| 摘要 |
A device for detecting the presence of an object in a region to be monitored of the kind comprising light emitting means, electronic light receiving means, and an optical system having means for focussing the light emitted by the light emitting means and means for focussing any light reflected by an object in the monitored region and passing to the light receiving means. The respective focussing means, which are preferably incorporated in a common block moulded from a light permeable material such as glass or plastics material, are so arranged relative to one another that the path of the light rays from the light emitting means and the path of any light rays passed to the light receiving means meet at an angle to one another in the region to be monitored.
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