FLIR was originally developed by the United States Navy to assist in the identification and targeting of opposition forces. FLIR units measure the amount of infrared energy emitted by various objects and life forms. Infrared energy is normally invisible to the human eye, but FLIR systems are able to detect and interpret emissions and provide the user with an image, even in complete darkness. Like conventional night vision enhancement systems, FLIR does not provide a perfect picture of what is in its field of view. What it does provide is an image with sufficient clarity to allow users to distinguish between cars, trees, and people.
FLIR systems are still highly effective under conditions of low-visibility or complete darkness.
While the effectiveness of the system may be diminished when viewing distant objects under obscured conditions, FLIR still provide better visibility than other night vision enhancement systems. Because it measures thermal energy, FLIR can be used to conduct surveillance without relying on search lights or special lighting, improving the user's ability to remain undetected. The military advantages of these early systems were significant. Naval vessels could still detect and target enemy ships, even on the darkest nights or under foggy conditions. As one might imagine, these early systems were large, heavy, and expensive.