FLIR PathFindIR LE, User Manual

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334-0001-00-10-LE, rev. 100 May ’08 23
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8 Infrared Technology
8.1 History of Infrared
Less than 200 years ago the existence of the infrared portion of the
electromagnetic spectrum wasn't even suspected. The original significance
of the infrared spectrum, or simply ‘the infrared’ as it is often called, as a
form of heat radiation is perhaps less obvious today than it was at the time
of its discovery by Herschel in 1800.
Figure 8-1: Sir William Herschel (1738–1822)
The discovery was made accidentally during the search for a new optical
material. Sir William Herschel—Royal Astronomer to King George III of
England, and already famous for his discovery of the planet Uranus—was
searching for an optical filter material to reduce the brightness of the sun’s
image in telescopes during solar observations. While testing different
samples of colored glass which gave similar reductions in brightness he
was intrigued to find that some of the samples passed very little of the
sun’s heat, while others passed so much heat that he risked eye damage
after only a few seconds’ observation.
Herschel was soon convinced of the necessity of setting up a systematic
experiment, with the objective of finding a single material that would give
the desired reduction in brightness as well as the maximum reduction in
heat. He began the experiment by actually repeating Newton’s prism
experiment, but looking for the heating effect rather than the visual
distribution of intensity in the spectrum. He first blackened the bulb of a
sensitive mercury-in-glass thermometer with ink, and with this as his
radiation detector he proceeded to test the heating effect of the various
colors of the spectrum formed on the top of a table by passing sunlight
through a glass prism. Other thermometers, placed outside the sun’s rays,
served as controls.