Fresnel lens
A Fresnel lens [ fʀɛˈnɛl ] or more precisely a Fresnel step lens is a volume and mass-reduced design of an optical lens . It was originally developed for lighthouses by the French physicist Augustin Jean Fresnel around 1822 . The functional principle was devised by Georges-Louis Leclerc de Buffon in 1748.
construction
Since light is only refracted when it passes through the lens surfaces , the angle of refraction does not depend on the thickness, but only on the angle between the two surfaces. The volume of the Fresnel lens is reduced by dividing it into ring-shaped areas, the maximum glass thickness of which is approximately the same. The required curvature of the surfaces gives the lens a series of annular steps. The lens retains its focal length, but the image quality is impaired by the step structure.
Today there are essentially two types of Fresnel lenses: headlight lenses and belt lenses .
- Like conventional lenses, headlight lenses have an optical axis . They focus the light in the direction of this axis.
- A belt lens is a hollow body of revolution . Its shape is created by rotating the cross section of a Fresnel lens around an axis that is perpendicular to the optical axis of the lens. The light source is arranged at the intersection of both axes. Belt lenses focus their light in a plane perpendicular to the axis of rotation and emit it evenly in all directions.
use
Lighthouse optics
In the 19th century, Fresnel lenses were mainly used in lighthouses . They consisted of hand-polished prisms made of glass, which were put together in a frame made of metal, usually brass . With their much higher light intensity compared to earlier beacon optics, they revolutionized lighthouse technology. The manufacture of the early Fresnel lenses was complex and expensive. The main production centers were in Great Britain and France, the main manufacturer in Germany was Weule .
Subdivision of lighthouse optics into orders
In lighthouse lenses , the term order is used to denote their geometric size - with which the optical size focal length is associated. Large lenses (large focal length) produce a brighter, more far-reaching lighthouse light than small lenses with the same light source. Six orders are standardized in the USA:
| properly planning |
Focal length (mm) |
Height (mm) |
Mass (kg) |
Typical use in the USA |
|---|---|---|---|---|
| 1. | 920 | 2590 | 5800 | Largest coastal lighthouses |
| 2. | 700 | 2069 | 1600 | Great lakes, islands off the coast, straits |
| 3. | 500 | 1576 | 900 | Coast, estuaries on straits, bays, canals |
| 4th | 250 | 722 | 200-300 | Shallows, reefs, harbors, river islands |
| 5. | 187.5 | 541 | 120-200 | Docks, rivers, smaller islands in straits |
| 6th | 150 | 433 | 100 | Docks and quays |
1st Order Fresnel Lens, Headlight Lenses, Point Arena Light Museum , California.
4th order Fresnel lens, belt lens, Museum of Point Sur Light Station , California.
Further examples
Fresnel lenses are still used when the weight of the lenses is crucial and the image quality is secondary.
With guiding lights only one lens is used, while with rotating lights several lenses are arranged in a circle around the light source.
A Fresnel lens was located behind rear projection screens to improve the illumination of the screen for the viewer.
Stage lights are mostly designed as stepped lens lights. This construction saves mass. The Fresnel lens of the headlights for film and TV lighting is frosted (“frosted”) in order to blur the projection of the step rings.
Fresnel lenses are often pressed from plastic and achieve a high level of precision. They are used in daylight projectors , with simple handheld magnifiers , with automobile taillights and as wide-angle lenses (foils) in automobile rear windows (blind spots) and checkouts (checking shopping carts). The focusing screens in the viewfinder of SLR cameras also have a Fresnel lens area.
Stepped lenses are also part of long focal length telephoto lenses to reduce their weight.
With concentrator solar cells , lenses are used to focus the sun's rays on a smaller transducer element, which reduces its area and thus the price.
Furthermore, the Fresnel lenses found in optical signal and special signaling systems use (Xenonblitz-, all colors rotating beacon ).
Headlight lens in the tail light of a car
Image of a frog (left, front) with a thin Fresnel lens (left, back); Image and lens (right)
Ship lantern with one-piece belt lens
Partial belt lens from the Loschen lighthouse in Bremerhaven (clamping angle 60 °)
Fresnel sheet for enlarging texts
literature
- Dietrich Kühlke: Optics. Basics and Applications . 2nd, revised and expanded edition. Harri Deutsch Verlag, Frankfurt am Main 2004, ISBN 3-8171-1741-8 , pp. 122 .
Web links / references
Remarks
- ↑ Traité élémentaire de physique, Volume 2 .
- ↑ Information on beacon optics: headlight lenses . Specialist for traffic engineering. Retrieved December 30, 2010.
- ↑ Information on beacon optics: belt lenses . Specialist for traffic engineering. Retrieved December 30, 2010.
- ↑ On this and the following cf. Ray Jones, The Lighthouse Encyclopedia. The Definitive Reference , Guilford, CT 2004, pp. 88f.
- ↑ With modern, brighter light sources, the range can be increased or a smaller lens (higher order) can be used with the same range.
- ↑ cf. Comparative Table of Lens Orders
- ↑ cf. Comparative Table of Lens Orders , in: Dennis L. Noble, Lighthouses & Keppers. The US Lighthouse Service and its Legacy , Annapolis, MD 1997, p. 24.
- ↑ United States Lighthouse Society 'Fresnel Lens Orders, Sizes, Weights, Quantities and Costs'
- ↑ Information on beacon optics: Rotary lenses: Fig . 3 . Specialist for traffic engineering. Retrieved December 30, 2010.
- ↑ Nikon: Phase Fresnel Lens (PF) , accessed September 25, 2018
- ^ A Closer Look at Nikon's New Phase Fresnel (PF) Lens Technology: A Closer Look at Nikon's New Phase Fresnel (PF) Lens Technology , accessed September 25, 2018
- ↑ Rolf Hug: Concentrator photovoltaics brings the sun to the point: new German technology for efficient solar power plants . The solar server. April 17, 2008. Retrieved December 30, 2010.
