Due to the progress of the diamond turning technique, the use of aspheric elements in optical systems is increasing. However, there are materials, like glass, which are not compatible with this technique especially when the shape of the aspheric element is not a simple conic. Using ion beam milling through a mask which modulates spatially the amount of ions impinging on the surface to be shaped, a piano-convex glass lens has been transformed into an aspheric element with a conical front surface. This technique is valuable for any material used either in the visible or in the IR spectrum.
N. P. Eisenberg,
"Aspheric Generation On Glass By Ion Beam Milling", Proc. SPIE 1038, 6th Mtg in Israel on Optical Engineering, (5 July 1989); doi: 10.1117/12.951063; https://doi.org/10.1117/12.951063