| 摘要 |
To detect an intrusion, the device projects a narrow beam (13) or, better, two beams (13) - (13') of infrared rays which are pulse-modulated, in the direction selected according to the circumstances by the user. An objective (19), mounted to the side of the exit aperture of the beam, concentrates part of the radiation which is back-scattered from the area of incidence (17) of the beam, into an almost point image the position of which is dependent upon the distance of the said point of incidence, this being a distance which must be within the useful range of the apparatus. A photocell (21) shielded by a thin slit (25) is positioned in such a manner as to collect the radiation forming the infrared image. As soon as the intruder intercepts the beam, the image of the region "illuminated" by the beam, which now is on the intruder, undergoes a movement as a result of which the rays, which cannot any longer pass through the slit, do not reach the sensitive surface of the photocell (21): the absence of the output signal of this photocell provides the information that the intrusion has taken place. <IMAGE>
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