LSO:Ce, LYSO:Ce and GSO:Ce single-crystal scintillator light emission characteristics were studied in the low γ-ray energy range (99mTc source) used in nuclear medical imaging. The absolute luminescence efficiency and the optical emission spectrum of the three scintillators were measured, under γ-ray excitation using an integration sphere coupled to a photomultiplier and an optical spectrometer, respectively. Spectral compatibility of all scintillators to optical sensors was also estimated. The absolute luminescence efficiency of all crystals was found adequately high (8.7 μW m−2/μGy s−1 for GSO:Ce, 15.3 μW m−2/μGy s−1 for LYSO:Ce and 20.0 μW m−2/μGy s−1 for LSO:Ce). Their emission spectra were found compatible (57–94%) to currently employed optical photon detectors.