| 摘要 |
<p>An element of an intrusion-alarm system, for example a passive infra-red sensor (1), has two subsystems which protect it against obstruction. The first subsystem (11, 13, 15) can detect attempts at obstruction by the removal of the lens or front screen (3) of the element (1) or by the spraying of the element (1) with a spray or a lacquer. The second subsystem (16a, 16b, 18) projects, towards the area under surveillance, radiation which can be reflected by a masking screen. The detection (18) of this reflected radiation gives rise to the emission of a warning signal. <IMAGE></p> |
