A chart is really all it is. A chart made up of increasingly darker shades, from white to grey to charcoal to black.
A chart that brought a consistent ranking system to something that had proven difficult to measure or quantify.
Maximilien Ringelmann, a French civil engineer, developed his Smoke Chart in 1897, largely in response to increasing air pollution following the Industrial Revolution. As more cities choked on smoke-filled air, Ringelmann wanted to create a way to gauge the pollution and particulates coming from industrial smoke stacks. He wanted something anybody could use, at any time, to gauge pollution.
His chart, with increasing gradients at equal intervals, was the answer. Anybody could hold up the chart and match the smoke to the color of a box. Light smoke indicated fewer particulates and more water particles, while black smoke indicated denser pollution caused by incomplete combustion.