Fresnel lens
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Fresnel Lens displayed in the Musée national de la marine in Paris, France |
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.
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.
["Fresnel lens." Encyclopædia Britannica. 2005. Encyclopædia Britannica Online. 11 November 2005 .] 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.
[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. .] Scottish physicist Sir
David Brewster is credited with convincing the British to use these lenses in their lighthouses.
["Brewster, Sir David." Encyclopædia Britannica. 2005. Encyclopædia Britannica Online. 11 November 2005 .]["David Brewster." World of Invention, 2nd ed. Gale Group, 1999. Reproduced in Biography Resource Center. Farmington Hills, Mich.: Thomson Gale. 2005. .]The Fresnel lens (pronounced
fre-NELL in scientific and theatrical and motion picture lighting applications, although often pronounced
FREZ-nell) 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.
Graphic examples
| 1: Cross section of a Fresnel lens
2: Cross section of a conventional plano-convex lens of equivalent power |
|  | Close-up of a lighthouse lens |
|
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.
Perhaps the most widespread use of Fresnel lenses 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 multi-faceted reflectors with plain
polycarbonate lenses.
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 (see
Fresnel lantern); 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.
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.
Sizes of lighthouse lenses
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.
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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Lighthouse Getaway: Fresnel lens (contains photographs)
