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.
The ring-down cavity is bounded by two confocal mirrors with a radius of curvature equal to or greater than the distance between them. Pulsed laser light is prepared for coupling into the cavity by spatial filtering followed by down-collimation to match the TEM00 mode of the cavity. A photomultiplier tube or an amplified positive/intrinsic/negative (PIN) silicon photodiode detects light coming out of the cavity. The output of the photodetector is sent to a transient-signal digitizer. The digitized signal is processed to determine its ring-down time (t) or, equivalently, its rate of decay (k).
The ring-down time is given by
fv = (λ/kΘL)[(d/cτ) + R – 1]
where d is the distance between mirrors, c is the speed of light, R is the reflectivity of the mirrors, ke is a constant of proportionality for the amount of absorption of light in soot, fv is the soot concentration (expressed as a volume fraction), L is the distance over which soot is distributed, and l is the wavelength of the laser light. One potential source of error is uncertainty in the value of ke; this value can be calculated from the index of refraction of soot, which is not known to better than about 10 percent. The equation can be solved for the concentration of soot:
The sensitivity of this CRD technique exceeds that of any other soot-measurement technique now in use; it enables measurement of soot concentrations in the range from 1 to 100 parts per billion. The temporal resolution of this technique is also greater than that of any other soot-measurement technique. CRD has a good spatial resolution in directions orthogonal to the cavity axis but suffers from a lack thereof along this axis. However, CRD could be used along with laser-induced incandescence (LII), which gives spatially-resolved relative concentrations. Because the CRD signal for a given path is proportional to the path-integrated LII signal, CRD measurements could be used to calibrate LII measurements. CRD, either alone or in combination with LII, could be used to measure concentrations of particulate matter in the atmosphere at large, in industrial aerosols and in the exhausts of gasoline engines, diesel engines, power plants, and incinerators.
This work was done by Randall L. Vander Wal of NYMA, Inc., for Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the category, or circle no. 59 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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