The literature abounds with equations and data that pertain to infrared system design. Comparatively little has been written about the process of the design, however. Infrared system design is not a synthetic process, as is some circuit design. You cannot start with the statement of the problem and proceed in an orderly fashion to one final solution. Rather, we "guess" a solution and explore its applicability and capability. Infrared system design is an iterative process. The iteration goes faster, of course, if the first guess is a good one. The quality of that first guess is a function of insight and experience, and insight comes largely from experience...