This study presents an experimental evaluation of photon absorption in a TiO2 slurry medium using an annular photoreactor. This photoreactor was equipped with several windows placed at equidistant axial positions. Tubular black collimators and inner polished-aluminum collimators were attached to these windows to measure the total transmitted radiation and the transmitted non-scattered radiation. Experiments were developed with six TiO2 commercial powders having differences in particle size, agglomerate particle size in water suspension, and specific surface area. Modeling allowed to establish that the forward-transmitted radiation can be represented by the difference of two exponential decay functions accounting respectively for the total transmitted radiation and the transmitted non-scattered radiation. These two exponentials are functions of particle concentration and extinction coefficients, with the extinction coefficient for the transmitted radiation being strongly affected by the particle agglomerate size.