Introduction to the function and principle of Fresnel lens, Fresnel lens (English: Fresnel lens), also known as threaded lens, is a kind of lens invented by French physicist Augustin-Jean Fresnel. This design was originally used in lighthouses. This design can build a lens with a larger aperture, which is characterized by a short focal length, and uses less material, weight and volume than ordinary lenses. Compared with earlier lenses, the Fresnel lens is thinner, so it can transmit more light, making the lighthouse visible even at a considerable distance.

Fresnel lens is a very widely used optical element. Its design and manufacturing design covers many technical fields, including optical engineering, polymer material engineering, CNC machining, diamond turning process, nickel plating process; molding, injection molding, Manufacturing processes such as casting.

Fresnel lens (Fresnel lens), also known as threaded lens, is mostly made of polyolefin material injection molding sheet, but also made of glass, one side of the lens surface is smooth, the other side is engraved with concentric circles from small to large , Its texture is designed according to the requirements of relative sensitivity and receiving angle using the interference of light, and the requirements of the lens are very high.

A high-quality lens must have a smooth surface and a clear texture. Its thickness varies with the application, mostly around 1mm. It is characterized by large area, thin thickness and long detection distance. Fresnel lens is equivalent to infrared and visible light convex lens in many cases, the effect is better, but the cost is much lower than ordinary convex lens. It is mostly used in occasions where accuracy is not very high, such as slide projectors, film magnifiers, infrared detectors, etc.

Fresnel lens uses the special optical principle of the lens to produce an alternating "blind zone" and "high sensitive zone" in front of the detector to improve its detection and reception sensitivity. When someone walks by the lens, the infrared rays emitted by the human body will alternately enter the “high-sensitivity area” from the “blind area”, so that the received infrared signal is input in the form of sudden strong and weak pulses, thereby increasing its energy amplitude .

Fresnel lens simply means that there are equidistant teeth on one side of the lens. Through these teeth, the optical bandpass (reflection or refraction) of the specified spectral range can be achieved. Bandpass optical filters for traditional polished optical equipment are expensive to manufacture. Fresnel lenses can greatly reduce costs.

The typical example is PIR. PIR is widely used in alarms. If you take one and take a look, you will find that there is a small plastic cap on each PIR. This is the 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 microns (the peak of infrared radiation from the human body).

The Fresnel lens can focus the light passing through the narrow-band interference filter on the photosensitive surface of the silicon photoelectric secondary detector. The Fresnel lens cannot be wiped with any organic solution (such as alcohol, etc.). Distilled water can be used to remove dust. Or rinse with ordinary clean water, then wipe with absorbent cotton.

Current camera focusing screens are all frosted frosted glass Fresnel lenses, which have the advantage of being bright and uniform in brightness. When the focus is not accurate, the image on the focusing screen is not clear. In order to coordinate with more precise focusing, a split image and micro-ring device are generally installed in the center of the focusing screen. When the focus is not accurate, the image of the subject in the center of the focusing screen splits into two images. When the two split images are merged into one, it indicates that the focus is accurate. The standard focusing screen of an AF SLR machine generally does not have a split image device, but is engraved with a small rectangular frame to indicate the AF area, and some focusing screens are also engraved with partial metering or spot metering areas. 

When the early AF SLRs were focusing in a dark environment, it was often difficult to see the focus frame, and it was difficult to determine which point the camera was using as the focus point. The focus point on the focusing screen of the new generation SLR camera would glow, or there would be a focusing sound. Prompt to confirm the focus in a complex environment. Different types of focusing screens have different purposes. For portraits, it is better to use split-image focusing screens. Focusing screens with horizontal and vertical lines or scales are suitable for building photography and document copying; there is no split image in the middle part but only micro-edge focusing The screen is suitable for small aperture lens, it will not have the shortcomings of split image which is bright and black. Many SLR cameras focus screens can be replaced by users. Also known as threaded lens.

Fresnel Lens is a fine-structured optical element, viewed from the front, it looks like a dart board, consisting of a ring of concentric circles. Its working principle is very simple: assuming that the refractive energy of a lens only occurs on the optical surface (such as the lens surface), remove as much optical material as possible while retaining the curvature of the surface.

Another understanding is that the continuous surface of the lens "collapses" onto a plane. Viewed from the cross section, its surface is composed of a series of zigzag grooves, with an elliptical arc in the center. Each groove has a different angle with the adjacent grooves, but they all concentrate the light in one place to form a central focal point, which is the focal point of the lens. Each groove can be regarded as an independent small lens, adjusting the light into parallel light or condensing light. This lens can also eliminate some spherical aberrations.

Round Fresnel lens, Fresnel lens array, cylindrical Fresnel lens, linear Fresnel lens, diffractive Fresnel lens, Fresnel reflective lens, Fresnel beam splitter and Fresnel prism.