Fresnel lens, also known as threaded lens, Fresnel lens is mostly made of polyolefin material, the surface of the lens is smooth, and the other side is filled with concentric circles from small to large. Its texture is designed to utilize the interference of light and to be based on relative sensitivity and acceptance angle requirements. The quality requirement of the lens is very high. A good lens must have a smooth surface, clear texture. Its thickness varies with the application and normally about 0.6 mm. The lens characteristics are wider surface, thinner and farther detection distance.
There are two functions of Fresnel lens: one is the focusing effect; the other is to divide the detection area into several bright and dark areas, so that the moving object entering the detection area can produce the change of heat release on the PIR in the form of temperature change. It is what we called Infrared signal.