Gd2O2S : Tb and CsI : Na phosphors mostly used in X-ray medical imaging detectors were compared at various X-ray tube voltages between 50 and 150 kVp. Laboratory prepared test screens of 80, 110, and 150 mg/cm2 coating weight were evaluated on the basis of the number of emitted photons per incident X-ray (NEP) and DQE, both expressing the signal-and-noise transfer properties of image receptors. NEP and DQE were determined by a method employing luminescence and emission spectrum measurements. The spectral compatibility of the two phosphors with films, photocathodes and the Si photodiode was also examined. CsI : Na exhibited high performance in the 60–90 kVp X-ray tube voltage range while Gd2O2S : Tb was found better at higher X-ray energies. The CsI : Na – film combination could provide an excellent X-ray radiographic detector, if effectively protected from humidity. Gd2O2S : Tb is more suitable for high tube voltage digital imaging but also adequate for conventional imaging.