Publ. No. TM 614 006 699 Rev B – ENGLISH (EN) – Oct 30. 2008 231
History of Infrared technology
Less than 200 years ago the existence of the infrared portion of the electro-
magnetic spectrum wasn’t even suspected. The original signicance 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.
16
Figure 16.1 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 lter material to reduce the brightness of the
sun’s image in telescopes during solar observations. While testing diffe-
rent samples of colored glass which gave similar reductions in brightness
he was intrigued to nd 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 nding 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 distri-
bution of intensity in the spectrum. He rst blackened the bulb of a sensi-
tive mercury-in-glass thermometer with ink, and with this as his radia-
tion 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.
As the blackened thermometer was moved slowly along the colors of the
spectrum, the temperature readings showed a steady increase from the
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