The Microsoft Kinect doesn't just work by using the cameras to read your body image, there's an additional IR projector that covers the room in dots that the other two cameras can see. This makes it easy for the system to decode your movements, but it does make us feel slightly paranoid knowing that we are all painted up as a target while playing games.
arstechnica.com - One of the three "cameras" on the face of the Kinect is actually an IR projector, throwing out infrared light so the other (non-RGB) camera can get a sense of your room in 3D space. It's invisible to the naked eye, of course, but if you happen to have a pair of those inexpensive night vision goggles sitting around—like the kind that came with the most expensive edition of Modern Warfare 2—you can see just how much IR light the Kinect throws out.
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