A number of techniques are available to assist the optical designer in the process of designing infrared systems. This chapter describes some of them and they are further illustrated in the detailed descriptions of infrared zoom lenses in Chapters 6 and 7. 4.1 Optical Design Starting Point The starting point for the design of an infrared optical system may be one of the following, in order of increasing difficulty: (1) commercially available zoom lens systems (Sec. 6.1.1) (2) zoom lenses described in patents or other reference literature (Sec. 6.5.2) (3) a thin lens solution (Sec. 6.1.2). Selection of the starting point is a very important decision to be made by the optical designer since it will strongly influence the direction in which the design activity will proceed and the likelihood of achieving the desired solution. Since zoom lenses in general are more complicated and time consuming to design than fixed focal length lenses, selection of the best starting point is more crucial to the duration and cost of the design task. 4.2 Scaling Scaling a lens is a simple but powerful tool for assisting in selecting a starting point. When a previously designed optical system is chosen as a candidate for the starting point, it is more than likely that the first-order parameters are somewhat different from the requirements for the new design. Scaling by the appropriate factor makes it possible to match the required focal length with that of the candidate starting point (Secs. 6.1.2 and 7.2.2). It is important to keep in mind what does and does not scale along with the focal length. What do scale are the longitudinal distances along the optical axis and the lateral dimensions perpendicular to that axis; thus, lens thicknesses and separations will scale along with total track length. Lens diameters and pupil diameters will also scale. The image size at the focal plane will scale.