FAG Detector III User Manual page 282

Target
Every form of matter with a temperature (T) above absolute zero emits infrared
radiation according to its temperature. This is called characteristic radiation. The
cause of this is the internal mechanical movement of molecules. The intensity of
this movement depends on the temperature of the object. Since the molecule
movement represents charge displacement, electromagnetic radiation (photon
particles) is emitted. These photons move at the speed of light and behave
according to the known optical principles. They can be deflected, focused with a
lens, or reflected from reflective surfaces. The spectrum of this radiation ranges
from 0.7 to 1000 µm wavelength. For this reason, this radiation cannot normally
be seen with the naked eye. This area lies within the red area of visible light and
has therefore been called "infra"-red after the Latin. (See Fig. 2).
Fig. 2 The electromagnetic spectrum, with range from around 0.7 to 14 µm useful for
measuring purposes
Fig. 3 shows the typical radiation of a body at different temperatures. As indicated,
bodies at high temperatures still emit a small amount of visible radiation. This is
why everyone can see objects at very high temperatures (above 600°C) glowing
somewhere from red to white. Experienced steelworkers can even estimate
temperature quite accurately from the color. The classic disappearing filament
pyrometer was used in the steel and iron industries from 1930 on. The invisible
part of the spectrum, however, contains up to 100,000 times more energy.
Infrared measuring technology builds on this. It can likewise be seen in Fig. 3 that
the radiation maximum move toward ever-shorter wavelengths as the target
temperature rises, and that the curves of a body do not overlap at different
temperatures. The radiant energy in the entire wavelength range (area beneath
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