Abstract: A system for sensing the presence of nearby objects includes an emitter-detector assembly and retroreflector. The emitter-detector assembly has a light emission area from which light is transmitted toward the retroreflector. The retroreflector returns that light to a light detection area of the emitter-detector assembly which produces a signal indicating an unbroken light path. Objects passing near the assembly interrupt the light path thereby providing an indication of their presence. The light emission area and the light detection area are concentric to each other which enables the light detection area to have close proximity to the light emission area. That parameter optimizes response to light from the retroreflector, while minimizing response to light reflected by other objects.