A plano concave lens is composed of a flat surface and a concave curved surface, with a negative focal length, commonly used as a reduced image or diverging collimated beam. In this case, the curved surface of the lens should be oriented towards the light source to minimize spherical aberration. In addition, they can compensate for the spherical aberration caused by other lenses in the optical system. The commonly used materials for flat convex lenses are H-K9L or JGS1. H-K9L glass is the most common optical glass that provides excellent optical performance and is reasonably priced. H-K9L has the characteristics of few bubbles and low impurities, and is widely used in the visible and near-infrared bands. JGS1 is applied in the wavelength range of 185 to 2100 nm, with excellent transmittance in the ultraviolet region, without emitting fluorescence under ultraviolet light, and is radiation resistant. Compared with other optical glasses (such as K9L), UV fused silica (JGS1) has lower refractive index, lower thermal expansion coefficient and better uniformity at a certain wavelength.