FlashSight™ User’s Guide
Copyright © 2006, FLIR Systems, Inc. 431-0002-09-10 Version 100
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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 15: 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.
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, however, who was the first to recognize that
there must be a point where the heating effect reaches a maximum, and
those measurements confined to the visible portion of the spectrum failed to
locate this point.
Figure 16: Marsilio Landriani (1746–1815)
