We calibrated a leadglass Pb-scintillator photon detector with a positron beam of 30 GeV. The detector consisted of a segmented leadglass counter in front and a Pb-scintillator sandwich counter in the back. The response of the sandwich counter exhibited an exponential dependence on the position of the incident particle. The calibration constants of the leadglass segments were obtained by an iterative method. The results of our analysis showed an energy resolution of σ(E)E = 24%/√E. A monitoring system using LED and \sp241Am sources regularly checked the stability of all modules. A typical photomultiplier's gain factor deviated no more than 6% from the mean value at any time during a period of three months.