A completely objective system for equivalent-neutral-density calibration of a set of photographic film images from a subtractive color process is described. It includes these elements: 1. Restrictions on spectral density distributions of possible component absorbers are established by determining eigenvectors of the spectral density variability of images from the process; 2. Estimates of spectral density distributions of individual process dyes are adjusted to conformance with the restriction; 3. Spectral density distributions of process-formed gray images are predicted by combining spectral densities of actual process images with just-sufficient amounts of (two of) the conforming dyes; 4. Equivalent neutral densities of the third dye in the images are colorimetrically computed from the predicted gray-image distribution.
W. L. BREWER and F. C. WILLIAMS, "An Objective Method for Determination of Equivalent Neutral Densities of Color Film Images. I. Definitions and Basic Concepts," J. Opt. Soc. Am. 44, 460-464 (1954)