Fresnel lens
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.
|
|
| Lighthouse in Rozewie |
(a) Cross section of a Fresnel lens
(b) Cross section of a conventional plano-convex lens of equivalent power |
The Fresnel lens (commonly pronounced FREZ-nell but originally fray-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.
For these reasons, Fresnel lenses tend to be used in applications where weight of the lens is at a premium and/or image quality is unimportant.
High quality glass Fresnel lenses are used in lighthouses and may include extra annular prismatic elements to improve the brightness of the light beam.
Cheap Fresnel lenses can be stamped or moulded out of transparent plastic and are used in overhead projectors and hand-held sheet magnifying glasses.
Fresnel lenses for lighthouses are classified in six orders based on the focal length of the lens. The largest (first order) lens stands 12 feet tall and 6 feet across, with a focal length of 36 inches. The smallest (sixth order) stands only 2 feet tall, with a focal length of 5.9 inches.
A related optical device is the Fresnel zone plate.
