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. The Fresnel lens is a special optical lens. It can now be made from plastic such as acrylic fresnel lens, PMMA, polyvinyl cholride (PVC), polycarbonate (PC) and HDPE. A traditional glass convex lens would be thick, heavy and very expensive, but a plastic Fresnel lens is a thin, flat, lightweight and low cost alternative.