Fresnel Lenses

A Fresnel lens is a type of lens invented by Augustin-Jean Fresnel (pronounced fre-NELL in scientific and lighting applications, although often incorrectly pronounced FREZ-nell). more...

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Originally developed for lighthouses, the design enables the construction of lenses of large aperture and short focal length without the weight and volume of material which would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner, thus passing more light and allowing lighthouses to be visible over much longer distances.

Development

The idea of creating a thinner, lighter lens by making it with separate sections is often attributed to Georges-Louis Leclerc, Comte de Buffon. However, it is difficult to find any other sources that link Buffon to work with optics. French physicist and engineer Augustin-Jean Fresnel is most often given credit for the development of this lens for use in lighthouses. According to Smithsonian, the first Fresnel lens was used in 1822 in a lighthouse on the Gironde River in France, Cardovan Tower; its light could be seen from more than 20 miles out. Scottish physicist Sir David Brewster is credited with convincing the British to use these lenses in their lighthouses.

Detailed information

The Fresnel lens reduces the amount of material required compared to a conventional spherical lens by breaking the lens into a set of concentric annular sections known as Fresnel zones.

In the first (and largest) variations of the lens, each of these zones was a different prism. Thus what looked like from the outside to be a single piece of glass, when approached closely could be seen to be many, many individual pieces. It's not until modern technology could turn out large complex pieces that these lens were single pieces of glass. The picture to the right allows you to see the separate prisms.

For each of these zones, the overall thickness of the lens is decreased, effectively chopping the continuous surface of a standard lens into a set of surfaces of the same curvature, with discontinuities between them. This allows a substantial reduction in thickness (and thus weight and volume of material) of the lens, at the expense of reducing the imaging quality of the lens.

Graphic examples

Uses

For the reasons given above, Fresnel lenses tend to be used in applications where imaging quality is not critical, or where the bulk of a solid lens would be prohibitive. Cheap Fresnel lenses can be stamped or moulded out of transparent plastic and are used in overhead projectors, projection televisions, and hand-held sheet magnifying glasses. Fresnel lenses are also used in traffic lights and solar forges.

Read more at Wikipedia.org