The effective quantum gain of digital imaging detectors, was evaluated in this study. Digital detectors, often consisting of a scintillator-optical sensor combination, are employed in a large variety of medical imaging systems (digital radiography, computed tomography-CT, nuclear imaging-SPECT and PET, portal imaging) . This gain was defined as the number of electrons at the output of the detector per incident x-ray. Scintillator layers were prepared and irradiated in laboratory using x-rays at the diagnostic energy range. Emitted light flux and emission spectra were experimentally determined and combined with sensitivity and detection efficiency data of optical sensors. Results showed that x-ray absorption efficiency, x-ray to light conversion efficiency, wavelength dependent optical attenuation effects, spetral compatibility and incident radiation (x-rays or gamma rays) energy are of importance for selecting the appropriate detector for each imaging technique.