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About Fresnel Lenses
Fresnel lenses resemble a planoconvex or planoconcave lens that is cut into narrow rings and flattened. If the steps are narrow, the surface of each step is generally made conical and not spherical. The convex surface is reduced to concentric ridges, to avoid overheating and to reduce weight. Fresnel lenses are flat rather than thick in the center and can be stamped out in a mold. Fresnel lenses are commonly used on spotlights. Lamps equipped with this lens are called Fresnels. Fresnel lenses can be positive or negative. A positive lens is a Fresnel lens with a positive focal length. Negative lenses are Fresnel lenses with a negative focal length. Lens application is an important parameter to consider when searching for Fresnel lenses. Lens application refers to the desired wavelength range for the lens. This wavelength range is mostly dependent upon the lens material. Near infrared Fresnel lenses are designed for use in the 750 nm to 2500 nm wavelength range. Visible Fresnel lenses are designed for use in the 380 nm to 750 nm wavelength ranges. Ultraviolet Fresnel lenses are designed for use in the 4 nm to 380 nm wavelength ranges.

Important lens geometry parameters to consider when specifying Fresnel lenses include effective focal length, grooves, round or square construction, and dimensions. The effective focal length is the distance from the lens the light converges. Focal lengths are positive for a converging system and negative for a diverging system. Fresnel lens use concentric grooves to focus light. The number of grooves is measured by grooves per millimeter. Fresnel lenses usually come square or round shapes. Round lenses are round when viewed from the front or back. Square or rectangular lenses are square or rectangular when viewed from the front or back. Dimensions to consider include the diameter for round lenses, the side for square lenses, and the length for rectangular lenses. Overall thickness of the lens is also important to consider.

Fresnel lenses resemble planoconvex or planoconcave lenses that have been cut into narrow rings and flattened. If the steps are narrow, the surface of each step is generally conical and not spherical. The convex surface is reduced to concentric ridges, to avoid overheating and to reduce weight.

Because Fresnel lenses are thin, very little light is lost through absorption. Fresnel lenses are a compromise between efficiency and image quality. High groove density in Fresnel lenses allows higher quality images, while low groove density in Fresnel lenses yields better efficiency (as needed in light gathering applications). Fresnel lenses are most often used in light gathering applications, such as condenser systems or emitter/detector setups. Fresnel lenses can also be used as magnifiers or projection lenses but often produce a high level of distortion.

The wave theory of light gave Fresnel rein to a number of inventions in optics. These included the Fresnel mirrors (canted to produce an interference pattern), the Fresnel rhomb (produces elliptical polarization using internal reflection), and most importantly, the Fresnel lens, which uses a stepped geometry to produce focussing. The lens shown above are plastic and can be used for magnification or for solar radiation concentration.Fresnel lens are used for overhead projector. Fresnel lenses are used in automobile headlights and large glass Fresnel lenses are used in lighthouses and have been instrumental in the prevention of shipwrecks

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