Construction and performance of a far-infrared diffraction-grating spectrometer are described and compared with those of the Michelson interferometer built by Richards. There is no marked difference between the spectral resolutions of these two instruments. The resolutions attained are between 0.1 and 0.05 cm-1 in the spectral region from 30 cm-1 to 60 cm-1. Some features of the spectrometer discussed in the article are the focusing of the beam on the Golay detector by a conical light pipe, the manufacture of long, thin conical light pipes and of Yoshinaga filters, a special design of the optical path, and a new optical arrangement for reflectance measurements.
F. K. KNEUBÜHL, J.-F. MOSER, and H. STEFFEN, "High-Resolution Grating Spectrometer for the Far Infrared," J. Opt. Soc. Am. 56, 760-763 (1966)