FLIR Photon User Manual

46
Appendix C
Moving the thermometer into the
dark region beyond the red end of the
spectrum, Herschel conrmed 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. Dierent investigators, in
attempting to conrm his work, used
various types of glass indiscriminately,
having dierent 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 reective elements exclusively
(plane and curved mirrors). Fortunately,
this proved to be true only until 1830,
when the Italian investigator Melloni
(Figure 3) 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 1930s.
Figure 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
infrared and a famous astronomer in his
own right. Based upon the dierential
evaporation of a thin lm of oil when
exposed to a heat pattern focused upon
it, the thermal image could be seen by
reected light where the interference
eects of the oil lm made the image

FLIR Photon Specifications

General

Brand	FLIR
Model	Photon
Category	IP Camera
Language	English