Fresnel lens, also known as screw lens, was invented by French physicist Augustine Fresnel. In 1822, he initially used the lens to build a glass Fresnel lens system, i.e., the lighthouse lens. Fresnel lenses are mostly thin sheets made of polyolefin materials and glass. One side of the lens surface is smooth, and the other side is burned with concentric circles from small to large. Its texture is designed according to the light interference and the requirements of the relative sensitivity and receiving angle. The requirement for the lens quality is very high. A high-quality lens must have a smooth surface and clear texture. Its thickness varies with the application, mostly about 1mm. It is charactered by large area, thin thickness and long detection distance.
There are two functions of Fresnel lens: one is focusing; the other is to divid the detection area into several bright and dark areas, so that the moving object entering the detection area can generate varying thermoluminescence infrared signal on PIR in the form of temperature change. Compared with the convex lens of infrared and visible light, Fresnel lens has better effect, but its cost is much lower than that of ordinary convex lens. Fresnel lens is generally used in situations with low accuracy requirement, such as slide projector, film magnifier, infrared detector, etc.