A theoretical formulation of diffuse reflection density of a glossy color print under perfectly diffuse illumination has been developed by taking into consideration the first-surface reflection from a gelatin layer and multiple internal reflections in a gelatin layer. The diffuse reflection density of a glossy color print under perfectly diffuse illumination is approximately equal to the specular reflection density of a color print adjusted for 5% surface reflection from the gelatin. The color gamuts obtainable in glossy color prints under perfectly diffuse illumination and under specular illumination (perpendicular to color prints) with 5% surface reflection are shown in comparison with those obtainable in color transparencies.
NOBORU OHTA, "Reflection Density of Multilayer Color Prints, II. Diffuse Reflection Density," J. Opt. Soc. Am. 62, 185-191 (1972)