9 – Infrared Primer
A6700sc/A6750sc User’s Manual
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9 Infrared Primer
9.1 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.
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.
