The use of thermal detectors for IR imaging has been the subject of research and development for many decades. Thermal detectors are not useful for high-speed scanning thermal imagers. Only pyroelectric vidicons have found more widespread use. These devices achieved their fundamental limits of performance by about 1970. However, the speed of thermal detectors is quite adequate for nonscanned imagers with 2-D detectors. Figure 20.1 shows the dependence of noise equivalent difference temperature (NEDT) on noise bandwidth for typical detectivities of thermal detectors [1]. The calculations have been carried out assuming 100 × 100 µm2 pixel size, 8-14 µm spectral range, f/1 optics and top = 1 of the IR system. With large arrays of thermal detectors the best values of NEDT below 0.1 K could be reached because effective noise bandwidths less than 100 Hz can be achieved. This compares with a bandwidth of several hundred kilohertz for conventional cooled thermal imagers with a small photon detector array and scanner. Realization of this fact caused a new revolution in thermal imaging, which is underway now. This is due to the development of 2-D electronically scanned arrays, in which moderate sensitivity can be compensated by a large number of elements. Large scale integration combined with micromachining has been used for manufacturing of large 2-D arrays of uncooled IR sensors. This enables fabrication of low cost and high-quality thermal imagers. Although developed for military applications, low-cost IR imagers are used in nonmilitary applications such as: drivers aid, aircraft aid, industrial process monitoring, community services, reghting, portable mine detection, night vision, border surveillance, law enforcement, search and rescue, and so on.