Scattering to extinction cross-section ratios, ρ_(se), were measured using the NIST Large Agglomerate Optics Facility for soot produced from ethene and acetylene laminar diffusion flames. Measurements were performed using light sources at 543.5 nm, 632.8 nm, and 856 nm. The average scattering to extinction cross-section ratios for these wavelengths are equal to 0.245, 0.195, and 0.195 for ethene and 0.311, 0.228, and 0.237 for acetylene. The 856 nm measurements represent the longest wavelength for which accurate scattering measurements have been performed for soot. The size distribution and fractal properties of the two soots were determined to assess the effects of limited acceptance angle range, finite size of the sensor, and departure from cosine response on the uncertainty in the measurement of ρ_(se). The expanded relative uncertainty (95% confidence level) was found to be +- 6% at the two visible wavelengths and +- 8% at 856 nm. Both the magnitude and wavelength dependence ofρ_(se) for the present experiments are significantly different from those reported by Krishnan et al. [7] for overfire soot produced using a turbulent flame. The results are compared with the predictions of fractal optics.
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