Optical surveillance esp. for secure doors or windows - relies upon variation of attenuation in light guide, caused by deformation through mechanical coupling to secured object

摘要

A light-emitting diode (3) is brightness-modulated by an amplifier (2) of square waves from a generator (1). A mode filter (4) excludes higher-order modes from the lightguide (5) which passes through a succession of mechanical movement detectors (6) coupled to the secured objects (7). The lightguide (5) is terminated at an optical receiver (8, 9) with a sample-and-hold circuit (10) switched in synchronism with the square wave generator (1). Any change in attenuation due to lightguide deformation activates an alarm (11). ADVANTAGE - Alarm installation cannot be influenced by electric or magnetic fields and can be made practically invisible.