FLIR Photon Manual Book page 53

visible to the eye. Sir John also managed
to obtain a primitive record of the
thermal image on paper, which he called
a thermograph.
Figure 4. Samuel P. Langley (1834–1906)
The improvement of infrared-detector
sensitivity progressed slowly. Another
major breakthrough, made by Langley
(Figure 4) 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.
History and Theory of Infrared Technology
Between the years 1900 and 1920, the
inventors of the world "discovered"
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 World War I, 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 world 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
(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 (search beam
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