9 – Infrared Primer
A6600/A6650 User’s Manual
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Figure 8-4: Samuel P. Langley (1834–1906)
The improvement of infrared-detector sensitivity progressed slowly. Another major breakthrough,
made by Langley in 1880, was the invention of the bolometer. This consisted of a thin blackened strip
of platinum connected in one arm of a 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 common ‘thermos bottle’, used for storing hot and cold drinks, is based upon his
invention.
Between the years 1900 and 1920, the inventors of the world ‘discovered’ the infrared. 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 experimental 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 military, 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 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.
