The image quality of modern infrared systems has almost to be diffraction limited. Therefore high demands in lens design, choice of materials and the manufacture are required. Typical advances in lens design using full advantage of aspherics are shown. For the materials used homogeneity and birefringence have to be in certain limits and kept under control. Optical surfaces for the IR may be manufactured applying the classical polishing technique or by single point diamond turning, whereby diamond turning is used especially for the manufacture of aspherics. Imperfect surface structure will cause veiling glare and thus reduce image quality. Typical examples are shown.
"Improvements In The Design And Manufacture Of Infrared Optical Systems", Proc. SPIE 0916, Infrared Systems--Design and Testing, (11 October 1988); doi: 10.1117/12.945554; https://doi.org/10.1117/12.945554