Fresnel lens
A Fresnel lens [ fʀɛnɛl - ] or more precisely, a Fresnel Fresnel lens is an optical lens, which was invented about 1822 by the French physicist Augustin Jean Fresnel and originally used for lighthouses. Due to the applied design principle weight and volume of large lenses are generally smaller, especially with lenses effects of short focal length, which are very thick and heavy in normal form.
Construction
The reduction in volume takes place, in the Fresnel lens by a division into annular regions. In each area the thickness is reduced, so that the lens is replaced by a series of annular steps. Since light is diffracted only when passing through the lens surfaces, the refractive angle is not in the thickness, but only upon the angle between the two surfaces depends. The lens maintains its focal length, but the picture quality is degraded by the step structure.
Today there are two types of Fresnel lenses in essence, headlight lenses and belts lenses.
- Headlight lenses are conventional lens having an optical axis. They focus the light in the direction of this axis.
- A cylindrical lens is a hollow cylinder, which is caused by rotation of the cross section of a headlamp lens at a plane perpendicular to the optical axis thereof axis. The step shape is usually outside. The intersection of the two axes, the light source is disposed, whose light is focused in the plane of the original optical axis, and emitted uniformly in all directions.
Use
Use in historic lighthouse optics
In the 19th century Fresnel lenses were used especially in lighthouses. They were made of hand-polished prisms of glass, which were joined together into a frame made of bronze. Through her compared to previous beacon optics much higher light intensity they revolutionized the lighthouse technology. The production of the early Fresnel lenses was complicated and expensive, and the main production centers were in the UK and France.
Subdivision of lighthouse optics in orders
The term order is at lighthouse lenses whose geometric size, which is accompanied by the optical size focal length marked. Large lenses ( long focal length ) yield for the same light source brighter, longer range lighthouse light as small lenses. In the U.S., six orders of between 2.6 m height are (first order: focal length 0.92 m) and 0.43 m height ( sixth order: focal length of 0.15 m) standardized.
4th order Fresnel lens, cylindrical lens, Museum of Point Sur Light Station, California.
Today's use
Fresnel lenses are used today where the weight of the lenses decisive, and the picture quality is secondary.
Belt lenses occur in ship lanterns and beacons. When beacons do not often encompass a full circle, but leave the area on the mainland freely and there are not many because of its size in one piece.
Headlight lenses come with beacons also common. With leading lights only one lens is used. For three fires several headlight lenses are arranged uniformly circular, each of which currently preferred radiates in a sector.
Headlight lenses are also used in headlights of automobiles.
Behind a rear projection screen is a Fresnel lens. In addition, this lens is used in specific fixtures for event and theater technology.
High quality projector for film and television lighting are mainly carried out as Fresnel. This construction permits, in addition to a uniform scale illumination with a soft edge, a continuous regulation of the light emission angle. The Fresnel lens of this headlight is matted ( " frosted " ) to avoid a projection of the stage collars.
Cheap headlight lenses are molded from plastic and place in daylight projectors, with passive infrared sensors with simple hand magnifiers, in automotive taillights and as wide- angle lenses ( foils) in automobile rear windows and Tills (control of the shopping cart) use.
Further applications is the principle at shims in the viewfinder of SLRs with more or less large Fresnel Fresnel lens range or scattering foils with small Fresnel lens squares to stick on windows as well as overhead projectors.
New applications arise in solar technology, as higher efficiencies can be achieved with solar power modules with lenses. With solar thermal power plants Fresnel mirror collectors are used.
Note: The Fresnel zone plate has technically nothing to do with the Fresnel lens, although it also acts as a lens.
Illustration of a frog ( left, front ) with thin Fresnel lens (left, hi); Image and lens: right
Ship lantern with one-piece cylindrical lens
Part - lens made of the Loschen Lighthouse Bremerhaven ( rake angle 60 °)