  Recent advances in far-infrared detector technology have led to increases in
raw sensitivity of more than an order of magnitude over previous
state-of-the-art detectors. With such sensitivity, photon noise becomes the
dominant noise component, even when using cryogenically cooled optics, unless a
method of restricting the spectral bandpass is employed. The leading instrument
concept features reflecting diffraction gratings which post-disperse the light
that has been modulated by a polarizing Fourier transform spectrometer (FTS)
onto a detector array, thereby reducing the photon noise on each detector. This
paper discusses the development of a cryogenic (4 K) diffraction grating
spectrometer which operates over the wavelength range from
\SIrange{285}{500}{\micro \meter} and was used to post-disperse the output from
a room-temperature polarizing FTS. Measurements of the grating spectral
response and diffraction efficiency are presented as a function of both
wavelength and polarization to characterize the instrumental performance.

Instrumentation and Detectors;Instrumentation and Methods for Astrophysics