An article on optics of paint layers

Abstract

The question of how the color of a substrate is changed by the application of a coat of paint of specified composition and thickness, and especially the ques- tion of what thickness of paint is needed to obscure the substrate, has, in view of its great practical importance, already been the subject of several experimen- tal studies. Theoretical studies of this question, on the other hand, have been lacking.1 The recently published computations of light scattering of opaque glasses etc.2, despite the apparent degree of analogy, cannot be applied to the case of paint coatings without further work. This is because these calculations consider only the quantity of light passing through the scattering layer; this has no direct relation to the quantity of light thrown back by a scattering layer over a reflective substrate, which is the only quantity of interest when considering coatings. The present article presents a first attempt at a theoretical treatment of the optics of coatings. Equations are derived from a general differential relation of the distribution of light within the coating that presents the optical behavior of a coating in detail and allow it to be quantified. The explanations relate primarily to matte, uncolored coatings—and among these preferably to white and light gray opaque coatings—and only touch on the general case of colored coatings. Glossy coatings are not considered.