The techniques generally used for measuring spectral transmission of lenses in the visible wavelength range are difficult to apply in the range of thermal wavelengths (2 to 14 Microns). The paper describes a more appropriate technique which can be used both for measuring the axial transmission factor of a lens system as well as the variation of transmission over the aperture. Two particular equipment configurations are described, one is designed to integrate with a computerised MTF instrument, whilst the other is based on the use of a commercial radiometer.
T. L. Williams,
"Measurement Of Spectral Transmission Of IR Lens Elements And Assemblies", Proc. SPIE 0590, Infrared Technology and Applications, (1 May 1986); doi: 10.1117/12.951973; https://doi.org/10.1117/12.951973