Fresnel lens, in short, has equidistant teeth on one side of the lens. Through these teeth, we can achieve the function of optical band-pass (reflection or refraction) in the specified spectral range. The traditional band-pass optical filter for polishing optical equipment is expensive. 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 a small plastic hat on each PIR. This is the Fresnel lens. The inside of the little cap is all carved with teeth. The Fresnel lens can limit the peak frequency of incident light to about 10 microns (the peak value of human infrared radiation).

     There are two functions of Fresnel lens in the induction of thermoluminescence infrared: one is the focusing function, that is, the refraction (reflection) of thermoluminescence infrared signal on PIR; the second function is to divide the detection area into several bright areas and dark areas, so that the moving objects entering the detection area can produce the varying thermoluminescence infrared signal on PIR in the form of temperature change.