Photon diffusion coefficient in an absorbing medium

A number of investigators have recently claimed, based on both analysis fro m transport theory and transport-theory-based Monte Carlo calculations, tha t the diffusion coefficient for photon migration should be taken to be inde pendent of absorption. We show that these analyses are flawed and that the correct way of extracting diffusion theory from transport theory gives an a bsorption-dependent diffusion coefficient. Experiments by two different set s of investigators give conflicting results concerning whether the diffusio n coefficient depends on absorption. The discrepancy between theory and the earlier set of experiments poses an interesting challenge. (C) 1999 Optica l Society of America [S0740-3232(99)02205-X].