A spectroscopic ellipsometer in which the polarizer and the analyzer are rotating synchronously in opposite directions at the same speed is proposed. The light intensity involves four components, one dc and three cosine terms, with frequencies of omega, 2 omega, and 3 omega. The main advantage of the proposed ellipsometer is that: it is feasible to extract the ellipsometric parameters psi and Delta from the even Fourier coefficients without relying on the dc component which is considered to be a serious problem in rotating-analyzer or -polarizer ellipsometer. This allows measurements in semi-dark room without worrying about stray light problems, dark currents in detectors, and long term fluctuations in light sources. The calculations of the optical parameters of c-Si, Au, and GaAs are in excellent agreement with the published data. (C) 2010 Elsevier B.V. All rights reserved.