The Kubelka–Munk Framework and Measuring Opacity

Abstract

So far, we have considered the scattering phenomenon itself—Mie’s theory of individual particles scattering light and the effects that different particle attributes have on maximum scattering strength, and the theory of groups of particles scattering light and the finding that close proximity of one particle to another leads to a loss of scattering strength (dependent scattering). We have, however, neglected a crucial aspect of this phenomenon—how to actually measure light scattering in the paint films of interest to us. To do this, we will consider scattering from the entire paint film, rather than from individual particles or groups of particles. Using this framework, we can transform intensity measurements of white light reflected from a paint film drawn down on a black and white chart into scattering values (S _TiO2 and S _coat) of the paint. Using Kubelka–Munk equations, we can calculate from the reflectance data the area that a liter of paint will cover at complete hide as formulated, and we can directly compare the light scattering and absorption efficiencies of different paints. In addition, we can use these equations as a guide to different ways of increasing the opacity of the paint and ensuring full advantage is taken of the pigment used.