Introduction to Fresnel lens

　　It was first used in navigation lighthouse in 1800, and has been used since then. Now, polyethylene plastic injection molding process is mostly used. Fresnel lens can greatly reduce the cost. A typical example is PIR (Passive Infrared Detector). PIR is widely used in alarm. If you take a look at one, you will find that there is a small plastic cap on each PIR. This is the Fresnel lens. The inside of the small cap is engraved with tooth patterns. This Fresnel lens can limit the frequency peak of incident light to about 10 microns (the peak value of human infrared radiation). The cost is quite low. Polyethylene Fresnel lens is widely used in detectors with detection distance less than 30 meters due to its convenient processing and low price.

　　The current Fresnel lens and focusing screen are all frosted ground glass, with the advantages of brightness and uniform brightness. The imaging on the focusing screen is not clear when the focus is not in time. In order to coordinate with more accurate focusing, generally a split image and micro edge ring device are installed in the center of the focusing screen. When the focusing is not correct, the image of the subject in the center of the focusing screen is split into two images. When the two split images are combined, it indicates that the focusing is accurate.

　　Most Fresnel lenses on the market can be divided into three colors: a primary color of polyethylene material, which is semi transparent or transparent, with good light transmittance. The infrared light emitted by the human body has strong penetrability and is not easy to be lost, and the signal received by the heat release sensor is strong. However, it has poor white light resistance and is easy to cause false alarms. Two kinds of white, opaque. It can resist the penetration of white light and prevent false alarm. But the disadvantage is that when the human body's infrared ray penetrates the lens, it will lose part of the infrared ray, and the signal received by the thermoluminescence sensor is weak, which is easy to cause the phenomenon that the infrared detector fails to report. (This phenomenon is more serious when detecting a long distance.) The three types are less used in the market, and can be matched according to the color of the product. The black is used for multiple points to prevent strong light interference.

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