A meniscus lens is a lens with a concave side and a convex side. If the focal length of a negative meniscus lens is negative and the radian outward is less than the radian inward, the lens has a negative focal length. Meniscus lenses can be used to correct phase differences, usually as corrective lenses, also can be used as a concentrator in the lighting system, and choose the right thickness can also have the effect of eliminating color differences.