Visual Determination of Opacity (EPA Method 9) - A trained and certified observer views the emission plume (smoke) emitted from a stack or other stationary emission source and determines the opacity (the degree to which the smoke obscures the background against which it is viewed). This opacity is recorded every 15 seconds. Most regulations use a six-minute average of opacity readings to determine compliance.
Fugitive Emissions (EPA Method 22) - A trained observer views a potential emission source continuously and records for what period of time any emission is visible. This method does not determine the opacity of emissions, but rather the amount of time that emissions are visible during the observation period.