An imperfection of time-dependent diffusion models for a determination of scattering medium optical properties

In the present work the absorption and reduced scattering coefficients of the homogeneous scattering medium are determined using three various types of the diffusion model (classical, refined and extrapolated) in the case of the scattering slab geometry. The expression for the refined diffusion model in this case was derived for the first time. These coefficients are determined from experimental temporal distributions of transmitted photons at different slab thickness (from 10 mm to 110 mm) with fitting procedure. The specific dependencies of the scattering and absorption coefficients on the slab thickness were obtained for each of the examined models, although it is physically impossible for the same substance. This effect is caused by the fact that the significant features of the scattering indicatrix are lost when the diffusion approximation has been applied to the time-dependent radiation transport equation. This explanation was confirmed by Monte Carlo simulation, where the similar dependencies were obtained. There are presented several remarks regarding the experimental measurements when diffusion models are used.