The accuracy of the Macbeth illuminometer was studied by measuring a known source. The mean of a number of measurements from a number of operators gives a precise estimate of brightness. The most adequate measures are obtained when “calibration” amperage of the working lamp is carefully determined and maintained with precision.
To establish the relation between the sensitivity of the eye and brightness measurements; the difference thresholds for lighter and darker were determined. The close similarity of the curves for the threshold data and for the brightness measurements demonstrates that “match” judgments are essentially discriminations of “lighter” and “darker.”
A. MORRIS, FREDERICK L. MCGUIRE, and HAROLD P. VAN COTT, "Accuracy of the Macbeth Illuminometer as a Function of Operator Variability, Calibration, and Sensitivity," J. Opt. Soc. Am. 45, 525-528 (1955)