Kortüm was a physical chemist who had a broad range of interests, and published prolifically.  His research covered many aspects of light scattering.  He began to pull together what was known in various fields into an understanding of how “reflectance spectroscopy” worked.  In 1969, the English translation of his book entitled Reflectance Spectroscopy (long in preparation and translation) was published.  This book came to dominate thinking of the day for 20 years in the emerging fields of both DRIFTS and NIR Spectroscopy.
Kortüms position was that since regular (or specular) reflection is governed by different laws than diffuse reflection, they should therefore be accorded different mathematical treatments.  He developed an approach based on Schusters work by ignoring the emissivity of the clouds in the "foggy atmosphere".  If we take  as the fraction of incident light absorbed and  as the fraction scattered isotropically by a single particle (referred to by Kortüm as the "true coefficients of single scatter"), and define the absorption and isotropic scattering for a layer as  and  then: 
This is the same "remission function" as used by Judd, but Kortüm's translator referred to it as "the so-called reflectance function".  If we substitute back for the particle properties, we obtain  and then we obtain the Schuster equation for isotropic scattering:
Additionally, Kortüm derived "the Kubelka-Munk exponential solution" by defining  and  as the absorption and scattering coefficient per centimeter of the material and substituting:   and , while pointing out in a footnote that  is a back-scattering coefficient.  He wound up with what he called the "Kubelka–Munk function", commonly called the Kubelka–Munk equation: 
Kortüm concluded that "the two constant theory of Kubelka and Munk leads to conclusions accessible to experimental test.  In practice these are found to be at least qualitatively confirmed, and suitable conditions fulfilling the assumptions made, quantitatively as well."
Kortüm tended to eschew the "particle theories", though he did record that one author, N.T. Melamed of Westinghouse Research Labs, "abandoned the idea of plane parallel layers and substituted them with a statistical summation over individual particles."