Figure 16.4 Samuel P. Langley (1834–1906).
The improvement of infrared-detector sensitivity progressed slowly. An-
other major breakthrough, made by Langley in 1880, was the invention of
the bolometer. This consisted of a thin blackened strip of platinum con-
nected 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, rst introduced the use of lique-
ed 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 lique-
ed 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 rst 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 exploita-
tion of the infrared. These programs included experimental systems for
enemy intrusion/detection, remote temperature sensing, secure commu-
nications, and ‘ying torpedo’ guidance. An infrared search system tested
during this period was able to detect an approaching airplane at a dis-
tance 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 con-
verter and the photon detector. At rst, the image converter received the
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