A method developed for the absolute determination of complex reflectivity is discussedin terms of reflection standards for the 3 to 600 cm-1 spectral region. These can beprimary standards based on transparent materials such as silicon, or secondary, opaquestandards such as thin aluminium films, with or without protective overcoating.
J. R. Birch,
"Far infrared reflectance standards", Proc. SPIE 2104, 18th International Conference on Infrared and Millimeter Waves, 210423 (30 August 1993); doi: 10.1117/12.2298507; https://doi.org/10.1117/12.2298507