This is a Ringelmann Chart:
The chart is used to visually assess smoke emissions by comparing the darkness of the smoke with standard shades of white (0) through to black (5) on a chart placed in a suitable position.
Each shade number represents a 20% increase in obscuration of the background.
The darkness of the smoke being emitted should be compared with the shades on the chart.
The darker the smoke the greater the polluting potential of the smoke.
Note: This post is for reference only - proper charts follow strict British Standards.