Editing Fresnel lens (section)

== Design ==
[[File:Fresnel lens.svg|thumb|left|upright|1: Cross-section of Buffon/Fresnel lens. 2:&nbsp;Cross-section of conventional [[lens (optics)#Types of simple lenses|plano-convex lens]] of equivalent power. (Buffon's version was [[lens (optics)#Types of simple lenses|biconvex]].<ref name=levitt-p59>Levitt, 2013, p.{{nbsp}}59.</ref>)]]
[[File:Flat flexible plastic sheet lens.JPG|thumb|Close-up view of a flat Fresnel lens shows concentric circles on the surface]]

The Fresnel lens reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections. An ideal Fresnel lens would have an infinite number of sections. In each section, the overall thickness is decreased compared to an equivalent simple lens. This effectively divides the continuous surface of a standard lens into a set of surfaces of the same curvature, with stepwise discontinuities between them.

In some lenses, the curved surfaces are replaced with flat surfaces, with a different angle in each section. Such a lens can be regarded as an array of prisms arranged in a circular fashion with steeper prisms on the edges and a flat or slightly convex center. In the first (and largest) Fresnel lenses, each section was actually a separate prism. 'Single-piece' Fresnel lenses were later produced, being used for automobile headlamps, brake, parking, and turn signal lenses, and so on. In modern times, [[CNC|computer-controlled milling equipment (CNC)]] or [[3-D printing|3-D printers]] might be used to manufacture more complex lenses.{{Citation needed|date=March 2023}}

Fresnel lens design allows a substantial reduction in thickness (and thus mass and volume of material) at the expense of reducing the imaging quality of the lens, which is why precise imaging applications such as photography usually still use larger conventional lenses.

Fresnel lenses are usually made of glass or plastic; their size varies from large (old historical lighthouses, meter size) to medium (book-reading aids, OHP viewgraph projectors) to small ([[Twin-lens reflex camera|TLR]]/[[Single-lens reflex camera|SLR]] camera screens, micro-optics). In many cases they are very thin and flat, almost flexible, with thicknesses in the {{convert|1|to|5|mm|in|frac=32|abbr=on}} range.{{Citation needed|date=March 2023}}

Most modern Fresnel lenses consist only of refractive elements. Lighthouse lenses, however, tend to include both refracting and reflecting elements, the latter being ''outside'' the metal rings seen in the photographs. While the inner elements are sections of refractive lenses, the outer elements are reflecting prisms, each of which performs two refractions and one [[total internal reflection]], avoiding the light loss that occurs in reflection from a silvered mirror.