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| Lighthouse Fresnel lens, on display at the Musée national de la Marine |
A Fresnel lens (pronounced /freɪˈnɛl/ fray-NELL) is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.
The design enables the construction of lenses of large aperture and short focal length without the mass and
volume of material that would be required by a lens of conventional design. Compared to conventional bulky lenses, the Fresnel lens is much thinner, larger, and flatter, and captures more oblique light from a light source, thus allowing lighthouses to be visible over much greater distances.
History
The idea of creating a thinner, lighter lens by making it with separate sections mounted in a frame is often attributed to Georges-Louis Leclerc, Comte de Buffon. The marquis de Condorcet (1743–1794) proposed grinding such a lens from a single thin piece of glass. French physicist and engineer Augustin-Jean Fresnel is most often given credit for the development of the multi-part lens for use in lighthouses. According to Smithsonian magazine, the first Fresnel lens was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary; its light could be seen from more than 20 miles (32 km) out. Scottish physicist Sir David Brewster is credited with convincing the United Kingdom to adopt these lenses in their lighthouses.
Description
The Fresnel lens reduces the amount of material required compared to a conventional spherical lens by dividing the lens into a set of concentric annular sections known as "Fresnel zones", which are theoretically limitless.
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| How a Fresnel lens works |
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| 1: Cross section of a Fresnel lens 2: Cross section of a conventional plano-convex lens of equivalent power |
In each of these zones, the overall thickness of the lens is decreased, effectively dividing the continuous surface of a standard lens into a set of surfaces of the same curvature, with stepwise discontinuities between them. In fact a Fresnel lens can be regarded as an array of prisms arranged in a circular fashion, with steeper prisms on the edges and a nearly flat convex center.
In the first (and largest) variations of the lens, each zone was actually a separate prism. Though a Fresnel lens might appear like a single piece of glass, closer examination reveals that it is many small pieces. It was not until modern computer-controlled milling equipment (CNC) could turn out large complex pieces that these lenses were manufactured from single pieces of glasss
Fresnel lens design allows a substantial reduction in thickness (and thus mass and volume of material), at the expense of reducing the imaging quality of the lens, which is why precise imaging applications such as photography still use conventional bulky (non-Fresnel) lenses.
Fresnel lenses are usually made of glass or plastic; their size varies from large (old historical lighthouses, meter size) to medium (book-reading aids, OHP viewgraph projectors) to small (TLR/SLR camera screens, micro-optics). In many cases they are very thin and flat, almost flexible, with thicknesses in the 1 to 5 millimeter range.
Examples and uses
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The use of a Fresnel lens for image projection reduces image quality, so they tend to be used only where quality is not critical or where the bulk of a solid lens would be prohibitive. Cheap Fresnel lenses can be stamped or molded of transparent plastic and are used in overhead projectors, projection televisions, and hand-held sheet magnifying glasses. Fresnel lenses have been used to increase the visual size of CRT displays in pocket televisions, notably the Sinclair TV80. They are also used in traffic lights. Fresnel lenses are also used to correct several visual disorders, including several ocular-motility disorders such as strabismus.
Since plastic Fresnel lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used to concentrate sunlight for heating in solar cookers, in solar forges, and in solar collectors used to heat water for domestic use.
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| Cape Meares Lighthouse (Oregon, USA) first-order Fresnel lens |
Perhaps the most widespread use of Fresnel lenses, for a time, was in automobile headlamps, where they allow the roughly parallel beam from the parabolic reflector to be shaped to meet requirements for dipped and main beam patterns, often both in the same headlamp unit (such as the European H4 design). For reasons of cost, weight, and impact resistance, newer cars have dispensed with glass Fresnel lenses, using multifaceted reflectors with plain polycarbonate lenses. However, Fresnel lenses continue to be widely used in automobile tail, marker, and backup lights.
Fresnel lenses are also used in left-hand-drive European lorries entering the UK and Republic of Ireland (and vice versa, right-hand-drive Irish and British trucks entering mainland Europe) to overcome the blind spots caused by the driver operating the lorry while sitting on the "wrong" side of the cab and driving on the "wrong" side of the road. They are applied to the passenger-side window.
High-quality glass Fresnel lenses were used in lighthouses, where they were considered state of the art in the late 19th and through the middle of the 20th centuries; 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. These lenses conferred many practical benefits upon the designers, builders, and users of lighthouses and their illumination. Among other things, smaller lenses could fit into more compact spaces. Greater light transmission over longer distances, and varied patterns, made it possible to triangulate a position.
Glass Fresnel lenses also are used in lighting instruments for theater and motion pictures (see Fresnel lantern); such instruments are often called simply Fresnels. The entire instrument consists of a metal housing, a reflector, a lamp assembly, and a Fresnel lens. Fresnel instruments usually have a convenient way of changing the focal distance between the lamp and the lens. As a result, they are very flexible, and can often produce a beam as narrow as 7° or as wide as 70°. The Fresnel lens produces a very soft-edged beam, so it is often used as a wash light. A holder in front of the lens can hold a colored plastic film (gel) to tint the light or wire screens or frosted plastic to diffuse it. Many Fresnel instruments allow the lamp to be moved relative to the lens' focal point, to increase or decrease the size of the light beam. The Fresnel lens is useful in the making of motion pictures not only because of its ability to focus the beam brighter than a typical lens, but also because the light is a relatively consistent intensity across the entire width of the beam of light.
Aircraft carriers and naval air stations typically use Fresnel lenses in their optical landing systems. The "meatball" light aids the pilot in maintaining proper glide slope for the landing. In the center are amber and red lights composed of Fresnel lenses. Although the lights are always on, the angle of the lens from the pilot's point of view determines the color and position of the visible light. If the lights appear above the green horizontal bar, the pilot is too high. If it is below, the pilot is too low, and if the lights are red, the pilot is very low.
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| Optical Landing System on US Navy aircraft carrier USS Dwight D. Eisenhower |
New applications have appeared in solar energy, where Fresnel lenses are used to concentrate sunlight (with a ratio of almost 500:1) onto solar cells. Thus the active solar cell surface can be reduced to a fraction compared to conventional solar modules. This offers a considerable cost-saving potential by low material consumption, and it is possible to use high-quality and expensive solar cells, which achieve a very high efficiency under concentration due to thermodynamic effects.
Fresnel reflectors are also currently being incorporated into next-generation solar thermal-energy systems. See solar power for more information.
In photography, Canon developed a diffractive optics (DO) version of their EF 70-300mm lens using Fresnel lenses. Also, the Polaroid SX-70 camera used a Fresnel reflector as part of its viewing system.
Multi-focal Fresnel lens are also used as a part of retina identification cameras, where they provide multiple in- and out-of-focus images of a fixation target inside the camera. For virtually all users, at least one of the images will be in focus, thus allowing correct eye alignment.
The Fresnel lens has seen applications for enhancing passenger reading lights on Airbus aircraft: in a dark cabin, the focused beam of light does not dazzle neighboring passengers.
Fresnel lenses have also been used in the field of popular entertainment. The British rock artist Peter Gabriel made use of them in his early solo live performances to magnify the size of his head, in contrast to the rest of his body, for dramatic and comic effect. In the Terry Gilliam film Brazil, plastic Fresnel screens appear ostensibly as magnifiers for the small CRT monitors used throughout the offices of the Ministry of Information. However, they occasionally appear between the actors and the camera, distorting the scale and composition of the scene to humorous effect.
Sizes of lighthouse lenses
Fresnel's lighthouse lenses ordinarily fell into six orders based on their focal length. The order of a Fresnel lens is approximately the dioptre or optical power of the lens. The dioptre (also spelled diopter) is the reciprocal of the focal length of the lens in meters. A Fresnel lens with a focal length of 50 cm or 0.5 m would be classified as a second-order lens.
However, there are a few hyperradiant Fresnel lenses—the largest ever made—'off the chart' of sixth through first order. One such lens was on hand when it was decided to build and outfit the Makapuu Point Light. Rather than order a new lens, the huge optic construction—twelve feet tall and with over a thousand prisms—was used there.
The largest (first-order) lens has a focal length of 920 mm (36 in) and an optical area 2590 mm (8.5 ft) high. The complete assembly is about 3.7 m (12 ft) tall and 1.8 m (6 ft) wide. The smallest (sixth-order) has a focal length of 150 mm (5.9 in) and an optical area 433 mm (17 in) high.
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.
Projection uses
Fresnel lenses of different focal lengths (one collimator, and one collector) 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). Fresnel lenses are also used as collimators in overhead projectors.
Generating solar power
More recently Fresnel reflectors are used in concentrating solar power (CSP) plants to concentrate solar energy.
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