Transmission measurement is the best method for characterization of highly transparent materials. In particular, transmission spectra of plane parallel plates exhibit a periodic transmission pattern caused by interference (Fabry-Perot etalon fringes). Real and imaginary parts of the dielectric constant can be determined from these measurements, as the period and amplitude of the etalon fringes depend on the material refractive index and absorption, respectively.
Characterization of semi-transparent materials requires a THz Mach-Zehnder interferometer (shown below), since no etalon fringes can be observed in the transmission spectra of such materials. The Mach-Zehnder setup enables measurements of a phase shift induced by the sample as a function of frequency. Combining this data with transmission spectrum, real and imaginary parts of the dielectric constant can be calculated