Absolute calibration of photomultiplier based detectors – difficulties and uncertainties

Photomultiplier manufacturers can provide a calibration of quantum efficiency over a range of wavelengths with an accuracy of up to 2%. To convert these figures to absolute counting efficiency requires knowledge of photomultiplier collection efficiency, F. Traditional methods for determining F are discussed with emphasis on sources of error. Light sources emitting at a known photon rate allow the absolute quantum efficiency to be determined directly. It is important in all attempts at absolute calibration to appreciate the conditions which manufacturers apply when calibrating photomultipliers.