Fresnel lens

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Image:MuseeMarine-phareFresnel-p1000466.jpg A Fresnel lens is a type of lens invented by Augustin-Jean Fresnel. Originally developed for lighthouses, the design enables the construction of lenses of large size and short focal length without the weight and volume of material which would be required in a lens of conventional design. Compared to earlier lenses, much less light was lost with this design and it was visible over much longer distances.

1 Development
2 Detailed information
- 2.1 Graphic examples
3 Uses
- 3.1 Sizes of lighthouse lenses
- 3.2 Projection uses
4 References
5 External links

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Development

According to the Encyclopædia Britannica, the idea of creating a thinner, lighter lens by making it with separate sections began with Georges-Louis Leclerc, Comte de Buffon.Template:Ref 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.Template:Ref Scottish physicist Sir David Brewster is credited with convincing the British to use these lenses in their lighthouses.Template:Ref Template:Ref

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Detailed information

The Fresnel lens (often pronounced FREZ-nell but usually the original fre-NELL in scientific and theatrical and motion picture lighting applications) 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. 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.

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Graphic examples

Image:Fresnellens1.png

Image:Lighthouse Lens.jpg

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Uses

Image:CapeMearesLens wb.jpg

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 solar forges.

High-quality glass Fresnel lenses were used in lighthouses; most are now retired from service. Lighthouse Fresnel lens systems typically include extra annular prismatic elements, arrayed in faceted domes above and below the central planar Fresnel, in order to catch all light emitted from the light source. The light path through these elements can include an internal reflection, rather than the simple refraction in the planar Fresnel element.

Glass fresnel lenses also are used in lighting instruments for theater and motion pictures; such instruments are often called simply fresnels. The entire instrument consists of a metal housing, reflector, lamp assembly, and fresnel lens. A slot on the housing can hold a colored film (gel) to tint the light. Many fresnel instruments allow the lamp to be moved relative to the lens focal point, which creates a more or less hard-edged light beam.

Fresnel reflectors are also currently being incorporated into next-generation solar thermal energy systems. See Solar power for more information. The Polaroid SX-70 camera used a Fresnel reflector as part of its viewing system.

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Sizes of lighthouse lenses

Image:Soczewka fresnela t.jpg Fresnel's lighthouse lenses fell into six orders based on their focal length. The largest (first order) lens stands 12 feet (3.7 m) tall and 6 feet (1.8 m) across, with a focal length of 36 inches (0.91 m). The smallest (sixth order) stands only 2 feet (0.60 m) tall, with a focal length of 5.9 inches (14.9 cm).

Subsequent development extended this to seventh and eighth orders, an intermediate three-and-one-half order, and two orders even larger than first: mesoradial and hyperradial.

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Projection uses

Fresnel lenses of different focal lengths (one collimator, and one collecter) are used in commercial and DIY projection. The collimator lens has the lower focal length, and is placed closer to the light source, and the collector lens, which focuses the light into the triplet lens, is placed after the projection image (an active matrix LCD panel in LCD projectors).

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References

Template:Note "Fresnel lens." Encyclopædia Britannica. 2005. Encyclopædia Britannica Online. 11 November 2005 <http://search.eb.com/eb/article-9035385>.
Template:Note Watson, Bruce. “Science Makes a Better Lighthouse Lens.” Smithsonian. August 1999 v30 i5 p30. Reproduced in Biography Resource Center. Farmington Hills, Mich.: Thomson Gale. 2005. <http://libproxy.uncg.edu:2088/servlet/BioRC>.
Template:Note "Brewster, Sir David." Encyclopædia Britannica. 2005. Encyclopædia Britannica Online. 11 November 2005 <http://search.eb.com/eb/article-9016395>.
Template:Note "David Brewster." World of Invention, 2nd ed. Gale Group, 1999. Reproduced in Biography Resource Center. Farmington Hills, Mich.: Thomson Gale. 2005. <http://libproxy.uncg.edu:2088/servlet/BioRC>.