Optical Cavity-Ring-Down Measurement of Soot Concentration

An optical technique for measuring the concentration of soot in air is based on the cavity-ring-down (CRD) principle, which is so named because it involves observation of the decay ("ring-down") of a signal echoing in a cavity. By use of (1) the observed rate of decay and (2) a known relationship between the rate of decay and the quantity of something that contributes to the decay, one calculates this quantity. In this case, the ring-down cavity is an optical one, the absorption and scattering of light by soot contributes to the decay of an optical signal, and the concentration of soot is the quantity to be determined from the measured rate of decay.