The measuring method has the light provided by a light source (1) passed through a rotating polariser (2), before reflection by a sample (3) onto a non-rotating analyser (4), or passed through a non-rotating polariser before reflection by the sample onto a rotating analyser. The analyser is coupled to via a monochromator (5) to a detector (6), with a lock-in amplifier (8) for amplifying the detector signal to provide the output signal. The rotation frequency of the polariser or analyser is used as the base for the lock-in reference frequency. USE - For providing optical characteristics of thin layers for optical components used in dielectric mirror system of excimer laser,or anti-reflection coatings for UV lithography.