9 - The bidirectional reflectance in other geometries

Summary

Introduction

Chapter 8 treated the bidirectional reflectance of an optically thick, plane-parallel particulate medium in which the particles were randomly oriented and could be regarded as embedded in a vacuum. In this chapter we will discuss the effects on the reflectance when each of these restrictions is removed.

Diffuse reflectance from a medium with a specularly reflecting surface

The upper surfaces of many particulate materials may be sufficiently smooth on a scale comparable to the wavelength that light is scattered both quasi-specularly from the surface and diffusely from below the surface. The specular component is known as regular reflection. Because a surface effectively becomes more optically smooth at large angles of incidence (Chapter 6), the regular component may become especially important at large phase angles.

The most familiar example of the combination of diffuse reflection and regular reflection is water containing suspended solids, as in rivers, lakes, and oceans. If a body of water is examined in the geometry for specular reflection from the surface, a bright glare is seen, which is the reflected image of the sun. However, if the same body is examined in an off-specular configuration, it looks dark and may be colored blue, brown, or green, depending on the nature of the suspended solids.