Photomultiplier tubes (PMTs) are used in biological detection systems in order to detect the presence of biological warfare agents. To ensure proper operation of these biological detection systems, the performance of PMTs must be characterized in terms of their responsivity and long-term stability. We report a technique for PMT calibration at the Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology (NIST). SURF III provides synchrotron radiation with a smooth and continuous spectrum covering the entire UV range for accurate PMT measurements. By taking advantage of the ten decade variability in the flux of the synchrotron radiation, we studied properties of commercial PMTs such as the linearity, spatial uniformity, and spectral responsivity. We demonstrate the degradation of PMTs by comparing new PMTs with PMTs that were used and operated in a biological detection system for a long period of time. The observed degradation is discussed.