A far ir spectrometer has been constructed with an Ebert-Fastie monochromator of 1.8-m focal length plane gratings 19 cm × 13 cm, and slit height of 10 cm. The source optics of the evacuable instrument are double beam, with an electronic system of recording the ratio of the radiant powers in each beam. The instrument has been used thus far over the range 20–300 cm-1 and the resolution achieved with the four gratings of 2 lines/mm, 4 lines/mm, 10 lines/mm, and 30 lines/mm varies from 0.1 cm-1 to about 0.4 cm-1, depending on the grating angle. The combinations of reflection and transmission filters used to eliminate higher orders reduce unwanted radiation to 1% or less in most ranges, though at a few wavenumbers it may be as large as 5%. A commercial bolometer of gallium-doped germanium cooled with liquid helium serves as the detector. Details of the operation of the instrument are given, and the results obtained are illustrated with the spectra of gaseous polyatomic molecules.
T. M. Hard and R. C. Lord, "A Double Beam, High Resolution Spectrometerfor the Far Infrared," Appl. Opt. 7, 589-598 (1968)