PTZ-35x140
Quick Start Guide 427-0011-00-12 Version 100
however, who was the first to recognize that there must be a point where the heating effect reaches a maximum, and
those measurements confined to the visible portion of the spectrum failed to locate this point.
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 ‘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. Different investigators, in attempting to
confirm his work, used various types of glass indiscriminately, having different 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
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 infrared optical material, and remained so for the next
hundred years, until the art of synthetic crystal growing was mastered in the 1930’s.
Macedonio Melloni (1798–1854)
