TABLE OF CONTENTS Our discussion of reflection (Fresnel equations) in Section 5.4.1 assumes that the object surface is perfectly smooth, flat, and isotropic. However, the surfaces of real objects are almost never like that. In order to characterize light reflection and scattering from surfaces, we need a more general way to describe the optical property of surface reflection.
Although surface reflection is a well-studied subject, the terms used in the literature have not yet been standardized. Difficulties arise not only with the definitions, but also with the underlying concept of measurement and the models of the assumed physical processes. Let us start by treating light as rays (geometric optics) and see what can happen as light interacts with a rough surface. [1] Figure 8.1 shows two light rays (ray 1 and ray 2) undergoing refraction, reflection, scattering, and absorption. Let us assume that the top half of the diagram is air and the bottom half is an object of some inhomogeneous material, say a painted surface. When light (ray 1 and ray 2) is incident on the rough surface, part of its energy is reflected (ray 3 and ray 7) and part of it penetrates into the surface (ray 4 and ray 6). The reflected light can undergo more than one reflection (ray 7). The penetrating light can undergo multiple scattering by pigment particles in the surface material, and eventually is absorbed (ray 6) or reflected back into the air (ray 5). The first reflection at...
