Abstract: An apparatus and method for measuring the thickness of a protective film on a sheet or substrate as the sheet moves past the apparatus. The apparatus includes a black-body radiator in the form of an isothermal cavity having an opening across which the sheet is moved. The cavity has an observation window opposite the opening through which window the surface of the moving sheet may be scanned. A radiation pyrometer scans the moving sheet through the observation opening. Rays from the radiator pass through the film to the substrate, are reflected by the substrate, and pass back through the film and through the window to the pyrometer. The absorption of rays from the black-body radiator as they pass through the film is a function of the thickness of the portion of the film being observed. The radiation pyrometer thus receives an output which is a function of the film thickness being measured.