| 摘要 |
PURPOSE:To determine a detector area widths in accordance with the area lengths of areas by reflecting heat rays passing a Fresnel lens by a reflecting mirror to be incident upon a pyroelectric element in case of a long distance wherein the area lengths are large and making the heat rays directly incident upon the element in case of the areas wherein a short distance is aimed at. CONSTITUTION:In case of long distance areas (1) and (2), heat rays coming from an object to be detected via a Fresnel lens 2 are reflected from a reflecting mirror 4 to be incident upon a pyroelectric element 1. In case of a short distance area (3), the heat rays are directly incident upon the pyroelectric element 1 without being reflected by the reflecting mirror 4. At this time, a reflecting mirror 6 is divided into a plurality of reflecting mirror elements 6a, 6b... in accordance with the number of the division of the Fresnel lens 2. Thus, constant and stable detecting sensitivity can be obtained in the areas covering long to short distances.
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