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.
The design enables the construction of lenses of large aperture and short focal length without the weight and volume of material that would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner, thus passing more light and allowing lighthouses to be visible over much longer distances.
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