Density measurements used in the sensitometry of color films can be divided into two broad classes: Integral densitometry, which provides a description of the multilayer image as a whole in terms of its absorptions; and analytical densitometry, which gives a description of the image in terms of its components by determining their individual density values. Each class contains subclasses formed by special definitions of density. Terms and units are defined for each class and subclass; for each one, the field of usefulness and methods of measurement are briefly discussed.
FRANKLIN C. WILLIAMS, "Objectives and Methods of Density Measurement in Sensitometry of Color Films," J. Opt. Soc. Am. 40, 104-111 (1950)