Kramers-Kronig analysis of spectra of reflectance has usually been done only with radiation incident as nearly normal to the sample surface as possible. Any effects of obliquity have been assumed to be negligible. However, it is not much more difficult or time consuming to do Kramers-Kronig analysis of spectra taken with almost any angle of incidence, provided that polarized radiation is used to obtain the data. The method for such analysis for radiation incident at almost any angle is described in this paper. The method fails with π polarized radiation if the angle of incidence lies between the two somewhat different Brewster angles at the high and low frequency ends of the spectrum. The rather small error in a typical analysis caused by neglecting a 15° angle of incidence is illustrated.
Dwight W. Berreman, "Kramers-Kronig Analysis of Reflectance Measured at Oblique Incidence," Appl. Opt. 6, 1519-1521 (1967)