Thermally induced defocusing exists in all optical systems, but is a particularly acute problem in imaging systems for the mid- to far-infrared. This is due to the fact that there are a limited number of optical materials for this spectral range, which are generally highly sensitive to changes in temperature. Most infrared imaging systems fall into two broad categories: scanned systems with a line detector, and staring- array systems. Each of these has components with fairly well- defined paraxial characteristics, so that the first-order data of the system as a whole is amenable to analysis during the preliminary stages of optical system design. For each of the various component sub-systems, the thermal variation of focal length can be calculated for any of several configurations (e.g. achromatic doublet, or diffractive element), based on the coefficients of thermal expansion and of thermal variation in refractive index of the optical materials. By combining the thermal characteristics of the optical sub-systems with the first-order optical characteristics, one obtains a first-order analysis of the optical system's thermal characteristics, including the rate of thermal defocus, and the range required for a particular thermal defocus adjustment.