This paper treats the fundamentals of infrared spectropolarimetry as a step in understanding electro-optical materials and designing better spatial light modulators. It describes the issues in converting a Fourier transform spectrometer to perform spectropolarimetric measurements and includes mathematics to interpret the resulting spectropolarimetric data. Two distinct differences exist between this proposed instrumentation and previous infrared crystal optics studies: (1) this instrument acquires data simultaneously at all wavelengths within its spectral range and (2) it measures Mueller polarization matrices. Conventional measure-ments with laser polarimeters take birefringence data with applied fields at a few laser wavelengths. With the spectropolarimeter, data are obtained over the entire spectrum, including on and near absorption bands where the most interesting phenomena occur. Measuring Mueller matrices as a function of wavelength provides data on polarization and scattering, effects that will ultimately limit the performance of a modulating crystal. Thus, more data are available with which to compare materials and optimize modulator designs. Better modulators must result from such investigations.