A Fresnel Lens is a lens that uses a series of annular rings of spherical lenses set at various angles to produce a lens with a very short focal length relative to its diameter. The lens will also be much thinner (using less material) than its corresponding spherical lens. Sometimes the spherical rings are replaced with precision optic flat surfaces. Fresnel lenses are very useful in gathering light using a thinner lens. This can be useful in long-wavelength applications or systems with a very short working distance. Fresnel lenses are used in solar energy products, virtual reality head-mounted displays, condenser systems, and illumination systems.