Abstract: 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.

Abstract: An automatic two wavelength photoelasticimeter comprising, in succession, a light source for emitting light having at least two wavelengths, .lambda..sub.1 and .lambda..sub.2 ; a polarizer rotating at constant speed, an orientable quarter-wave plate of orientation .beta. with respect to a reference axis; a model to be analyzed exhibiting double-refraction and having a fast axis which forms an angle .theta. with respect to the reference axis and which provides a phase shift .phi. (which parameters .theta. and .phi. are to be measured) and three photodetectors 5, 6, and 7 preceded by analyzers 10, 11 and 12 respectively. The photodetectors 6 and 7 are preceded by filters 8 and 9 of wavelengths .lambda..sub.1 and .lambda..sub.2 respectively. The difference between the phases of the signals of the frequency 2.omega.