A researcher whose joint work on NIST calibrations in photometry explains how to maintain an optical measurement scale in practice. Repeated measurements of check lamps, subtraction of detector background and corrections for instrument response help distinguish a real change in a test source from drift in the measuring system.[ref]
NIST Measurement Services: Photometric Calibrations by Yuqin Zong and others. §§3.2–3.5.6: dark subtraction, check-lamp measurements and readout corrections
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