| Augustin Fresnel's Design |
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Augustin Fresnel (1788-1827)
In 1819, Augustin Fresnel, a French physicist invented a lens system that was one of the greatest advancements in lighthouse illumination. Fresnel’s design is based on refraction or the bending of light through glass. Before the Fresnel lens, lighthouse illumination consisted of an oil lamp mounted in front of a parabolic reflector. Fresnel's system was far more efficient, collecting and redirecting light from a single oil lamp 20 miles out to sea.
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Lamplight collected by the Fresnel Lens
This section view of a Fresnel lens shows how light is collected from a single oil lamp placed in the center of the lens. The lamp radiates light in all directions. The prisms are positioned to collect all light directed toward them, not allowing any light to escape. The areas shown in red are intentionally left open. The area at the top of the lens is reserved for exhausting the lamp. The area toward the floor is blocked by the shadow cast by the base of the oil lamp. |
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Bending of the Light
This standard cross section of a Fresnel lens shows how the light is refracted and reflected through the lens. All the light rays exit the lens horizontal, directing them seaward. There are two basic types of Fresnel lenses. Fixed, producing a steady glow, and Rotating, producing a flash as the lens rotates in an observer's line of sight. Each have the same cross section. The Fixed lens have prisms generated around a vertical axis forming belt lenses. The Rotating lens have prisms generated around a horizontal axis forming bullseye lenses. The Fixed/Flashing lens is a combination of the two, producing a steady glow varied by flashes.
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Types of Prisms
The Fresnel lens contains two basic prism designs. The area near the center is referred to as the lens section. The technical term is "dioptric", which means the light is bent or refracted twice. Once as it enters the glass and again as it exits. Above and below this section, a different style prism was designed, to use refraction and reflection to direct the light. These prisms are called reflecting or "catadioptric" prisms. The light is refracted as it enters the prism, reflected off the second face, and refracted as it exits. These prisms were necessary because the angle of light from the lamp was too steep to direct the light seaward using simple refraction. |
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