For a wide gamut of dye combination s for a reflection print material, equivalent neutral densitometry is shown to compare favorably with colo rimetric measurements for the evaluation of color prints. Over a limited range of normal printing conditions, there is a linear relation between reflection density (END) and distance from the illuminant point along the vectors of a calorimetric dye grid.
Both neutral scale and picture-area densitometry offer advantages for the measurement of matched and rejected prints. It is necessary to bear in mind the possible limitations of neutral scale densitometry and to utilize picture area measurements for reproductions in which conditions vary considerably from “normal.”
JUDITH W. ONLEY, "Analytical Densitometry for Color Print Evaluation," J. Opt. Soc. Am. 50, 177-179 (1960)