Abstract: The device operates by means of the X-ray fluorescence principle wherein a beam of primary X-rays is directed on to a coated test specimen and X-rays emitted back by the test specimen are counted to determine the thickness of the coating. The device has a stage for supporting test specimens, an X-ray generator, a collimator device for the primary X-rays, a counter for X-rays emitted back by the test specimens, and a displacement device for moving the stage in at least one direction at right angles to the collimator device, and in the direction of the X-ray beam. A rod composed principally of aluminum of a material of low atomic number is positioned at a safe distance beneath the collimator device, and approximately at right angles to the X-ray beam. The shaft of an electric motor imparts periodic motion to the rod. An evaluating device monitors the motion of the rod and outputs a signal when the rod is momentarily stopped.