Fresnel Lens

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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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

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.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
References in periodicals archive ?
As is well known, a two-level Fresnel reflector focus the signal because the level separation is designed to be [lambda]/4, so the reflected signals in the even zones are in phase with the reflected signal in the odd zones.
As we have seen in the previous section, the objective of the experimental system is to measure the transmission coefficient between two horn antennas in the W band (from 75 to 110 GHz), one of them being the feeder of a Fresnel reflector. The idea is to place both horn antennas at different angular positions with respect to the reflector, so we have separated the design of the system in the following steps: design of the Fresnel reflector, design of the experimental set-up and definition of the measurement process.
In Figure 5, a scheme of the experimental system is shown, where a dielectric arm can be seen to support the reception horn whose mission is to focus the signal reflected by the Fresnel reflector, which arrives from the far-field horn emitter.
--Having the same radiating aperture, the directivity of a Fresnel reflector is lower than a parabolic reflector.
This paper describes an exhaustive study to evaluate the behaviour of a Fresnel reflector in W band.
Figure 13 shows the ratio between the Fresnel reflector and the metallic surface, versus frequency.
These results provide very clear evidence of the better behaviour of the Fresnel reflector compared to a metallic surface.
As the results in Figure 14 are obtained from measurements using the Fresnel reflector itself, this figure also demonstrates the evolution with frequency of the limit of the angular coverage, maintaining all geometric characteristics of the reflector.
In Figure 15 the radiation pattern of the Fresnel reflector and the plane metallic surface is shown for the case when the feeder is placed at 45[degrees] of elevation.
The measurements made in this work have demonstrated that the focal position of a Fresnel reflector is not a single point and it depends on the direction of arrival of the signal.