Opacity is formally defined as the measure of the impenetrability of electromagnetic radiation through a sample. The opacity instruments described in this guide are primarily used to analyze air and smoke; these media types can be informally described as "clear" or "dirty" depending on their opacity.
Opacity measurement largely depends on the concentration of particles within a sample. When electromagnetic radiation—in this case visible light or infrared radiation—is emitted toward a sample, suspended solids will scatter, reflect, and absorb the radiation, preventing it from passing through. Opacity sensors measure the quantity of transmitted light and convert it to a relevant value.
Measurement Principles
Particles within a sample typically absorb or scatter emitted radiation. In general, particles which are smaller than the wavelength of emitted radiation absorb it, while particles larger than the wavelength scatter it. The latter scenario is preferable for opacity measurement. Because particles within industrial emissions are usually between 0.1 and 50 microns in size, opacity sensors typically use short wavelengths of between 0.38 and 0.75 μm.
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