Fresnel lens

Fresnel lens is equivalent to infrared and visible light convex lens in many cases, the effect is better, but the cost is much lower than ordinary convex lens. It is mostly used in occasions where accuracy is not very high, such as slide projectors, film magnifiers, infrared detectors, etc.
The Fresnel lens uses the special optical principle of the lens to produce an alternating "blind zone" and "high-sensitivity zone" in front of the detector to improve its detection and reception sensitivity. When someone walks in front of the lens, the infrared rays emitted by the human body will alternately enter the [high-sensitivity area" from the [blind area", so that the received infrared signal is input in the form of sudden strong and weak pulses, thereby increasing its energy amplitude .
Fresnel lens
Fresnel lens
The Fresnel lens has two functions: one is focusing, that is, the pyro-infrared signal is refracted (reflected) on the PIR, and the second is to divide the detection area into several bright and dark areas, so that it can enter the detection area A moving object can produce a changing pyro-infrared signal on the PIR in the form of a temperature change.
The Fresnel lens, in simple terms, has equidistant tooth patterns on one side of the lens. Through these tooth patterns, the optical bandpass (reflection or refraction) of the specified spectral range can be achieved. Bandpass optical filters for traditional polished optical equipment are expensive to manufacture. Fresnel lenses can greatly reduce costs.