1. A calibration method for a gamma ray detector including a scintillator array for emitting scintillation photons at photo conversion positions in response to incident gamma rays and a photodetector array coupled thereto in light-sharing mode for determining a spatial intensity distribution of scintillation photons, said method comprising the steps of recording spatial intensity distributions of scintillation photons emitted by the scintillator array in response to multiple incident gamma rays; determining sets of coincidently emitted scintillation photons from the recorded spatial intensity distributions; determining for the sets of coincidently emitted scintillation photons center-of-gravity positions and cumulative energies; performing a clustering analysis based on the determined center-of-gravity positions and cumulative energies to obtain clusters of gamma ray events attributed to a scintillator array element; cumulating for a cluster the spatial intensity distributions to determine a cumulative spatial intensity distribution of scintillation photons emitted in response to incident gamma rays in the scintillator array element; and determining a light matrix including expected spatial intensity distributions of scintillation photons for different scintillator array elements based on the cumulative spatial intensity distributions.