A general analysis of the effect of cell windows shows more error terms in each of the four ellipsometry zones than is found in the conventional small-retardation wave-plate approximation. The value of ψ from a two-zone average requires a correction that is generally dependent on both the window and the reflecting surface under study. A difference of the flux transmission in the s direction from that in the p direction as small as 10-6 leads to an appreciable correction ~0.06°. An experimental procedure to correct for window error is described and is applied to three different types of windows with Δ errors from 0.05° to 1.5°.
R. M. A. AZZAM and N. M. BASHARA, "General Treatment of the Effect of Cell Windows in Ellipsometry," J. Opt. Soc. Am. 61, 773-776 (1971)