So far, the evaluation of extinction measurements has mostly relied on Bouguer-Lambert-Beer＇s law (BLBL), even though BLBL is known to be valid in the limit of low concentrations only. With higher particle concentrations, particle-particle interactions such as multiple scattering; dependent scattering and steric interactions can lead to important deviations from BLBL. In a discussion of the various interaction mechanisms, steric interactions are shown to be dominating in a wide and practically important range of parameters. For the theoretical description of steric interactions, different complementary models are introduced, including a statistical model, a layer model and a numerical simulation model. The various approaches coincide to yield a simple extinction equation in two versions, applying to the regimes of Fraunhofer diffraction and straight ray propagation, respectively. In any case, BLBL is found as the low concentration limit of the extinction equation. All considerations are restricted, so far, to the case of monodisperse spherical particles.