Ranger HRC™ operator´s manual – History of Infrared technology
Publ. No. TM 614 006 699 Rev B – ENGLISH (EN) – Oct 30. 2008 233
in large enough natural crystals to be made into lenses and prisms – is
remarkably transparent to the infrared. The result was that rock salt be-
came 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 16.3 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 rst 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 rst 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 evapora-
tion of a thin lm of oil when exposed to a heat pattern focused upon it,
the thermal image could be seen by reected light where the interference
effects of the oil lm 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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