Traditional introduction to Fresnel lens

You'll find a little plastic hat on each PIR.This is the Fresnel lens, which is still in use today.This Fresnel lens can limit the peak frequency of incident light to around 10 microns (the peak of infrared radiation in the body), which can greatly reduce the cost.The typical example is the PIR, which is pretty cheap, if you look at one.Because of the convenience of polyethylene Fresnel lens processing, the plastic injection molding process is mostly used.The inside of the cap is engraved with teeth, which are widely used in detectors within 30 meters.First used in navigational lighthouses in the 1800s, PIR is widely used in sirens.

Today's Fresnel lenses, which are inexpensive, have the advantage of being bright and even in brightness, and of being out of focus. In order to coordinate more accurate focus, the image of the subject in the center of the focus screen is split into two images.The image on the focus screen is not clear, indicating that the focus is accurate, the transmittance is good, when the focus is not accurate.The pyroelectric sensor receives a strong signal and is prone to false positives.In general, the center of the focusing screen is equipped with a split image and a micro-edge ring device, which can resist the penetration of white light.But the disadvantage is that the human body will lose part of the infrared ray when the infrared ray penetrates the lens, (this phenomenon is more serious when detecting a long distance) three kinds are used less on the market, the human body sends out the infrared ray penetrating power is strong.The function is anti - interference of strong light, but its ability to resist white light is poor.

Most Fresnel lenses on the market come in three colors: a polyethylene primary color.Two whites.Opaque.Translucent or transparent.Prevent false positives, focus screen is frosted glass.The pyroelectric sensor receives a weak signal.It is easy to cause the phenomenon of infrared detector missing.Not easily lost.Can be color according to the product color, black with the multi-point, when the two split images into one.