Emerson's Rosemount Analytical OPM 3000 Opacity Dust Density Monitor stackmounting transmissometer sensor system includes a retro reflector mounted on one side of the stack and an optical transceiver on the other. The OPM 3000 ratio measurement technique gives incessant automatic compensation to secure prolonged instrument stability and precision for variations in light source intensity. The OPM 3000 is unaffected by light source aging due to its insensitivity to the absolute intensity of the light source.