How Fresnel lenses work

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Fresnellens (Fresnellens) is a sheet made of polyolefin material. The surface of the lens is smooth on one side and the concentric circles on the other side are small to large. Its texture is the use of light to involve refraction and The relative sensitivity and receiving angle requirements are designed. The requirements of the lens are very high. A high-quality lens must have a smooth surface and a clear texture. The thickness is generally about 1 mm. The characteristics are large area, thin thickness and long detection distance.

There are two Fresnel lens functions: one is the focusing effect, that is, the pyroelectric infrared signal is refracted (reflected) on the PIR, and the second function is to divide the detection area into several bright and dark areas to enter the detection area. The moving object can produce a varying pyroelectric infrared signal on the PIR in the form of a temperature change.

The Fresnel lens simply means that there are equidistant ridges on one side of the lens, through which the bandpass (reflection or refraction) of the specified spectral range can be achieved. Traditional band-pass optical filters for polishing optical equipment are expensive, and Fresnel lenses can greatly reduce costs. A typical example is the PIR (Passive Infrared Detector), which is widely used on alarms. If you take a look, you will find a small plastic hat on each PIR. This is the Fresnel lens, and the inside of the small hat is engraved with a tooth pattern. This Fresnel lens can limit the frequency peak of incident light to about 10 μm (the peak of human infrared radiation), and the cost is quite low.

The Fresnel lens can focus the light transmitted through the narrow-band interference filter on the photosensitive surface of the silicon photodiode detector. The Fresnel lens is made of plexiglass and cannot be wiped with any organic solution (such as alcohol). When removing dust, rinse with distilled water or ordinary clean water first, then wipe with absorbent cotton.

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