optris PI 160 Operator's Manual page 58

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He blackened the peak of a sensitive mercury thermometer. This thermometer, a glass prism that led sun
rays onto a table made his measuring arrangement. With this, he tested the heating of different colors of
the spectrum. Slowly moving the peak of the blackened thermometer through the colors of the spectrum,
he noticed the increasing temperature from violet to red. The temperature rose even more in the area
behind the red end of the spectrum. Finally he found the maximum temperature far behind the red area.
Nowadays this area is called "infrared wavelength area".
Figure 29: The electromagnetic spectrum and the area used for temperature measurement
For the measurement of "thermal radiation" infrared thermometry uses a wave-length ranging between
1 µ and 20 µm. The intensity of the emitted radiation depends on the material. This material contingent
constant is described with the help of the emissivity which is a known value for most materials
(see enclosed table emissivity).