The function of Fresnel lens

Fresnel lens uses the special optical principle of lens to produce 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 camera, the infrared emitted by the human body continuously enters the "high sensitive area" alternately from the "blind area", so that the received infrared signal is input in the form of strong and weak pulses, thereby enhancing its energy amplitude.

Fresnel lens has two functions: one is to focus, that is, to refract (reflect) pyroelectric infrared signals onto PIR, and the other is to divide the detection area into several bright areas and dark areas, so that moving objects entering the detection area can generate changing pyroelectric infrared signals in the form of temperature changes on PIR. Fresnel lens is simply that one side of the lens has equidistant teeth. Through these teeth, it can transmit (reflect or refract) light in a specific spectral range. Traditional bandpass filters used for polishing optical equipment are expensive. Fresnel lens can greatly reduce the cost.

A typical example is PIR. PIR is widely used in alarms. If you take a look, you will find a small plastic cap on each PIR. This is a Fresnel lens. The inside of the little hat is engraved with teeth. This Fresnel lens can limit the frequency peak of incident light to about 10 micron (the peak of human infrared radiation).

Fresnel lens can focus the light passing through narrow-band interference filter on the photosensitive surface of silicon photoelectric secondary detector. Fresnel lens can not be wiped with any organic solution (such as alcohol), but can be washed with distilled water or ordinary water and then wiped with absorbent cotton.

At present, the focusing screen of the camera is frosted glass Fresnel lens, which has the advantages of bright and uniform brightness. When the focus is not correct, the image on the focused screen is not clear. In order to cooperate with more accurate focusing, image splitting and micro-edge ring devices are generally installed in the center of the focusing screen. When the focus is not correct, the object image in the center of the focused screen is divided into two images. When two segmented images are merged into one, it shows that the focus is accurate. The standard focusing screen of AF SLR is generally not equipped with image splitting device, but is engraved with a small rectangular frame to represent the autofocus area, and some focusing screens are also engraved with partial metering or spot metering areas. When the early autofocus SLR focused in a dark environment, it was often difficult to see the focus frame clearly, and it was difficult to judge which point the camera focused on. The focus point on the new generation SLR focusing screen will glow or have a focus sound prompt, which is convenient to confirm the focus in a complex environment. Different types of focus screens have different uses, such as split focus screens, which are suitable for architectural photography and document remake. There is no split image in the middle, only the micro-edge focusing screen is suitable for small aperture lens, and there will be no defect that the split image is bright and black at the same time. The focal screen of many SLR cameras can be replaced by users themselves. Also known as threaded lens.