An optical null spectrophotometer having means for setting up the instrument gain for optimum response prior to insertion of a sample in the sample beam, the means including a voltage source to provide an unbalancing voltage to close the reference beam attenuator, a gain adjustment potentiometer in the detector output circuit and a visual readout device for indicating optimum gain with the reference beam attenuator in a closed position.
TOTAL LOOP GAIN INDICATOR FOR OPTICAL NULL SPECTROPHOTOMETER