  An extension of the Kramers-Kronig method for treatment of polarized infrared
reflectance spectra from the face of low-symmetry crystals, where directions of
principal dielectric axes depend on frequency, is proposed. It is shown, how to
obtain the frequency dependencies of the complex reflectivity tensor
components, using three reflectance spectra measured for different directions
of linear polarization of the incident wave, when reflected wave is immediately
sent to the detector. The problem is formulated in a form of the system of
integral equations, and effective numerical technique is found for solving it.
The question of the further recovery of the complex dielectric tensor on the
base of the reflectivity tensor, is discussed. The case of the monoclinic
crystals is considered in details. An example of the application of the
extended Kramers-Kronig method to the reflectance spectra from the (ac)-face of
the single-crystal bismuth oxide with the monoclinic lattice is given.
