A single vane shutter flag is asynchronously controlled so that a measuring system light source is interrupted for a minimum necessary amount of time for standardization/calibration and normalization of InGaAs system detectors. Source/detector hemispheres or serially connected randomly oriented fiber bundles homogenize light passing to the detectors. Light source testing is performed by measuring spectral power distributions at a plurality of light source power levels and comparing the measurements to baseline characteristics established for the light source. Calibration sample life is extended indefinitely by controlling the shutter flag to block source light except for short calibration time periods during which standard light levels are provided.