Opacity

Opacity is the degree to which the transmission of light is reduced or the degree to which visibility of a background, as viewed through the diameter of a plume, is reduced.  A transmissometer (or opacity analyzer) measures the amount of light that is lost to scattering or absorption in the emission's effluent.  It is expressed in percentage usually, but sometimes in Ringelmann numbers.

Rosemount OPM2000      (still supported with parts & service by RIGAS) 
Rosemount OPM2000A   (still supported with parts & service by RIGAS) 
Rosemount OPM2000R   (still supported with parts & service by RIGAS) 
Rosemount OPM2001     (still supported with parts & service by RIGAS)