UV-grade fused silica offers high transmission in the deep UV and extremely low fluorescence levels compared to natural quartz, making it an ideal choice for applications from the UV to the near IR. In addition, UV fused silica has better homogeneity and a lower coefficient of thermal expansion than BK7.
Bi-concave lenses have negative focal lengths and are often used to increase the divergance of a converging beam.
Both of a plano-concave lens and a bi-concave lens can cause the incident light to diverge, but it is usually preferrable to choose a bi-concave lens if the absolute conjugate ratio (object distance divided by image distance) is close to 1. When the desired absolute magnification is either less than 0.2 or greater than 5, the tendency is to choose a plano-concave lens instead.