The problem of spectral filtration and beam splitting in the far infrared portion of the spectrum is, at present, a more difficult problem than in the shorter wavelength region of the spectrum multilayer dielectric stacks are used almost exclusively in the short wavelength region. The theory for the design of this type of filter is well known and the behavior of a filter can be predicted to a high degree of accuracy. Even though interference dielectric stacks offer better characteristics than other filtering methods presently used in the far IR, the use of this form of filter has not been given a lot of consideration above 30 to 40 μm. This is due, in part, to several problems arising from material characteristics and the inherent difficulty in vacuum deposition of very thick layers. This paper reports on a preliminary experimental study to examine the feasibility of constructing multilayer dielectric interference filters for the 2 to 1000 μm region of the spectrum. Test results are given for two filters which were constructed to operate at 100 μm.
R. J. Serviss,
"Preliminary Report On Interference Filters For The 20 To 1000 Micron Spectral Region", Proc. SPIE 0067, Long-Wavelength Infrared, (10 November 1975); doi: 10.1117/12.954534; https://doi.org/10.1117/12.954534