Determination of the Refractive Indices of Soot Particles from the Reflectivities of Compressed Soot Pellets

Measurements of the polarized components of the reflectivity of a compressed propane soot pellet are presented which demonstrate that, for wavelengths in the infrared, the reflection is specular and is described by the Fresnel equations. By matching these equations to the full angular variations of the specular reflectivities, an effective refractive index of the soot/air mixture is determined at each wavelength. To deduce the refractive indices of the particles themselves, theories for the optical properties of composites are considered and a new relationship based upon the effective medium theory is introduced. The void fraction near the surface of the pellet must be known. A technique is presented for measuring this voidage by performing image analyses of scanning electronmicrographs of the pellet surface. This value of void is found to be substantially less than the bulk void. The inferred indices of the particles are seen to be sensitive to the value of the void fraction but somewhat insensitive to the particular theory adopted for the optical properties of the soot/air mixture.