History Of Infrared Technology; Figure 17.1 Sir William Herschel (1738-1822) - FLIR ThermaCAM P20 Operator's Manual

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History of infrared technology

Less than 200 years ago the existence of the infrared portion of the electromag-
netic 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.
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Figure 17.1 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 experi-
ment, 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.
As the blackened thermometer was moved slowly along the colors of the spectrum,
the temperature readings showed a steady increase from the violet end to the
red end. This was not entirely unexpected, since the Italian researcher, Landriani,
in a similar experiment in 1777 had observed much the same effect. It was Herschel,
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