When a monochromatic light beam traverses through gas containing particulate matter and color, the intensity of the beam will decrease by absorption and scattering processes within the particulate distribution. The net effect is described by the Lambert-Beer law.
Transmissometer, also termed opacity monitor, measure the transmittance of light that passes through a flue gas.
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In the early days of emissions measurements, the opacity of the smoke leaving a stack was measured by the Ringelmann method in which a trained observer made a visual estimate of its appearance. Even to this day, the opacity monitor is certified by a trained "smoke reader" to verify the instrument under the guidelines of 40 CFR Part 60, Appendix A, Reference Method 9. |
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Opacity = 1.0 - Transmittance
Opacity can be defined as "the fraction of incident light that is attenuated by an optical medium of interest" (US EPA 40 CFR 60, Appendix B).