Absolute Calibration of PMTs with Laser light

We report on the results from calibrating photo-multiplier tubes (PMTs) absolutely. Obtaining the quantum efficiency measurements of PMTs to better than 10% is very difficult, but this level of accuracy is necessary to make an absolute measurement of cosmic ray air shower fluorescence yield. As a calibration light source we use photons from a 337.1 nm nitrogen laser passed through an integrating sphere and pinhole collimator. This gives a low enough energy pulse to measure the single photoelectron spectrum for the PMT with same accuracy as the calibrated energy probes.