Fresnellinsejä

Fresnellinsejä, known in English as Fresnel lenses, are a type of composite lens designed to reduce the thickness and weight of a lens compared to a conventional plano-convex or plano-concave lens. They were invented by Augustin-Jean Fresnel in 1822. Instead of a continuously curved surface, a Fresnel lens consists of a series of concentric annular segments, each with a different angle. These segments refract light in the same direction as a conventional lens but do so with much less material.

The design of a Fresnel lens can be visualized as taking a thick conventional lens and slicing it into many thin rings. Each ring is then placed on a flat surface, with the curvature of the original lens now concentrated on the edges of these rings. This segmentation significantly reduces the amount of glass or plastic needed, making the lens much lighter and thinner, especially for large diameters.

Fresnel lenses are commonly used in applications where large lenses are required and weight or thickness is a concern. Prominent examples include lighthouses, where their ability to focus a powerful beam of light over long distances is crucial. They are also found in overhead projectors, magnifying glasses, and in some optical instruments. The stepped surface of a Fresnel lens can sometimes lead to diffraction effects, which are a characteristic feature of their optical performance.