The lack of equivalence between polarizer extinction aximuths in different zones is the result of relative amplitude attenuation in some mica wave plates. A dependence of transmittance on the orientation of the electric-field vector relative to the compensator axes is caused either by interference, or by pleochroism, or both. Experimental data establish these conclusions. The correct method for interpreting measurements made with such non-ideal compensators is presented.
R. J. ARCHER and C. V. SHANK, "Ellipsometry with Non-Ideal Compensators," J. Opt. Soc. Am. 57, 191-193 (1967)