Fresnel lens function

Fresnel lenses often serve as convex lenses for both infrared and visible light, delivering good performance at a significantly lower cost than conventional convex lenses. They are commonly used in applications where high precision is not required, such as slide projectors, thin-film magnifiers, and infrared detectors.

A Fresnel lens leverages its unique optical principles to create alternating “blind zones” and “high-sensitivity zones” in front of the detector, thereby enhancing its detection sensitivity. When a person walks past the lens, the infrared radiation emitted by the human body continuously transitions between these blind and high-sensitivity zones, causing the received infrared signal to be modulated into pulses of fluctuating intensity and thus increasing the signal’s energy amplitude.

The Fresnel lens serves two functions: first, it focuses the pyroelectric infrared signal by refracting (or reflecting) it onto the PIR detector; second, it divides the detection area into a series of bright and dark zones, enabling moving objects that enter the detection zone to generate varying pyroelectric infrared signals on the PIR as temperature changes.

A Fresnel lens, in simple terms, features equidistant grooves on one of its surfaces; these grooves enable bandpass filtering—either by reflection or refraction—of light within a specified spectral range. Conventional polished optical bandpass filters are costly to manufacture, whereas Fresnel lenses can significantly reduce production costs.

Key words:

Fresnel lens