We employ a physical theory to construct a computational model that accounts for both multiple scattering and absorption of light. The approach does not require a calibration model. Mie theory to describe single particle scattering, which is combined with the diffuse approximation to the radiative transfer equation to provides an analytical prediction of the reflectance. This approach is applied to experimental reflectance measurements on polystyrene particle suspensions with a wide range of particle radii and volume fractions. The method provides good estimates of the suspension properties from a full NIR-vis-UV spectrum.