A plano concave lens diverges parallel light outward, with a negative focal length and spherical aberration. It can be used to correct phase difference, spherical aberration, coma, and distortion of other lenses. Similar to a flat concave mirror, the side is generally facing away from the object surface or infinity. The flat concave lenses produced by Dingxinsheng Optics are usually made of K9 glass, and their anti reflective coating is designed for the wavelength range of 400-700 nanometers. UV quartz is also used as the substrate for processing. When UV quartz is used as the substrate, it has good transmittance and low thermal expansion coefficient in the UV region, and can be coated with UV AR film for special use.