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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).
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