The diffusion approximation to the Boltzmann transport equation is commonly used to analyze data obtained from biomedical optical diagnostic techniques. Unfortunately, this approximation has significant limitations which constrains its applicability to highly scattering systems and over length scales much larger than the transport mean free path l*. Using an approach formulated independently by Prahl and Star [11, 13, 14], we present a form of the diffusion approximation which adds a delta function term to both the radiance and phase function expressions. This formulation is presented and solved for steady illumination in infinite media with a collimated source of finite size exhibiting spherical symmetry. The solution is compared to results given by standard diffusion theory and to measurements made in a strongly scattering and strongly absorbing turbid phantom with reduced single scattering albedos a’ of 0.997 and 0.248, respectively. The results show that this approach provides accurate predictions of optical dosimetry in both low and high scattering media and at positions proximal to collimated light sources.

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