A hollow flexible vinyl or other plastic tube having a shiny inner wall is disposed along a boundary where intrusion detection is desired. A beam of infrared radiation from an LED at one end of the tube is passed through the tube by reflections from the wall to a detector at the other end. The radiation...http://www.google.ca/patents/US4829174?utm_source=gb-gplus-sharePatent US4829174 - Flexible tube optical intrusion detector
