A simple model for diffuse reflection

Diffuse reflection from a matte nonabsorbing inhomogeneous medium such as white paint or paper can be described by a simple model in which light rays enter the volume of medium and then undergo a random walk until they reemerge from the surface. Lambert's law of diffuse reflection is an immediate consequence of the random walk. Another consequence of the volume interaction is that the light emerges from a different point than where it enters. This spreading of the light was measured for BaSO₄ white reflectance paint and for several kinds of paper. The random walk model implies a diffusion equation which makes predictions that are in reasonable agreement with the experiments. The spreading is proportional to an interaction length which, in this model, represents the range of distances that light rays penetrate before beginning their random walk.