Cylindrical lenses have a cylindrical surface, unlike spherical lenses, which act symmetrically on incident light in both directions, while cylindrical lenses act in only one direction on incident light. Cylindrical lenses are generally used to focus parallel or divergent beams onto a line or to change the aspect ratio of the image. Cylindrical lenses are generally designed for applications where one-dimensional shaping of the light source is required. They have two structures, flat - concave and convex, which are used for divergent or converging beams respectively. Plano-convex cylindrical lenses focus the incoming light onto the line and have a positive focal length consisting of one plane and one convex cylinder, while plano-concave cylindrical lenses have a negative focal length consisting of one plane and one concave cylinder.