In this chapter, starting from analytical solutions of the RTE for the infinite medium that are more accurate with respect to the DE, a hybrid heuristic approach is implemented to derive solutions for the slab geometry. The proposed model makes use of
• The method of images together with the EBC to obtain the Green’s function for the fluence inside the slab as superposition of Green’s functions for a series of positive and negative point-like isotropic sources in a homogeneous infinite medium;
• The PCBC or Fick’s law to obtain the diffuse reflectance and transmittance.

For the Green’s function of the infinite medium, the idea is to use some RTE solutions that are more accurate than the DE solution with confidence that a better description of light propagation at small distances and short times could be obtained. The solutions of the RTE for the infinite medium that we report for the hybrid model are
• The solution of the time-dependent RTE obtained by Paasschens;
• The solution of the telegrapher equation (TE) (an equation less approximate than the DE) obtained by Durian and Rudnick.

Examples of comparisons reported in Ref. 29 and in Ch. 17 show that significantly more accurate results with respect to the DE can be obtained using the hybrid model. Since the hybrid solutions are in any case obtained by making use of approximations (EBC, PCBC, and Fick’s law arise from the diffusion approximation) their accuracy should be tested before using them in specific applications instead of the well-tested solutions of the DE.