9 – Infrared Primer
A6600/A6650 User’s Manual
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Figure 8-2: Marsilio Landriani (1746–1815)
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 8-3: 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 made the image visible to
the eye. Sir John also managed to obtain a primitive record of the thermal image on paper, which he
called a ‘thermograph’.
