Linear photomultiplier bridge circuit for the measurement of small changes in absorption

Abstract

In the circuit described, the voltage applied to the two photomultipliers is controlled by means of a negative feedback loop so that the anode current of the monitor photomultiplier is held approximately constant. The ratio of the anode currents of the two photomultipliers is then linearly proportional to the transmission of the sample and is unaffected by fluctuations in the luminous output of the source and variations in the mains voltage. In addition, the sensitivity of the bridge remains constant, so that it is possible to estimate small changes in absorption directly from readings of the out of balance current.