We present a spectroscopic, autocollimating ellipsometer capable of operating at arbitrary angles of incidence. Linearly polarized light incident on a sample is circularly polarized on reflection, ensuring that the retroreflected beam is orthogonal to the input polarization state. In order to achieve this at arbitrary angles of incidence, a Soleil–Babinet compensator (SBC) is introduced with its fast axis fixed horizontally. Nulling is achieved by varying the SBC delay and the azimuthal angle of the input linear polarization. A single calibration equation at a fixed wavelength and a knowledge of the wavelength dependence of the compensator birefringence enables the delay to be accurately calculated at any wavelength. Single- wavelength, variable angle of incidence measurements made on a thick gold film are in excellent agreement with those obtained with a traditional null ellipsometer. Spectroscopic measurements at a fixed angle of incidence of a silicon dioxide film on a silicon substrate yield thicknesses that are in excel lent agreement with independent measurements made with a null ellipsometer and a commercial instrument.
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