The theory conventionally used to describe the operation of Berek and Ehringhaus rotary compensators is based on geometrical optics and, hence, is not exact. When rotary compensators are analyzed within the framework of classical electromagnetic theory, exact solutions for the phase difference and amplitude ratio of the transmitted light can be determined. The approximate and exact solutions differ in some interesting ways which become of substantial importance in the examination of monochromatic plane waves of light. In particular, the discrepancies between exact and approximate solutions become more pronounced at high angles of incidence. Exact theory predicts the possibility of using a high-refractive index isotropic plate for measuring small phase differences at relatively long wavelengths.
D. A. HOLMES, "Wave Optics Theory of Rotary Compensators," J. Opt. Soc. Am. 54, 1340-1347 (1964)