January 11, 2010

The Fresnel Lense

Above: The view from the top of the lighthouse.

Above: The view of the Fresnel Lighthouse Lens from below.

Above: A diagram of what the lens looks like.

Above: A diagram showing how the lens works.

The St. Augustine lighthouse contains the only First Order Fresnel Lens still in use. In my previous post I told you that I had especially looked forward to the lighthouse in St. Augustine because of the lens.
Most people probably have no clue what a Fresnel lens is, much less why I would be so excited about seeing one. Well, I may not be able to explain why I was excited, but at least I can tell you what makes a Fresnel lens differ from others. Or at least try to- we'll see. Anyway, here goes.

A Fresnel lens (pronounced 'fray-NELL') is a type of lens developed by French physicist Augustin-Jean Fresnel for lighthouses.

The Fresnel Lens is designed to be much thinner than other lenses, thus providing more light over a longer distance, which is why it was designed for lighthouse use.

It is generally made of many prisms (pieces of glass) stacked together in a frame. See above photo.

"Though a Fresnel lens might look like a single piece of glass, closer examination reveals that it is many small pieces. It was not until modern computer-controlled milling equipment (CNC) could turn out large complex pieces that these lenses were manufactured from single pieces of glass.

"For each of these zones, the overall thickness of the lens is decreased, effectively chopping the continuous surface of a standard lens into a set of surfaces of the same curvature, with discontinuities between them. This allows a substantial reduction in thickness (and thus weight and volume of material) of the lens, at the expense of reducing the imaging quality of the lens.

"High-quality glass Fresnel lenses were used in lighthouses, where they were "state of the art" in the late 19th and through the middle of the 20th Centuries; most are now retired from service. Lighthouse Fresnel lens systems typically include extra annular prismatic elements, arrayed in faceted domes above and below the central planar Fresnel, in order to catch all light emitted from the light source. The light path through these elements can include an internal reflection, rather than the simple refraction in the planar Fresnel element.

"These lenses conferred many practical benefits upon the designers, builders, and users lighthouses and their illumination. Among other things, smaller lenses could fit into more compact spaces. Greater light transmission over longer distances, and varied patterns made it possible to triangulate a position." - Fresnel Lens, Wikipedia

Read the full article for some really neat info, such as:

The Fresnel lens is not only used in lighthouses, but also some things you may see everyday- like the headlights on your car, flashlights, theatre lighting, and some TV screens!

Whether you enjoyed any of that or not, I don't know- but I thought that it was really interesting!


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