More About Photoelectric Beam Detectors
Most of these devices feature two pieces. One piece is the side that contains the beam emitters and the other piece will be a reflector of sorts that stops the beam from going on infinitely. These also have their own power supplies and will not be powered from an alarm system.
Most of these units can be connected to a zone on an alarm system and used to make the system go into alarm when the beam is tripped. If the panel is tied to central monitoring then the Police will be called. You can also just use these as notification devices as well. Place them at an entrance to a store and have them ring a bell or sound a piezo when someone enters.