Calibration and comparison of laser-induced incandescence with cavity ring-down

Laser-induced incandescence (LII), a technique that determines relative soot volume fraction, requires calibration to achieve quantitative results. Although not spatially resolved, cavity ring-down (CRD), an absorption-based method, provides an integrated measure of f_v along the line-of-sight. Here, spatially resolved LII signals from soot within a methane/air diffusion flame are calibrated using CRD, which avoids extrapolation required of less sensitive methods in current use. Comparison of CRD with traditional light extinction and path-integrated LII verifies its accuracy for f_v determination. Using CRD, quantification of LII for parts per billion (ppb) f_v levels is demonstrated. Experimental tests demonstrate the accuracy of CRD for a single laser-pulse to be better than ± 5% for measurement of ppb soot volume fraction levels over a 1-cm pathlength. Using calibrated detector characteristics and a predetermined f_v level, the absolute LII signal level within a detection bandwidth of 405-415 nm produced by a laser fluence of 0.25 J/cm² at 1064 nm within a laminar ethylene/air diffusion flame was calculated. This value is 5 × 10 ⁵ photons/sr-nm per ppm of soot, collected over a 10-ns interval centered at the peak of the LII signal. Comparison of LII with CRD reveals that CRD may be used to advantage in applications where spatially resolved information is not necessary and/or achieving high geometric collection efficiency is impractical. LII's chief advantages are the spatially resolved f_v visualization and geometric versatility.