Comparing the P3-approximation with diffusion theory and with Monte Carlo calculations of light propagation in a slab geometry

Abstract

The energy fluence rate in a slab of tissue has been calculated for a monodirectional and unpola- rized incident light beam of infinite diameter perpendicular to the tissue surface. Published values for the optical constants of human dermis are used and the angular dependence of scattering is de- scribed by the Henyey-Greenstein function. Results are given for index matched and index mis- matched situations. The P3-approximation to the transport equation appears to be a considerable improvement over the P1 or diffusion approximation. Another improvement is obtained when a delta function is added to the scattering function. In the diffusion model this is called the delta- Eddington approximation. In the P3-approximation this addition yields a near perfect agreement with results of Monte Carlo calculations. The improvements are particularly apparent at the boundary where the incident beam enters the tissue. The relevance of these results for dosimetry in Photodynamic Therapy is discussed.