An energy field disturbance sensor comprises an energy field emission means such as a light emitting diode; the emitted field may contain a time-varying component. An energy sensing device such as a photodiode receives the field energy to produce a sensing signal related to the intensity of the emitted...http://www.google.de/patents/US4879461?utm_source=gb-gplus-sharePatent US4879461 - Energy field sensor using summing means
