Photon migration within the P3 a

摘要

Abstract: The diffusion approximation to the photon transport equation is the simplest model for photon migration. The applicability of the diffusion approximation, however, is limited. For example, it is strictly valid only when the absorption coefficient is small compared to the scattering coefficient and the source modulation frequency is small compared to the scattering rate. In this contribution we establish the theoretical range of validity of the diffusion approximation, review a frequency-dependent solution of the transport equation within the P$-3$/ approximation, and establish the significance of the P$-3$/ solution using Monte Carlo computer simulations. We find that the P$-3$/ approximation, in most cases, permits a more accurate determination of the optical properties of highly absorbing media as well as from data obtained at modulation frequencies greater than 3 GHz. Interestingly, for highly anisotropic scattering (g $EQ 0.9), we find that diffusion theory and the P$-3$/ approximation predict approximately the same values for the reduced scattering coefficient. !14