What is fresnel lens?
Fresnel lens Introduction
The Fresnel lens was invented by French physicist Augustin Fresnel, who originally used this lens design in 1822 to build a glass Fresnel lens system – a lighthouse lens. The Fresnel lens is a microstructured optical element that looks like a dartboard from the front and consists of concentric circles.
A Fresnel lens, in simple terms, has equidistant teeth on one side of the lens, through which the light bandpass (reflection or refraction) of a specified spectral range can be achieved.
Traditional bandpass optical filters for polished optical equipment are expensive. Fresnel lenses can greatly reduce costs. A typical example is PIR (Passive Infrared Detector).
PIR is widely used in alarms. If you take a look, you will find that there is a small plastic cap on each PIR. This is a Fresnel lens. The inside of the small cap is engraved with teeth. This Fresnel lens can limit the frequency peak of the incident light to about 10 microns (the peak value of infrared radiation from the human body).
Fresnel lens should not only focus the infrared radiation from the monitoring space to the sensor, but also be able to keenly detect the changes in these infrared energies.
When designing a lens, the smooth mirror surface is often processed to produce a series of alternating “high-sensitivity sensing areas” and “blind areas” in the monitored space. When someone walks in front of the lens, the infrared rays of the human infrared sensor will continuously enter the “blind area” from the “high-sensitivity sensing area”, and the infrared rays transmitted to the infrared sensor will be intermittent, that is, a large number of light pulses enter the infrared detection element, and after conversion, the correspondingly changing electrical pulses are output, thereby improving its receiving sensitivity and greatly improving its detection distance.
