An analytical and experimental study of an automatic ellipsometer is reported. The polarized beam is modulated simultaneously by two ADP cells, and the corresponding two signals are separated and are used to adjust polarizer and analyzer settings in any of the four zones. The reproducibility of the setting has been demonstrated to be of the order of 0.01°.
Hiroshi Takasaki, "Automatic Ellipsometer. Automatic Polarimetry by Means of an ADP Polarization Modulator III," Appl. Opt. 5, 759-764 (1966)