In this chapter, we deal with optical techniques in nondestructive evaluation (NDE). This field falls within the general category of using light to discern certain properties of matter. Figure 10 . 1 illustrates this concept of using light in this manner. This illustration also serves to describe the general approach of the chapter. We begin with a description of the properties and behavior of light, for instance, how light propagates though space as well as a dielectric medium. Incidentally, the use of the term light in no way restricts our attention to the visible spectrum. Optical engineers refer to electromagnetic energy in general as "light."