An ellipsometer for measuring the polarization parameters .alpha. and .lambda. of an elliptically polarized light wave. The light wave passes successively through an orientable quarter-wave plate, a birefringent plate rotatable at a constant speed .omega. and a polarizer and then impinges upon a photodetector. Reference signals having angular frequencies of 2.omega. and 4.omega. are generated, one of these signals being employed to synchronously detect the signal at the output of the photodetector by adjusting the quarter-wave plate until the synchronously detected signal component is zero, the orientation of the quarter-wave plate then corresponding to the polarization parameter .alpha.. The parameter .lambda. is obtained by measuring the phase of the component at the output of the photodetector having an angular frequency of 4.omega..The invention applies in particular to photoelasticimetry.