Do motion detectors work in complete darkness?
Sorry, Dave, but… The typical IR motion detectors (like the things with the little milky-white “window” on security lights) emit no light or infrared of their own; they rely on picking up infrared radiation given off by people. So, yes, they work just fine in complete darkness. They “view” the scene through a special type of Fresnel lens and an aperture grill that is something like a picket fence. These are arranged so that as a person crosses the area “seen” by the detector, the IR they give off alternately comes through and does not come through the vertical openings in the grill, so it alternately reaches and does not reach the sensor. Thus movement across the field results in a changing output of the sensor. The sensor does not need to see anything like images – all it detects is changes in the level of received IR. A sensor with an ordinary lens and no grill would NOT detect any changes as someone moved across the field. These sensors do, however, rely on the people being *alive
