What Is a Fresnel Lens?

Fresnel lens, also known as threaded lens, is mostly a thin sheet made of polyolefin material. It is also made of glass. One side of the lens surface is smooth and the other side is recorded with small to large concentric circles Its texture is designed according to the interference of light and the requirements of relative sensitivity and receiving angle. ^[1]

Fresnel lenses are often equivalent to infrared and visible light.

Fresnel lenses are currently used in projection and solar photovoltaic applications. Fresnel lenses are often used on stained lights because they have softer edges. A colored plastic film is placed on the bracket in front of the lens to dye the light, or a metal gauze or frosted plastic can be placed to diffuse the light. Many devices that contain Fresnel lenses allow the lamp to move back and forth in focus to magnify or reduce the size of the beam. It is ideal for use in lenticular projectors, rear-projection televisions, slide projectors, and collimators, not only because of its transmission The brightness of the light is higher than that of the ordinary lens, and the brightness of the entire light passing through it is relatively consistent in all parts.

In the field of solar photovoltaic, Fresnel is mainly used as a light-concentrating component in a concentrated photovoltaic system, converting light from a relatively large area to a relatively small area. Cheap Fresnel lenses are generally die-cast or molded from transparent plastic. Their size can be made larger than glass while being lighter and more economical. Therefore, large Fresnel lenses are also widely used in solar stoves. Sun or solar water heater. In addition, Fresnel lenses are also widely used in car headlights, car taillights and reversing lights. It can tilt the parallel light originally reflected by the concave mirror from the headlight downward. Therefore, the Fresnel lens is also used to correct some visual impairments, such as strabismus. ^[2]

Some people in the world have developed large Fresnel lenses in an attempt to make solar concentrators. Fresnel lens is a flat condenser lens, which is light in weight and relatively low in price. It is also somewhat focused and line focused. It is generally made of plexiglass or other transparent plastics. Power system.

From the 1970s to the 1990s, China has developed Fresnel lenses for solar devices. Some people use a compression molding method to process large-area flexible transparent plastic Fresnel lenses, and some use a combination molding tool to process a point-focusing Fresnel lens with a diameter of 1.5m, but the results are not ideal. Recently, some people have used a molding method to process linear glass Fresnel lenses, but the accuracy is not enough and needs to be improved. There are also two new types of solar concentrators designed using the principle of total reflection. Although they have not yet been practically used, they are instructive. One is an optical fiber concentrator, which is composed of an optical fiber lens and an optical fiber connected to the optical fiber lens. The sunlight is focused by the optical fiber lens and transmitted by the optical fiber to the place of use. The other is a fluorescent concentrator, which is actually a transparent plate (usually plexiglass) with fluorescent pigments, which can absorb the part of the sunlight that is consistent with the wavelength of the fluorescent absorption band, and then has a longer wavelength than the absorption band. The emission band wavelength emits fluorescence. Due to the difference between the plate and the surrounding medium, the emitted fluorescence is guided to the edge surface of the plate in a total reflection manner within the plate. Its concentration ratio depends on the ratio of the plate area and the edge area, and it is easy to reach 10 to 100. This kind of plate It can absorb incident light in different directions, and it can also absorb scattered light, without tracking the sun.