Who Invented the Lens Used at the Pigeon Point Lighthouse?
Augustine Jean Fresnel (pronounced fray-nell)
Fresnel (born May 10, 1788, died July 14, 1827), a French physicist, was commissioned by France in 1822 to develop a better lighting system for the French lighthouses. Rather than try to develop a brighter light source, Fresnel set about designing a better, more efficient method of using the light which 1820's technology could produce.
19th Century lighthouses used silvered-metal parabolic reflectors, placed behind a lamp, to direct the light seaward. This system was not very efficient, and worked poorly as an aid to navigation. Remember that light produced by a lamp, or any source, radiates out in all directions. Fresnel's task was to find the most efficient method to direct all, or nearly all, of the lamp's light rays out to sea.
To improve upon the parabolic reflector, Fresnel looked to glass lenses for a method of directing more of the light from a lamp seaward. Molding a single lens to do the job was impractical. A lens suitable for a lighthouse would be far too large to be cast as a single lens. Instead Fresnel designed a system of smaller lens and prisms, arranged in a stair-step configuration. He used this system to bend, fold, and focus the light out to sea. The result was a lens that was able to use about 80 percent of the light available from the lamp! In the case of the lens used at Pigeon Point, about 70,000 candlepower was produced by the original lamp.
This type of lens, called a Fresnel lens, was a technological breakthrough! The new lens was far more efficient in its use of the small amount of light produced by a lard oil lamp. In addition, a Fresnel lens could be disassembled and shipped in sections and configured into virtually limitless numbers of light characteristics, that is, patterns of flashes of light divided by periods of darkness.