PURPOSE: To propose an image quality index, the brightness-sharpness index (BSI), for assessing the quality of the image produced by phosphors of medical imaging detectors. MATERIAL AND METHODS: BSI was evaluated by experimental X-ray luminescence and modulation transfer function measurements. BSI was determined for a number of test phosphor screens prepared from Gd2O2S:Tb, La2O2S:Tb, and Y2O2S:Tb phosphor materials. The screens covered a wide range of coating thicknesses from 50 to 150 mg/cm2 and measurements were performed for X-ray tube voltages between 50 and 120 kVp. RESULTS: Gd2O2S:Tb phosphor exhibited higher brightness and sharpness, as compared to the other phosphor materials, for all screens and X-ray tube voltages used. Best Gd2O2S:Tb performance was observed for thin screens and high tube voltages. La2O2S:Tb exhibited higher BSI values than Y2O2S:Tb for medium and high tube voltages. CONCLUSION: Results showed that phosphor materials of high X-ray detection and X-ray-to-light conversion properties exhibit high BSI values indicating that BSI may provide a means of phosphor performance evaluation for imaging applications.