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Technical Science——Fresnel lens in pyroelectric sensor

Fresnel lens was invented by French physicist Augustin Fresnel (Augustin. Fresnel). Assuming that the refraction energy of a lens only occurs on the optical surface (such as: the surface of the lens), remove as much optical material as possible, while retaining the curvature of the surface, the thickness of the lens will also become thinner.

May 23,2023

Fresnel lens was invented by French physicist Augustin Fresnel (Augustin. Fresnel).

Assuming that the refraction energy of a lens only occurs on the optical surface (such as: the surface of the lens), remove as much optical material as possible, while retaining the curvature of the surface, the thickness of the lens will also become thinner. As shown below:

Fresnel lens

The continuous surface portion of the lens "collapses" into one plane. Viewed in section, its surface is composed of a series of zigzag grooves, and the central part is an elliptical arc. Each groove has a different angle from the adjacent grooves, but they all concentrate the light in one place to form a central focus, which is the focal point of the lens. Each groove can be regarded as an independent small lens, which adjusts the light into parallel light or concentrated light. This lens also eliminates some of the spherical aberration.

Fresnel lenses are mostly made of polyolefin materials, and can also be made of glass. One side of the lens is a smooth surface, and the other side is concentric circles from small to large. Its texture is based on the interference, diffraction and combination of light. It is designed according to the requirements of relative sensitivity and receiving angle.

Fresnel lens

In a passive infrared detector (PIR), a Fresnel lens can divide the detection area into several bright and dark areas. The moving object that enters the detection area can generate a changing pyro-infrared signal on the passive infrared detector (PIR) in the form of temperature change.

Fresnel lens

We know that the pyroelectric sensor works by detecting the infrared rays emitted by the human body at around 10µm. The 10µm infrared rays emitted by the human body are enhanced by the Fresnel lens and then gathered to the pyroelectric element. When this kind of component receives changes in the infrared radiation temperature of the human body, it will lose the charge balance and release the charge outward, and the subsequent circuit will generate an alarm signal after detection and processing.

pir sensor

Pyroelectric module with Fresnel lens

Fresnel lenses have different focal lengths (sensing distances) according to different performance requirements, resulting in different monitoring fields of view.

Infrared Fresnel lenses are easy to mold, milky white plastic, with the least absorption loss in the range of 8-14um, and can achieve consistent size, large aperture and minimum expansion coefficient.

Fresnel lens Other uses

1) Fresnel magnifying glass
The Fresnel magnifying glass can be made ultra-thin, and the minimum thickness can be between 0.45-0.90mm. Different from ordinary magnifying glass, its surface is covered with tiny fine lines, each ring zone is equivalent to an independent refraction surface, and the cross section is like many small prisms, and these ring zones can make the incident light converge to a common focus .

Because the Fresnel lens is very thin, very little light energy is lost due to absorption. The disadvantage is that the distortion is relatively large and the image distortion is serious.

2) LED collimation
Fresnel lenses can better collimate point light sources into parallel light sources.

In practical applications, it is mostly used for collimation of LED light sources.

3) Concentrating solar energy

One of the most common applications of Fresnel lenses is to collect sunlight, which is generally considered to be very close to parallel (infinite conjugate system). The light-harvesting properties of Fresnel lenses are ideal for concentrating light onto photovoltaic cells or heating surfaces.

Concentrating solar energy

In solar cells, Fresnel lenses are an important part of concentrated solar systems (CPV)

Fresnel lenses

4) Projector
The advantage of Fresnel lenses used in projection systems is to increase display brightness by focusing or collimating light.

Projector