optris PI 200 Operator's Manual page 74

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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 44: 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 µm
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 (►8 Emissivity).
Infrared thermometers are optoelectronic sensors. They calculate the surface temperature on the basis of
the emitted infrared radiation from an object. The most important feature of infrared thermometers is that
they enable the user to measure objects contactless. Consequently, these products help to measure the
temperature of inaccessible or moving objects without difficulties.