you're waiting for the target to break the beam
and cause an output. If the target happens to be a bright shiny object,
such as a white phone book, for example, the target itself can reflect
light back to the sensor. As far as the sensor is concerned, the beam
was never broken. The reflector was bouncing light back to the receiver
lens, then the shiny target came along and itself served as a second
reflector! The receiver lens never stopped getting light back, so no
output occurs.
