Direct experimental verification of light transport theory in an optical phantom

Abstract

Starting from light transport theory, we present an ab initio calculation of the fluence rate that is due to an isotropic point source in an infinite, anisotropically scattering medium. To verify the results experimentally, a latex suspension with uniform particle size, corresponding to g=0.734, was prepared. In the suspension an isotropic light source and an isotropic light detector were placed, and the fluence rate as a function of distance was measured. We observed good agreement in absolute values between the calculated and the observed fluence rate over distances ranging from 1/4 to ∼8× the total mean free path, which corresponds to a fluence rate varying over some five orders of magnitude. Furthermore, the calculated fluence was obtained from measured values of μs and μa and a calculated phase function without any kind of fitting, and thus the calculation was completely independent from the measurement. This is the first time that the fluence was measured quantitatively with an isotropic probe and found to agree within experimental error with transport theory. The experimental results indicate that, far from the source, the behavior is diffusionlike, even for low albedos, albeit with a corrected effective extinction coefficient.