Since we are looking at thermal or heat sources with a thermal-imaging system, for maximum system sensitivity most thermal imaging systems use cryogenically cooled detectors which operate at the liquid nitrogen temperature of 77 K or even lower. If these detectors, or focal plane arrays (FPAs), are allowed to "see" any thermal energy other than the energy contained within the scene being viewed, then the sensitivity is reduced. In addition, if the magnitude of this nonscene energy changes or modulates over the field of view, then we often see cosmetically undesirable image anomalies. In order to achieve maximum sensitivity and avoid image anomalies, the IR FPA is cryogenically cooled and mounted into a thermally insulated "bottle," or dewar, assembly.

Figure 12.3 shows a typical generic detector/dewar assembly intended for an IR-imaging application. Before we show how the dewar works, we need to see just how it interfaces with the rest of the optical system. The smaller figure on the upper right of Fig. 12.3 shows an entire scanning-imaging IR system. Light (actually infrared radiation) enters from the left into the larger lens generally called the collecting optics. After forming an intermediate image, the light is collimated by the second smaller lens. A further purpose of the second lens is to form an image of the larger collecting optics element, which is the system aperture stop, onto the scan mirror. After the light reflects from the scan mirror, it...