FLIR ThermaCAM P20 Operator's Manual page 98

Wheatstone bridge circuit upon which the infrared radiation was focused and to
which a sensitive galvanometer responded. This instrument is said to have been
able to detect the heat from a cow at a distance of 400 meters.
An English scientist, Sir James Dewar, first introduced the use of liquefied gases
as cooling agents (such as liquid nitrogen with a temperature of -196 °C (-320.8 °F))
in low temperature research. In 1892 he invented a unique vacuum insulating
container in which it is possible to store liquefied gases for entire days. The com-
mon 'thermos bottle', used for storing hot and cold drinks, is based upon his in-
vention.
Between the years 1900 and 1920, the inventors of the world 'discovered' the in-
frared. Many patents were issued for devices to detect personnel, artillery, aircraft,
ships – and even icebergs. The first operating systems, in the modern sense, began
to be developed during the 1914–18 war, when both sides had research programs
devoted to the military exploitation of the infrared. These programs included ex-
perimental systems for enemy intrusion/detection, remote temperature sensing,
secure communications, and 'flying torpedo' guidance. An infrared search system
tested during this period was able to detect an approaching airplane at a distance
of 1.5 km (0.94 miles), or a person more than 300 meters (984 ft.) away.
The most sensitive systems up to this time were all based upon variations of the
bolometer idea, but the period between the two wars saw the development of
two revolutionary new infrared detectors: the image converter and the photon
detector. At first, the image converter received the greatest attention by the mili-
tary, because it enabled an observer for the first time in history to literally 'see in
the dark'. However, the sensitivity of the image converter was limited to the near
infrared wavelengths, and the most interesting military targets (i.e. enemy soldiers)
had to be illuminated by infrared search beams. Since this involved the risk of
giving away the observer's position to a similarly-equipped enemy observer, it is
understandable that military interest in the image converter eventually faded.
The tactical military disadvantages of so-called 'active' (i.e. search beam-equipped)
thermal imaging systems provided impetus following the 1939–45 war for exten-
sive 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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Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004