Ringelmann scale

The Ringelmann scale is a tool for judging the opacity of smoke, created in 1888 by Maximilien Ringelmann, a French professor of agricultural engineering at La Station d'Essais de Machines in Paris. It consists of five shades of grey, produced by grids of black lines on a white background that visually blend into a solid tone when viewed from a distance, running from about 20 percent opacity at shade 1, generally accepted by air pollution regulators, up through 40, 60, 80 and 100 percent opacity at shade 5; shades 2 through 5 are generally classed as black smoke by regulators in most countries. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/