A computer-controlled rotating-detector ellipsometer (RODE) has been constructed using a windowless planar-diffused Si photodiode, a stepping motor, an operational amplifier, and a personal computer (PC) equipped with an A/D-DI A converter. The detector is mounted to the output shaft of the stepping motor at an optimum angle ofincidence of ~ 59" and is rotated through 360 0 about the incident beam as an axis. The detector output signal voltage is measured and stored in the PC at every 2° increment ofthe RODE angle O. Fourier analysis of the recorded data provides a handedness-blind determination of the state of polarization of the incident ligh t. A beam from a ReNe laser (A = 632.8 nm) is transmitted through the polarizing optics of an ellipsometer to provide the polarization states needed for calibration and testing. The calibration parameters m Land e r are determined by rotating the detector about a linearly polarized beam of light of zero reference azimuth. The RODE is subsequently tested and found to correctly measure the first two normalized Stokes parameters of a number of states with a residual rros error of -0.002. This limit on precision is dictated mainly by the 12-bit AID converter. A small angular misalignment of the rotation axis of the detector with respect to the light-beam axis introduces odd harmonics in the detector signal; hence, its effect is readily isolated by appropriate data reduction. Thin-film coatings on the detector surface that significantly improve the performance of the RODE are proposed.