A Fresnel lens, passive infrared detector and security system

Abstract

The invention is applicable in the optical field and provides a Fresnel lens, a passive infrared detector and a security system. The Fresnel lens comprises a plurality of strip-shaped Fresnel lens units which are the same in structure; the plurality of strip-shaped Fresnel lens units are arranged in parallel in turn along a width direction thereof; each Fresnel lens unit comprises an exit surfaceand a smooth incidence surface; an arc-shaped exit region is arranged on the exit surface; a plurality of first concentric circular arc tooth grains are arranged above the arc-shaped exit region in turn along the height direction of the Fresnel lens unit; circle centres of the first tooth grains are superposed with the circle centre of the arc-shaped exit region; the depths of the plurality of first tooth grains are equal; and the distances between every two adjacent first tooth grains are progressively decreased outwardly along the centre of the arc-shaped exit region. Because the arc tooth grains are arranged above the arc-shaped exit region, the effective detection height of the lens within a detectable distance is higher. Because infrared ray emitted by a lower movable object cannot be received by the lens, unnecessary alarm is avoided.

Description

technical field [0001] The invention belongs to the field of optics, and in particular relates to a Fresnel lens, a passive infrared detector and a security system. Background technique [0002] The Fresnel lens is mainly made of polyolefin material by injection molding. One side of the lens surface is smooth, and the other side is engraved with multiple concentric scallops from small to large. The Fresnel lens is equivalent to a convex lens for infrared and visible light to a certain extent. It has a good light-converging effect, and its cost is much lower than that of an ordinary convex lens. It is easy to form in production, so it is widely used. The most common one is installed in the detector of the security system as a focusing lens. The Fresnel lens divides the detection area into several bright areas and dark areas. Moving objects entering the detection area pass through the bright and dark areas. The infrared light is focused on the passive infrared sensing element...

AI Technical Summary

Problems solved by technology

However, the existing detectors will generate an alarm signal for any moving object that intrudes into the detection area. When a small animal or child breaks in, the alarm will also alarm, which will bring unnecessary trouble to the staff.

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