A Fresnel lens is a type of compact lens originally developed by French physicist Augustin-Jean Fresnel for headlamps.
The design allows the construction of large aperture and short focal length lenses without the mass and volume of material that would be required by a conventional design lens. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a headlamp equipped with one to be visible over greater distances.
History
The idea of creating a thinner and lighter lens when making it with separate sections mounted on a frame is often attributed to Georges-Louis Leclerc, the Earl of Buffon. The Marquis de Condorcet (1743-1794) proposed to rectify such a lens from a single thin piece of glass. The French physicist and engineer Augustin-Jean Fresnel is often given credit for the development of the multi-part lens for use in headlights. According to the Smithsonian, the first Fresnel lens was used in 1823 at the Cordouan lighthouse at the mouth of the Gironde estuary; Its light could be seen from more than 20 miles (32 km) outwards. Scottish physicist Sir David Brewster is credited to convince the UK to adopt these lenses in their headlamps.
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 infinitely many such sections. In each section, the total thickness decreases compared to a single equivalent lens. This effectively divides the continuous surface of a standard lens into a set of surfaces of the same curvature, with discontinuities staggered therebetween.
In some lenses, curved surfaces are replaced by flat surfaces, with a different angle in each section. Said lens can be considered as a series of prisms arranged in a circular way, with sharper prisms at the edges and a flat or slightly convex center. In the first (and largest) Fresnel lenses, each section was actually a separate prism. "One piece" Fresnel lenses were subsequently produced, being used for auto headlights, brakes, parking, and turn signal lenses, and so on. In modern times, computer-controlled milling equipment (CNC) could be used to manufacture more complex lenses.
The design of the Fresnel lens allows a substantial reduction in the thickness (and thus the mass and volume of material), at the expense of reducing the image quality of the lens, so that precise image applications such as Photography usually use larger conventional lenses.
Fresnel lenses are usually made of glass or plastic; Its size varies from large historical lighthouses (old historical headlamps, size meters) to medium (book reading assistants, OHP viewgraph projectors) to small (TLR / SLR cameras, micro-optics). In many cases they are very thin and flat, almost flexible, with thicknesses in the range of 1 to 5 mm (0.039 to 0.197 in).
Modern Fresnel lenses generally consist of all refractive elements. However, many of the headlights have refracting and reflective elements, as shown in the photographs and diagram. That is, the outer elements are sections of reflectors while the inner elements are sections of refractive lenses. Total internal reflection was often used to avoid loss of light in the reflection of a silver mirror.