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X-ray fluorescent screens consisting of a powder embedded in a silicone resin matrix and deposited on an aluminum plate have been prepared with the phosphors Gd2O2S:Tb and (ZnCd)S:Ag(P-20) at a series of different screen thicknesses. The x-ray conversion efficiency of the screens was measured using both radioisotopes and a dental radiography unit as x-ray sources. To simulate screen performance in an x-ray image intensifier tube, the screens were optically coupled to the photomultiplier which served as the detector. We found that Gd2O2S:Tb screens were substantially superior to P-20 screens in both x-ray conversion efficiency and x-ray energy absorption efficiency. Experimental screen efficiencies agree with those derived from theory by an analysis in which the efficiency of the conversion of absorbed x-ray energy to light is assumed equal to the energy efficiency of the phosphor under cathode ray excitation and optical parameters of the screens are used to estimate the efficiency with which the generated light is transmitted to the detector.