The mathematical treatments of absorption spectroscopy for scattering materials were originally largely borrowed from other fields.  The most successful treatments use the concept of dividing a sample into layers, called plane parallel layers. They are generally those consistent with a two-flux or two-stream approximation.  Some of the treatments require all the scattered light, both remitted and transmitted light, to be measured.  Others apply only to remitted light, with the assumption that the sample is "infinitely thick" and transmits no light.  These are special cases of the more general treatments.
There are several general treatments, all of which are compatible with each other, related to the mathematics of plane parallel layers.  They are the Stokes formulas, equations of Benford, Hecht finite difference formula, and the Dahm equation.  For the special case of infinitesimal layers, the Kubelka–Munk and Schuster–Kortüm treatments also give compatible results.  Treatments which involve different assumptions and which yield incompatible results are the Giovanelli exact solutions, and the particle theories of Melamed and Simmons.