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Figure 17.3 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 3 meters away (10 ft.).
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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Figure 17.4 Samuel P. Langley (1834–1906)
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17 – History of infrared technology
