A measurement device for measuring a photocathode current of a photomultiplier tube, the measurement device includes: (a) a voltage amplifier configured to amplify a shunt resistor voltage proportional to the photocathode current; (b) a leakage tolerant circuit configured to generate a first leakage tolerant voltage, based on a first amplified voltage and a first reference voltage having an absolute value that exceeds an absolute value of a leakage associated with the measurement device; (c) a voltage to current transducer that is configured to convert the first leakage tolerant voltage to a first leakage tolerant current; and (d) an output unit that is configured to convert the first leakage tolerant current to an output voltage that is indicative of the photocathode current.