The concepts of "initial rate of photon absorption" (IRPA), "dimensionless boundary layer of photon absorption" and "apparent optical thickness (tau(app))" are presented to evaluate the radiative transfer phenomena in solar, slurry, planar, photocatalytic reactors. The radiation field produced by suspensions of TiO2 and goethite, two photocatalysts with profoundly different optical properties used in heterogenous photocatalysis and heterogeneous photo-assisted Fenton reactions, was determined by the six-flux radiation absorption-scattering model coupled to the Henyey-Greenstein scattering phase function (SFM-HG). The concept of IRPA, defined by the differentiation at the local volumetric rate of photon absorption (LVRPA) at the reactor window boundary, is proposed as a new approach to determine the impact of catalyst loading and optical properties on the extinction of light inside a photoreactor. The IRPA showed that the extinction of light follows a second order dependency on the photocatalyst concentration while the impact of the optical properties can be expressed by a decoupled function (Psi function). The Psi function increased with photocatalyst concentration and approached a maximum at the same optimal photocatalyst concentration determined from the analysis of the total rate of photon absorption (TRPA) in the reactor. The analysis of TRPA and boundary layer of photon absorption redefined here in dimensionless form, as a function of tau(app), determined that the most efficient rate of radiation absorption in solar powered planar reactors occurs at tau(app) = 4.1-4.4, with approximately 10% of the reactor width under darkness. tau(app) is a similarity dimensionless parameter exclusively derived from the SFM approach, which clusters the effects of photocatalyst loading, reactor dimension and photocatalyst optical properties, providing an ideal parameter for designing and scaling photocatalytic reactors operated with any kind of photocatalytic material.