You will find a plastic hat on each PIR. This is the Fresnel lens, which has been used up to now. This Fresnel lens can limit the peak frequency of incident light to about 10 microns (the peak of human infrared radiation), and the Fresnel lens can greatly reduce the cost. A typical example is the PIR (Passive Infrared Detector), which is quite low cost if you take a look. Polyethylene Fresnel lenses are easy to process and form, and polyethylene plastic injection molding processes are now used. The inside of the small hat is engraved with a tooth pattern and is widely used in detectors with a detection distance of less than 30 meters. Originally used in navigation lighthouses in 1800, PIR is widely used on alarms.

The current Fresnel lens is inexpensive, and its advantages are brightness and uniform brightness, and the focus is not on time. In order to match more accurately, the image of the subject in the center of the focusing screen is split into two images. The imaging on the focusing screen is unclear, indicating that the focus is accurate and the light transmittance is good when the focus is not accurate. The signal received by the pyroelectric sensor is strong and easily causes false alarms. Generally, a split image and a micro-ring device are installed in the center of the focusing screen to resist the penetration of white light. However, the disadvantage is that the human body will lose some of the infrared light when it penetrates the lens. (This phenomenon is more serious when detecting long distances.) The three types are relatively rare on the market, and the infrared light emitted by the human body has strong penetrating power. The function is to prevent strong light interference, but its ability to resist white light is poor.

Most Fresnel lenses on the Fresnel lens market can be divided into three colors: a primary color of polyethylene. Two kinds of white. opaque. It is translucent or transparent. To prevent false alarms, the focusing screens are frosted frosted glass. The signal received by the pyroelectric sensor is weak. It is easy to cause the infrared detector to report the phenomenon. Not easy to be lost. It can be color-matched according to the color of the product, and the black is used for multiple points, when the two split images are combined into one.