Let There Be Light
by Mike Flynn
Title
Let There Be Light
Artist
Mike Flynn
Medium
Photograph - Photo Art
Description
What makes a lighthouse, a lighthouse.
History
The idea of creating a thinner, lighter lens by making it with separate sections mounted in a frame is often attributed to Georges-Louis Leclerc, Comte de Buffon. The marquis de Condorcet (1743–1794) proposed grinding such a lens from a single thin piece of glass. French physicist and engineer Augustin-Jean Fresnel is most often given credit for the development of the multi-part lens for use in lighthouses. According to Smithsonian magazine, the first Fresnel lens was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary; its light could be seen from more than 20 miles out. Scottish physicist Sir David Brewster is credited with convincing the United Kingdom to adopt these lenses in their lighthouses.
How a Fresnel lens focuses light
1: Cross section of a Fresnel lens
2: Cross section of a conventional plano-convex lens of equivalent power
The Fresnel lens reduces the amount of material required compared to a conventional spherical lens by dividing the lens into a set of concentric annular sections known as "Fresnel zones." In theory there are infinitely many such zones.
In the first (and largest) variations of the lens, each zone was actually a separate prism. Though a Fresnel lens might appear like a single piece of glass, closer examination reveals that it is many small pieces. 'Single-piece' Fresnel lenses were later produced, being used for automobile headlamps, brake, parking, and turn signal lenses, and so on. In modern times, computer-controlled milling equipment (CNC) might be used to manufacture more complex lenses.
In each of these zones, the overall thickness of the lens is decreased, effectively dividing the continuous surface of a standard lens into a set of surfaces of the same curvature, with stepwise discontinuities between them. A Fresnel lens can be regarded as an array of prisms arranged in a circular fashion, with steeper prisms on the edges and a nearly flat convex center.
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 still use conventional bulky (non-Fresnel) 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 (TLR/SLR camera screens, micro-optics). In many cases they are very thin and flat, almost flexible, with thicknesses in the 1 to 5 mm (0.039 to 0.20 in) range.
Uploaded
September 27th, 2012
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Comments (6)
Mike Flynn
Originally, James this was a B&W that I converted into a hi contrast image in my darkroom (with actual chemicals and such). Later, I added the color. It's all fun. And, of course, you knew exactly what this was because of your nice work with lighthouses. BTW, thanks.
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