Screwpiles: The Forgotten Lighthouses

Fresnel Lens Design, Perhaps the Single Greatest 19th Century Achievement The lens system consisted of concentric rings of prisms. The

outer circle of prisms were catadioptric, meaning light is refracted, reflected, and then refracted again. Dioptric refers to a prism that refracts twice. The bull’s eye is a standard dioptric convex lens. One important feature of the Fresnel design is that the light source, in all cases, a single lamp, is almost completely encased within the lens.

The lens captured more than 80 percent of the available light from that single lamp. In contrast, the parabolic reflector lamps captured—at most—39 percent of the available light. Another important feature of the larger Fresnel lenses was that they could be assembled and tested at the factory, then carefully disassembled, shipped, and reassembled at the remote lighthouse site.

National Archives

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