FlashSight™ User’s Guide
Copyright © 2006, FLIR Systems, Inc. 431-0002-09-10 Version 100
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Moving the thermometer into the dark region beyond the red end of the
spectrum, Herschel 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’.
When Herschel revealed his discovery, he referred to this new portion of the
electromagnetic spectrum as the ‘thermometrical spectrum’. The radiation
itself he sometimes referred to as ‘dark heat’, or simply ‘the invisible rays’.
Ironically, and contrary to popular opinion, it wasn't Herschel who originated
the term ‘infrared’. The word only began to appear in print around 75 years
later, and it is still unclear who should receive credit as the originator.
Herschel’s use of glass in the prism of his original experiment led to some
early controversies with his contemporaries about the actual existence of the
infrared wavelengths. Different investigators, in attempting to confirm his
work, used various types of glass indiscriminately, having different
transparencies in the infrared. Through his later experiments, Herschel was
aware of the limited transparency of glass to the newly-discovered thermal
radiation, and he was forced to conclude that optics for the infrared would
probably be doomed to the use of reflective elements exclusively (i.e. plane
and curved mirrors). Fortunately, this proved to be true only until 1830, when
the Italian investigator, Melloni, made his great discovery that naturally
occurring rock salt (NaCl) – which was available in large enough natural
crystals to be made into lenses and prisms – is remarkably transparent to the
infrared. The result was that rock salt became the principal infrared optical
material, and remained so for the next hundred years, until the art of synthetic
crystal growing was mastered in the 1930’s.
Figure 17: Macedonio Melloni (1798–1854)
Thermometers, as radiation detectors, remained unchallenged until 1829, the
year Nobili invented the thermocouple. (Herschel’s own thermometer could be
read to 0.2 °C (0.036 °F), and later models were able to be read to 0.05 °C
(0.09 °F)). Then a breakthrough occurred; Melloni connected a number of
thermocouples in series to form the first thermopile. The new device was at
least 40 times as sensitive as the best thermometer of the day for detecting
heat radiation – capable of detecting the heat from a person standing three
meters away.
The first so-called ‘heat-picture’ became possible in 1840, the result of work
by Sir John Herschel, son of the discoverer of the infrared and a famous
astronomer in his own right. Based upon the differential evaporation of a thin
film of oil when exposed to a heat pattern focused upon it, the thermal image
could be seen by reflected light where the interference effects of the oil film
