FLIR FlashSight User Manual page 22

FlashSight™ User's Guide
The tactical military disadvantages of so-called 'active' (i.e. search beam-
equipped) thermal imaging systems provided impetus following the 1939–45
war for extensive secret military infrared-research programs into the
possibilities of developing 'passive' (no search beam) systems around the
extremely sensitive photon detector. During this period, military secrecy
regulations completely prevented disclosure of the status of infrared-imaging
technology. This secrecy only began to be lifted in the middle of the 1950's,
and from that time adequate thermal-imaging devices finally began to be
available to civilian science and industry.
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