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History of infrared technology 24
Before the year 1800, the existence of the infrared portion of the electromagnetic spec-
trum 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 to-
day than it was at the time of its discovery by Herschel in 1800.
Figure 24.1 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 re-
duce 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 ac-
tually 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 sensi-
tive mercury-in-glass thermometer with ink, and with this as his radiation detector he pro-
ceeded 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 experi-
ment 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 maxi-
mum, and that measurements confined to the visible portion of the spectrum failed to lo-
cate this point.
Figure 24.2 Marsilio Landriani (1746–1815)
Moving the thermometer into the dark region beyond the red end of the spectrum, Her-
schel confirmed that the heating continued to increase. The maximum point, when he
found it, lay well beyond the red end – in what is known today as the ‘infrared
wavelengths’.
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