An interferometric ellipsometer (100) for measuring ellipsometric parameters of a sample. The ellipsometer comprises a light source (12) that is arranged to direct a light beam (14) onto the sample (16) for reflection. An interferometer (20) is arranged to receive and split the reflected light beam (18) from the sample (16) into two beams, one split beam (26) being modified by an optical system to generate a reference beam (30) that has p- and s-polarisations with a common phase and with a fixed relative amplitude and which is recombined with the remaining split beam (24) to generate an output beam (42) having temporal p- and s-fringes. An output detector (44) is arranged to detect the output beam (42) and output signals representing the p- and s-components of the output beam from which the ellipsometric parameters of the sample (16) can be determined.
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