Fresnel Lens

Fresnel lens

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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Fresnel lens [frā′nel ′lenz]

(optics)

A thin lens constructed with stepped setbacks so as to have the optical properties of a much thicker lens.

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Fresnel lens

Fresnellens

In lighting, a lens that concentrates light from a small source such as an incandescent filament; similar to but thinner and lighter than a plano-convex lens owing to steps on the convex side; used in many types of luminaires, esp. downlights and spotlights.

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Fresnel Lens 

a complex compound lens used in lighthouse and signal beacons. It was proposed by A. J. Fresnel. The lens does not consist of a single polished piece of glass with spherical or other surfaces like conventional lenses but instead is made of a

Figure 1. Cross section of ring-type Fresnel lens. In the center are rings whose outer surfaces are parts of toroidal surfaces. Along the edges of the lens are rings in which full internal reflection occurs in addition to refraction.

succession of individual, thin concentric rings, which in cross section have the shape of a special type of prism (see Figure 1). Such a design gives the Fresnel lens very low thickness, and therefore also very low mass, even for a large coverage angle. The sections of the rings are such that the spherical aberration of the Fresnel lens is slight and beams of light from point source S placed in the focus of the lens emerge, after refraction in the rings, in a practically parallel beam (in ring-type Fresnel lenses).

There are two types of Fresnel lenses: ring and belt. The ring-type (circular) lens is the system obtained by rotating the type shown in Figure 1 around optical axis SO; these lenses direct the stream of light in one particular direction. Belt-type (cylindrical) Fresnel lenses are obtained by rotating the type shown in Figure 1 around axis ASA’, perpendicular to SO; they propagate light from the source in all horizontal directions. The diameters of Fresnel lenses range from 10–20 cm to several meters.