When a light beam is shone across a duct through which particle-carrying gases are flowing the transmitted light intensity exhibits short time scale, high frequency fluctuations (1Hz). This opacity fluctuation, sometimes called scintillation, is the basis of a device which is increasingly challenging traditional opacimeters as a means of monitoring particulate emissions. The paper outlines some features of the underlying theory of the instrument and highlights both the advantages and the uncertainties which remain to be cleared up by further experimental and theoretical studies.
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