History And Theory Of Infrared Technology - FLIR Photon Manual Book

Appendix C
History and Theory of
Infrared Technology
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 Sir William Herschel in 1800
(Figure 1).
Figure 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
History and Theory of
Infrared Technology
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 (Figure 2), 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
that measurements confined to the
visible portion of the spectrum failed to
locate this point.
Figure 2. Marsilio Landriani (1746–1815)
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