The present invention relates to ellipsometer and polarimeter systems, and more particularly is an ellipsometer or polarimeter or the like system which operates in a frequency range between 300 GHz or lower and extending to higher than at least 1 Tera-hertz (THz), and preferably through the Infra-red (IR) range up to, and higher than 100 THz, including: a source such as a backward wave oscillator; a Smith-Purcell cell; a free electron laser, or an FTIR source and a solid state device; and a detector such as a Golay cell; a bolometer or a solid state detector; and preferably including at least one odd-bounce polarization state image rotating system, and optionally including a polarizer, at least one compensator and/or modulator, in addition to an analyzer.