Limits of the small-angle approximation to the radiative transport equation

The small-angle approximation to the radiative transport equation is used e xtensively in imaging models in which the transport medium is optically thi ck. The small-angle approximation is generally considered valid when the pa rticles are very large compared with the wavelength, when the refractive-in dex ratio of the particle to the medium is close to 1, and when the optical thickness is not too large. We report results showing the limits of the va lidity of the small-angle approximation as a function of particle size and concentration for a particle-to-medium fixed refractive-index ratio of 1.19 6. This refractive-index ratio is comparable with that of minerals or diato ms suspended in water. (C) 2001 Optical Society of America OCIS codes: 290. 7050, 290.5850, 290.4020, 290.5820, 290.4210, 110.4850.