Could someone help me understand this paragraph about smoke meters ?

[LCform]

Hi All

I don't know whether I don't know the terms or the language is difficult, here is the paragraph :

Typical of this class, in which comparison of the visual appearance
of a smoke plume is compared directly with a standard scale of
grey, is the Ringelmann Chart4. In this, a white card, on which
has been printed a series of black grids obscuring, respectively,
20%, 40%, 60% and 80% of the surface, is viewed in optical
proximity to the plume. The degrees of obscuration listed are
arbitrarily numbered 1,2, 3 and 4 Ringelmann. On this scale the
white card is numbered O and a totally black card 5 Ringelmann.

I appreciate your attention. I just need it in more simple words.
Thank you

 

[catserveng]

Try to Google "ringelmann smoke chart", you can find images of several different types of "smoke charts" that were produced to provide a reference to the amount of opacity in 5 steps.

Here is an older but pretty good handout on the history and use of the Ringelmann chart,

https://www3.epa.gov/ttnemc01/methods/VECourse.pdf

Hope that helps, MikeL.

 

[LCform]

Thank you very much for your great help