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8 – Infrared Technology
30 04/07 334-0001-00-10, version 100
As the blackened thermometer was moved slowly along the colors of the
spectrum, the temperature readings showed a steady increase from the
violet end to the red end. This was not entirely unexpected, since the
Italian researcher, Landriani, in a similar experiment in 1777 had
observed much the same effect. It was Herschel, 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.
Figure 8-2: 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 ‘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