There are two main reasons for including a chapter on the characteristics of human vision in a book that is primarily concerned with computer processing and measurement of digital images. First, much of image processing is concerned with enhancing the visibility of details and features within images, and this depends upon some understanding of what people see in images (and what they overlook). Second, many of the algorithms described in subsequent chapters for image processing and detecting objects in scenes are based to a greater or lesser extent on our understanding of how human visual processes work. That is not the only source of processing algorithms, of course. Some are based on the physics of light (or other signals) interacting with specimens. And some are simply ad hoc procedures that have been found useful. But the understanding gained from the study of vision has been an important source of methods for computer processing as well.