Abstract: Apparatus and method for continuous measurement of thickness of a sheet at a rolling mill or the like without contacting the sheet. A system directing radiation through the sheet in two energy bands and providing a measure of change in composition of the material as it passes the thickness gauging station. A system providing for changing the absorption coefficient of the material in the thickness measurement as a function of the change in composition so that the measured thickness is substantially independent of variations in composition.

Abstract: Apparatus for measuring the thickness of the plastic film of a blown film machine, making the measurement adjacent the extruder die without contacting the film. A radiation source and detector spaced from one side of the plastic film and a metal member spaced from the other side, with the detector receiving fluorescent x-rays from the metal member produced by source photons and receiving back scattered source photons from the film and the metal member, with both the fluorescent x-rays and the scattered photons producing indications of thickness.