My wife has lovely colored eyes. I particularly like the blue one. - Bob Monkhouse
Scatter plays a major role in how things appear; therefore, appearance deserves mention in this book. This chapter concentrates on some of the issues related to scatter that affect appearance; however, there is a lot more to appearance than just scatter. Color is a major factor in appearance, and the quantification of color is not an issue in this text. Gloss (specular reflection measured under special circumstances) is another appearance quantifier. The response of the human eye also modifies appearance. For a full treatment of these other issues, The Measurement of Appearance by Hunter and Harold (1987) is a good place to start. Nevertheless, scatter measurements are an excellent way to monitor appearance (and texture) in manufacturing situations, where circumstances do not allow the use of a well-defined relationship (such as the Rayleigh-Rice golden rule) to quantify surface statistics. Because the CCBRDF is a measure of scattered light as seen by the eye, it (as opposed to the BRDF) is usually used for appearance. (The differences are explained in Section 1.5.)
10.1 Beauty is in The Eye of the Beholder-And What We See is Scattered Light
That statement pretty well summarizes a main point of this chapter. The CCBRDF changes dramatically with the light source (wavelength, incident angle, polarization) as well as with surface variables (roughness, shape, material, etc.). As a result, providing a measured appearance number (like rms), or even an appearance function, is not really practical. Nevertheless, appearance can be monitored, and it is important to do so.