While at Vienna in 1897, Schwarzschild developed a formula to calculate the optical density of photographic material.
From the en.wikipedia.org
He noted that clusters of medium density sagebrush on the photographic maps fell in the same area as known uranium deposits.
From the time.com
Similarly, the density range of a traditional photographic print depends on the processes used as well as the paper characteristics.
From the en.wikipedia.org
One of the most common is silver halide photographic emulsion which uses the same materials as photographic film but with much higher grain density, i.e. of much higher resolution.