Screwpiles: The Forgotten Lighthouses

Fresnel Lens French physicist Augustin-Jean Fresnel’s invention of the Fresnel lens in 1822 revolutionized lighthouses in the nineteenth century. The Fresnel lens focused 85 percent of a lamp’s light versus the 20 percent focused with the parabolic reflectors of the time. The United States did not use the Fresnel lens, however, until the new 1851 U.S. Lighthouse Board recognized the value of the lens and made efforts to install those lenses in all new U.S. lighthouses.

The Fresnel lens requires optical-quality glass prisms, poured with exacting specifications, then precisely ground and polished. A Fresnel lens captures nearly all light emanating from a light source, yielding a much brighter concentration of light. For comparison, the largest first-order Fresnel lenses, reserved for the coast lighthouses such as Cape Hatteras Light, were approximately 8.5 feet tall and weighed 12,800 pounds. The sixth-order Fresnel lenses used in the James River lighthouses were just 17 inches tall and weighed 220 pounds.

Comparative size of a first-order Fresnel lens used in the largest coastal lighthouses

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