In this system for the securing of rooms, one or more transmitting elements emit suitably modulated light pulses into given regions or sectors which are produced by specially shaped optical components and/or by a special alignment of the transmitting elements and/or by the application of absorbent layers...http://www.google.ca/patents/US4760381?utm_source=gb-gplus-sharePatent US4760381 - Intruder-detection system for room security
