Fresnel lens

Optics. a large lens with a surface of concentric grooves of prismatic profile, used in automobile headlights, searchlights, spotlights, etc.

[1840-50; see FRESNEL]

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Series of concentric rings, each consisting of a thin part of a simple lens, assembled on a flat surface.

G.-L.-L. Buffon (1748) first had the idea of dividing a lens surface into concentric rings to reduce the weight. In 1820 his idea was adopted by Augustin-Jean Fresnel (1788–1827) for the construction of lighthouse lenses. Fresnel lenses have the optical properties of much thicker and heavier lenses. They are used in spotlights, floodlights, railroad and traffic signals, and decorative lights. Some thin Fresnel lenses are molded in plastic, the width of the rings being only a few thousandths of an inch; such lenses are used in cameras and small projectors.

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 succession of concentric rings, each consisting of an element of a simple lens, assembled in proper relationship on a flat surface to provide a short focal length (see illustration—>). The Fresnel lens is used particularly in lighthouses and searchlights (searchlight) to concentrate the light into a relatively narrow beam. It would be almost impossible to make a large lighthouse lens of the usual solid glass-disk type because the thickness and weight would be prohibitive; the lighter Fresnel lens is constructed of elements that are separately ground and polished from suitable glass blanks and assembled to make up the complete lens.

      A one-piece molded-glass Fresnel lens is convenient for spotlights (spotlight), floodlights, railroad and traffic signals, and decorative lights in buildings. Cylindrical Fresnel lenses are used in shipboard lanterns (lantern) to increase visibility.

      A wide variety of thin Fresnel lenses are molded in plastic, the width of the rings being only a few thousandths of an inch, for use as field lenses with ground-glass screens in cameras and small projectors to increase the brightness of the outer parts of the screen.

      Georges-Louis Leclerc (Buffon, Georges-Louis Leclerc, count de) de Buffon (1748) originated the idea of dividing a lens surface into concentric rings in order to reduce the weight significantly. In 1820 this idea was adopted by Augustin-Jean Fresnel in the construction of lighthouse lenses.

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