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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.
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."Fresnel lens." Encyclopædia Britannica. 2005. Encyclopædia Britannica Online. 11 November 2005
Detailed information
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
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 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 (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).
References
External links
- Lighthouse Getaway: Fresnel lens (contains photographs)
Lent de Fresnel | Fresnellinse | Lentille de Fresnel | Lente di Fresnel | Fresnellens | フレネルレンズ | Soczewka Fresnela | Fresnel-lins
"Fresnel lens"