Absolute efficiency and modulation transfer function of laboratory prepared La2O2S:Tb phosphor screens of various coating thickness were studied. Detailed experimental data on the variation of absolute efficiency with x-ray tube voltage up to 200 kVp and screen coating thickness in transmission and reflection observation mode are given. Data were compared with similar results from other rare earth phosphor materials. Theoretical calculations were in good agreement with experimental data to permit estimation of intrinsic efficiency and coefficients related to light scattering and absorption within the phosphor material. The MTF of La2O2S:Tb screens was also experimentally and theoretically evaluated.

The angular distribution of the light emitted from a fluorescent screen under x-ray excitation is measured as a function of the thickness of the screen for a number of granular phosphors. This angular distribution is found to have a systematic divergence from the Lambertian one. Experimental results are discussed on the basis of a plausible physical model which takes into account the absorption and scattering of the light in the screen. A report is given concerning the magnitude of the errors appearing in the fluorescent screen calculations which are due to using the Lambertian distribution instead of the real one.

The absolute efficiency of Gd,O,S : Tb screens under fluoroscopic conditions was studied and found to vary between 0.5 and 9.0 x W m-' mR" S depending on the x-ray tube voltage and on the screen's weight. Theoretical calculations based on the uniform screen model gave results in close agreement with experimental data.

The absolute efficiency of X-ray phosphor screens was studied in reflection mode observation for two phosphor materials widely used in X-ray applications, namely Y2O2S:Tb and Gd2O2S:Tb. The efficiency proved to depend on the tube voltage and on the screen thickness. Comparisons are given between the efficiency of the same screens in reflection and transmission mode observation. Theoretical calculations based on a uniform screen model give results that are in very good agreement with the experimental data.

No abstract