| 摘要 |
Array of Fresnel lenses moulded in a single sheet of plastic 29 are used in conjunction with a thermal radiation detector 1 to provide intruder alarm apparatus. Each lens defines one detection direction in a fan of such directions 26,27,28. Such arrays have been moulded as a curved surface 22 centered on the detector. To reduce moulding costs while maintaining the optical efficiencies of all lenses in the array, a flat array of lenses is provided in which each lens is moulded as an angled facet 30,31,32. The poles 36,37,38 of the lenses lie in a plane.
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