Figure 17.2 Marsilio Landriani - FLIR ThermaCAM P20 Operator's Manual

however, who was the first to recognize that there must be a point where the
heating effect reaches a maximum, and that measurements confined to the visible
portion of the spectrum failed to locate this point.
10398903;1
Figure 17.2 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 'in-
frared'. 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 in-
frared. Through his later experiments, Herschel was aware of the limited trans-
parency 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 in-
frared optical material, and remained so for the next hundred years, until the art
of synthetic crystal growing was mastered in the 1930's.
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Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004