  Kramers-Kronig type dispersion relations for integer powers of complex
reflection coefficient are introduced for testing the consistency of terahertz
reflection spectra. By using numerical simulations we show that such dispersion
relations can be applied for distillation from data with some experimental
artifacts without data extrapolations beyond the measured spectral range. These
dispersion relations, due to causality, provide a powerful and yet uncommon
tool to examine the consistency of the spectroscopic data obtained in
reflection spectroscopy at terahertz range. In particular we show that real and
imaginary parts of the complex reflection coefficient obtained from raw data
with systematic phase error caused by sample misplacement, not necessarily obey
dispersion relations, while the ones corrected with maximum entropy method obey
these relations.
