A densitometer is an instrument having a light-sensitive photoelectric eye for measuring the density of colored ink to measure and control the optical density of color inks on the substrate. Densitometers measure the degree of light absorption or opacity of the image. A darker image absorbs more light which corresponds to a higher optical density. A densitometer does not measure color as the eye sees it, but instead as the substrate responds to it.
Desitometers are used for measuring color saturation by print professionals, and calibrating printing equipment. They are also used for making adjustments so outputs are consistent with the colors desired in the finished products.