Objective: to study various phosphors used in detectors of medical imaging systems by the gain transfer function (GTF), defined in terms of X-ray luminescence efficiency, light spectrum and modulation transfer function. Materials and methods: four phosphor materials, La2O2S:Tb, Y2O2S:Tb, Y2O2S:Eu and Y2O3:Eu were used in the form of fluorescent layers prepared in the laboratory. The GTF was determined at 30 kVp and 80 kVp X-ray tube voltages for various phosphor coating weights. Results: La2O2S:Tb, which was the highest density and effective atomic number phosphor used, was found to exhibit the best GTF performance at 80 kVp. At 30 kVp, the yttrium based phosphors were found of increased performance. This is mainly due to the proximity of the X-ray energy to the K-absorption edge of yttrium at 17 keV. Europium activated phosphors were found to perform very well when combined with the red sensitive film and the silicon photodiode. Conclusion: The GTF may be a useful method for comparing and selecting phosphor materials for use in various medical imaging applications.