an x - ray fluorescence thickness measuring devise includes a primary x - ray beam collimation and workpiece positioning system that markedly increases the detectable fluorescent x - radiation from diverse specimen calibration standards and workpieces subjected to measurement . the positioning system includes an optical viewing system that provides a video signal image of the specimen surface prior to and during specimen radiation without hazard to the unit operator and independent of the collimator bore selected . a further feature of the measuring device is that it includes a system which assures repetitive and accurate positioning of a selected collimator relative to the axis of the x - ray beam to obtain maximum beam transmission therethrough .