Exact method for in situ correction of windows' linear birefringence in ellipsometry measurements with an isotropic sample is presented. The method is based on the idea that an unpolarized light is unaffected by a retarding optical element. An optical system was built, in order to simulate an ellipsometry measurement cell with windows with large retardation. The system consisted of two configurations, an "ideal" configuration in which optical retarders were used as windows, and a real life configuration in which sapphire slabs were used as windows. For the measurements with sapphire windows, the ellipsometric parameters of the gold mirror and the nickel foil without the correction yields (psi, DELTA)_(Au) velence (0.66(+-0.005), 1.56(+-0.05)), (psi, DELTA)_(Ni) velence (-0.713(+-0.005), 1.4(+-0.05)). While after the correction the parameters yields (psi, DELTA)_(Au) velence (0.704(+-0.006),2.15(+-0.1)), (psi,DELTA)_(Ni) velence (0.619(+-0.005), 2.33 (+-0.05)), which are in good agreement with our calibration measurements: (psi,DELTA)_(Au) velence (0.709(+-0.005), 1.98(+-0.06)) and (psi, DELTA)_(Ni) velence (0.615(+-0.005),2.32(+-0.05)).
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