A Fresnel lens is a type of composite
compact lens developed by the French physicist Augustin-Jean Fresnel
(1788–1827) for use in lighthouses.It has been called "the invention that
saved a million ships.
The design allows the construction of
lenses of large aperture and short focal length without the mass and volume of
material that would be required by a lens of conventional design. A Fresnel
lens can be made much thinner than a comparable conventional lens, in some
cases taking the form of a flat sheet. The simpler dioptric (purely refractive)
form of the lens was first proposed by Count Buffon,and independently
reinvented by Fresnel. The catadioptric form of the lens, entirely invented by
Fresnel, has outer elements that use total internal reflection as well as
refraction; it can capture more oblique light from a light source and add it to
the beam of a lighthouse, making the light visible from greater distances.
Types
There are two main types of Fresnel lens:
imaging and non-imaging. Imaging Fresnel lenses use segments with curved
cross-sections and produce sharp images, while non-imaging lenses have segments
with flat cross-sections, and do not produce sharp images.[11] As the number of
segments increases, the two types of lens become more similar to each other. In
the abstract case of an infinite number of segments, the difference between
curved and flat segments disappears.
Imaging
Spherical
A spherical Fresnel lens is equivalent to a
simple spherical lens, using ring-shaped segments that are each a portion of a
sphere, that all focus light on a single point. This type of lens produces a
sharp image, although not quite as clear as the equivalent simple spherical
lens due to diffraction at the edges of the ridges.
Cylindrical
A cylindrical Fresnel lens is equivalent to
a simple cylindrical lens, using straight segments with circular cross-section,
focusing light on a single line. This type produces a sharp image, although not
quite as clear as the equivalent simple cylindrical lens due to diffraction at
the edges of the ridges.
Non-imaging
Spot
A non-imaging spot Fresnel lens uses
ring-shaped segments with cross sections that are straight lines rather than
circular arcs. Such a lens can focus light on a small spot, but does not
produce a sharp image. These lenses have application in solar power, such as
focusing sunlight on a solar panel. Fresnel lenses may be used as components of
Köhler illumination optics resulting in very effective nonimaging optics
Fresnel-Köhler (FK) solar concentrators.
Linear
A non-imaging linear Fresnel lens uses
straight segments whose cross sections are straight lines rather than arcs.
These lenses focus light into a narrow band. They do not produce a sharp image,
but can be used in solar power, such as for focusing sunlight on a pipe, to
heat the water within.
