The earliest development of thermal IR imaging systems was based on military requirements for night vision capability. The first night vision devices were of the "star scope" variety, based on radiance amplification or "image intensification" of available light. Early military star scopes used NIR illuminators that could be detected by enemy sensors, and so offered limited military advantage. These were eventually replaced by "undetectable" passive image intensifiers based on high sensitivity GaAs photocathodes. The first truly passive thermal night vision devices operated in the IR and were based on the detection of thermal radiation from the target surface. For more than half a century, from the introduction of the first thermal IR devices through the current development of uncooled IR FPAs, the military has continued to be a driving force in this evolution. In the aftermath of the 9/11 attack in the U.S. and the worldwide SARS outbreak in 2002-2003, increased awareness of the need for homeland security has placed a strong emphasis on wider deployment of IR instrumentation for surveillance and threat detection.
Unlike the needs of most industrial applications, thermal imagers for night vision, security, and surveillance applications require little or no temperature measurement capability. The key requirement here is to present an image of the target with the best possible spatial and thermal resolution, at the greatest possible distance from the target, under the worst conditions of obscuration and absorption in the intervening atmospheric path, and without the possibility of detection by enemy sensors.
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