Fresnel Lens - PosterSpy

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A Fresnel lens is a flat lens with a number of concentric grooves engraved on one side of a plastic sheet, unlike ordinary spherical and aspherical lenses.

The cross section is shaped like a saw tooth. The name Fresnel comes from Augustin-Jean Fresnel, a French physicist in the 1800s who invented the lens.

Compared to ordinary lenses, it is possible to reduce the weight and thickness of the lens, which leads to cost reduction.

Usage of Fresnel lens

Fresnel lenses are often used when large lenses are required due to the advantage of being lightweight.

Lighthouses and floodlights require very large lenses, but Fresnel lenses were used for lighthouses at the time of their invention because ordinary lenses would be enormous in thickness and weight.

Nowadays, it is also used as an illumination lens for camera flashes and as a field lens for viewfinders of single-lens and twin-lens reflex cameras.

It is also used as a lens for relatively simple magnifying glasses such as loupes and magnifying glasses.

Fresnel lens principle

Needless to say, the groove carved in the lens is important for the Fresnel lens to function as a lens in the same way as a normal optical lens.

In a normal curved lens, the light incident on the lens is refracted by the lens and the light forms an image, but in the Frenel lens, the incident light can be refracted and condensed by this carved groove, so it is a normal lens. Even thinner and lighter Frenel lenses have the same or better light collection function.

Also, by adjusting the depth of the groove and the number of grooves, it is possible to focus at a focal length shorter than the aperture that is impossible with a normal spherical lens, so it is used for loupes and magnifying glasses.

Narrowing the sensation of concentric grooves (increasing the density of the grooves) improves the image quality, and conversely, widening the sensation of the grooves (lowering the density) improves the light collection efficiency.

Most Fresnel lenses are generally made of glass or resin (acrylic, polystyrene, polycarbonate).

Acryloyl Fresnel lenses are widely used because they have high transmittance in the visible and ultraviolet regions, while polycarbonate lenses are used in more harsh environments due to their high impact resistance and heat resistance. increase.

Advantages and disadvantages of using Fresnel lenses

As mentioned above, using a Fresnel lens leads to thinner and lighter weight. However, the imaging performance is inferior to that of singlet lenses. This is due to the fact that the groove itself carved on the surface of the Fresnel lens is observed, and in addition, the effect of diffraction occurs.

It is true that Fresnel lenses basically utilize the refraction phenomenon, but the grooves may cause diffraction, causing light in the near area to leak out and blurring the image.

Due to this demerit, it is not actively adopted when performance is required as an imaging lens. One point is a supplement, but it is not that the diffraction phenomenon is bad. There is also a so-called diffractive lens that actively utilizes the diffraction phenomenon.

Fresnel lenses and diffractive lenses have very similar structures and are often confused, but it should be noted that the physical phenomena are different.

Fresnel lens as a lens for lighting

According to DZOptics, fresnel lenses are used in various fields, and LED lighting is a typical example. In recent years, it has also been used as a condensing lens for solar panels, and is also applied to projector screens and optical sensors.

When using a Fresnel lens as an illumination lens, it is often used for the purpose of arranging a light source at the focal length position and emitting parallel light. It is often used for lighting used in stages and outdoor festivals, and is suitable when you want to softly illuminate a specific area.

Although it is inferior to singlet lenses in imaging performance, it can be said to be a good example of using it in the wrong direction.

There is also a linear Fresnel lens as an illumination lens similar to a Fresnel lens. A Fresnel lens has a part of the spherical surface arranged concentrically, while a linear Fresnel lens has a cylindrical lens arranged on a flat surface. It is suitable for applications such as condensing and diffusing in one direction, and is used when a linear light source is required.

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