A plane–parallel mesh Fabry-Perot resonant cavity has been used to determine the indices of refraction of a number of low-loss materials at 245.351 GHz using a C13H3F far-IR laser. The necessary apparatus is readily available in many far-IR laser laboratories and is readily extendable to other laser and millimeter-wave source frequencies. The values were compared to the measurements of other researchers at neighboring frequencies as available.
G. J. Simonis and R. D. Felock, "Index of refraction determination in the near-millimeter wavelength range using a mesh Fabry-Perot resonant cavity," Appl. Opt. 22, 194-198 (1983)