Explains the geometric conditions for measuring ISO diffuse and f/4,5 and f/1,6 projection transmission densities. Diffuse density is applicable to measurements of photographic images viewed on a transparency illuminator employing diffuse illumination. Projection density is applicable to measurements of photographic images projected with systems employing optical condensers. Though intended for the measurement of photographic images, the densitometric methods specified are often applied to optical filters and other sheet materials.