Editing Fresnel lens (section)

== 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.<ref>R. Winston, J. C. Miñano, and P. G. Benítez, ''Nonimaging Optics'', Academic Press, 2005.</ref> 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 lenses can be classified as:

; Spherical: A spherical Fresnel lens is equivalent to a [[simple lens|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. This type is sometimes called a [[kinoform]] when the ridges are microscopic, at the wavelength scale.
; 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 lenses can be classified as:

; 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.<ref name="IntroNio2e">{{cite book|mode=cs2 | first = Julio | last = Chaves | title = Introduction to Nonimaging Optics, Second Edition | url = https://books.google.com/books?id=e11ECgAAQBAJ | publisher = [[CRC Press]] | year = 2015 | isbn = 978-1-4822-0673-9}}.</ref>
; 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.<ref>{{Cite web|title=Linear Concentrator System Concentrating Solar-Thermal Power Basics|url=https://www.energy.gov/eere/solar/linear-concentrator-system-concentrating-solar-thermal-power-basics|access-date=2021-05-31|website=Energy.gov|language=en}}</ref>