Formulas are presented for the use of optical compensators in ellipsometers. Nothing need be known about the compensator characteristics other than the directions of “fast” and “slow” axes. The formulas can be used, without modification, when the compensator is not an exact quarter-wave plate, when multiple internal reflections are present, and when the compensator is absorbing. Also described is a method for calibrating the compensator at the same time it is being used in ellipsometry measurements.
D. A. HOLMES and D. L. FEUCHT, "Formulas for Using Wave Plates in Ellipsometry," J. Opt. Soc. Am. 57, 466-471 (1967)