10. Basics of Infrared Thermometry
10.1 Introduction
Depending on the temperature each object emits a certain amount of infrared radiation. A change in the tem-
perature of the object is accompanied by a change in the intensity of the radiation.
Searching for new optical material William Herschel by chance found the infrared radiation in 1800.
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 maxi-
mum temperature far behind the red area.
Nowadays this area is called “infrared wavelength area”.
Fig. 21 William Herschel (1738 – 1822)
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