平凸透鏡，S1-UV 等級熔融石英

The flat back surface made aligning it in my setup far easier than I expected, and the positive focal length is spot on for collimating my beam. Even under constant use, the fused silica stays crystal clear with no hazing or burn marks.

The fused silica handles UV way better than the borosilicate lens I was using before, no yellowing after a few weeks under the lamp. Edge is clean and the flat side sits flush in my mount. Focal spot is tight enough that I stopped hunting for a better optic.

I bought two of these to focus a UV laser beam and the fused silica handles the deep UV without any of the solarization I got with cheaper glass. The plano side made mounting in my optical cage simple, and the focal spot is tight and clean.