Forty-Four Fewer Shades of Gray

Photographic color test cards having four rows of six squares include a six-square row that has six gradations of gray, including pure white and pure black at the ends. The intervening values are different manifestations of gray, going from lighter to darker. But how are the intervening values selected? What determines how “gray” they are? It turns out that they are not steps of equal change in transmission (or reflection, depending upon the type of chart), nor are they steps of equal change in optical density. The size of the gray “steps” are chosen on a somewhat different scale of values. Who came up with them, and how did they decide which values to use?