Diffusion coefficient in photon diffusion theory

The choice of the diffusion coefficient to be used in photon diffusion theory has been a subject of discussion in recent publications on tissue optics. We compared several diffusion coefficients with the apparent diffusion coefficient from the more fundamental transport theory, D-app. Application to point sources in turbid media, for which exact solutions are available, showed that D-app has to be preferred. We give a simple equation to approximate D-app for several phase functions that apply to tissue optics. Reasons for the remaining discrepancies in diffusion coefficients applied to time-resolved and time-averaged descriptions of photon propagation in homogeneous turbid media are discussed. (C) 2000 Optical Society of America.