In a polarizing Fourier’ spectrometer the spectral information is contained in the interference pattern of polarization states as the function of the path difference. This is especially useful at low frequencies, where polarizing beamsplitters are much more efficient than non-polarizing ones.
In Faraday configuration light propagates along the magnetic field while the propagation of light is perpendicular to the magnetic field in Voigt configuration.
The TeslaFIR spectrometer is an automated instrument which enables the measurement of spectra in the energy range from about 250cm-1 down to 2cm-1. It uses a mercury arc lamp as the light source in a polarizing (Martin-Puplett) interferometer SPS-200, and two bolometers that are cooled to 0.3K by pumping on He3. The samples can be cooled down to 2.5K. The cryostat is equipped with a 12T magnet, where samples can be mounted in Faraday and Voigt configurations. More details can be found here.
There is a separate insert for reflection measurements. In the picture below one can see a relative reflectance spectrum of bulk Pb. The superconducting gap of Pb is clearly visible. To obtain this test spectrum several hours of measurements were averaged.
The high resolution of an instrument with a 5 cm travel stage is seen from the sharp absorption lines of low pressure water vapor lines.