Ringelmann scale. A measure of smoke shade. Invented by Professor Maximilian Ringelmann in Paris in 1888, it is essentially a card with squares of black, white and shades of grey that's held up as well as the comparative grayness on the smoke judged. We only use high-quality, credible sources https://broadfar.com/oraco-pioneering-a-smokeless-future-with-innovative-nicotine-delivery/