The concept of using a parametric representation of wavelength rather than actual wavelength in a polynomial relating wavelength to refractive index is applied to a wide range of IR materials. Excellent agreement between calculated and published measurements of index is found. With such data it is possible to design lenses which are well-corrected with wavelength and temperature.
D. R. Barron, D. R. Barron,
"Buchdahl's Chromatic Co-ordinate Concept Applied To IR Materials", Proc. SPIE 0805, Optical Components and Systems, (28 September 1987); doi: 10.1117/12.941368; https://doi.org/10.1117/12.941368