In employing ellipsometry to determine the complex refractive index of a reflecting surface, a small systematic error in the zero-angle positions of the polarizers is shown under certain circumstances to have a large effect on the values of the optical constants calculated. The effect of superficial optical anisotropy on ellipsometry in general, and on the zero position determination in particular, is analyzed and a test for the presence of such anisotropy presented.
M. J. Dignam and M. Moskovits, "Azimuthal Misalignment and Surface Anisotropy as Sources of Error in Ellipsometry," Appl. Opt. 9, 1868-1873 (1970)