FLIR PathFindIR User Manual page 33

8 – Infrared Technology
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'.
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