Abstract: A coating thickness gauge for measuring the thickness of a coating of chromic oxideon a chromium layer on a substrate has a light source 17 for generating linearly polarized light, means 19 for splitting elliptically polarized light reflected through the chromic oxide coating into a plurality of beams 20, 21, detectors 22, 23 for intensity of each of the beams and means 24 for calculating the thickness of the coating of chromic oxide from the measured intensities. To simplify the device and the calculation, the splitting means are arranged to split the reflected elliptically polarized light into two partial beam 20, 21 polarized at a known angle relative to each other and the calculation means calculate the ellipticity from the measured intensity of the two partial beams and the thickness from the ellipticity.