The improvement of infrared-detector sensitivity progressed slowly. Another major break-
through, 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 re-
sponded. 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 cool-
ing agents (such as liquid nitrogen with a temperature of -196 °C (-320.8 °F)) in low tem-
perature 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 tor-
pedo’ 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 bolome-
ter 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 im-
age converter was limited to the near infrared wavelengths, and the most interesting mili-
tary 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) ther-
mal 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 peri-
od, 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.
#T559498; r.22370/22370; en-US
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