H-K9L (N-BK7) plano-convex lenses are often used to focus parallel light or collimate point light sources, with excellent transmittance from visible to near infrared (350-2000nm). H-K9L is a commonly used optical glass. When the additional advantages of UV fused silica (good transmittance in the UV band and low thermal expansion coefficient) are not required, H-K9L is generally selected. When converging light, if an absolute magnification of less than 0.2 or greater than 5 is obtained, a plano-convex lens is usually selected. When the magnification is between 0.2 and 5, a biconvex lens is more suitable. To reduce spherical aberration, parallel light should be incident on the curved surface when focusing. A variety of coating specifications can be selected.