Densitometry is the quantitative measurement of optical density in light-sensitive materials, such as photographic paper or photographic film, due to exposure to light.
Overview
Optical density is a result of the darkness of a developed picture and can be expressed absolutely as the number of dark spots (i.e., silver grains in developed films) in a given area, but usually it is a relative value, expressed in a scale.
Since density is usually measured by the decrease in the amount of light which shines through a transparent film, it is also called absorptiometry, the measure of light absorption through the medium. The corresponding measuring device is called a densitometer (absorptiometer). The decadic (base-10) logarithm of the reciprocal of the transmittance is called the absorbance or density.
DMax and DMin refer to the maximum and minimum density that can be produced by the material. The difference between the two is the density range.