FLIR E75, User Manual

Page: 270 / 300

History of infrared technology 35
Before the year 1800, the existence of the infrared portion of the electromagnetic spectrum
wasn't even suspected. The original significance of the infrared spectrum, or simply ‘the in-
frared’ 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 35.1 Sir William Herschel (1738–1822)
The discovery was made accidentally during the search for a new optical material. Sir Wil-
liam 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 in-
trigued 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 distri-
bution 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
that measurements confined to the visible portion of the spectrum failed to locate this
point.
Figure 35.2 Marsilio Landriani (1746–1815)
#T810190; r. AJ/46209/46209; en-US
258