Theory of thermography
18
18.1
Introduction
The subjects of infrared radiation and the related technique of thermography are still new
to many who will use an infrared camera. In this section the theory behind thermography
will be given.
18.2
The electromagnetic spectrum
The electromagnetic spectrum is divided arbitrarily into a number of wavelength regions,
called
bands
, distinguished by the methods used to produce and detect the radiation.
There is no fundamental difference between radiation in the different bands of the elec-
tromagnetic spectrum. They are all governed by the same laws and the only differences
are those due to differences in wavelength.
Figure 18.1
The electromagnetic spectrum. 1: X-ray; 2: UV; 3: Visible; 4: IR; 5: Microwaves; 6:
Radiowaves.
Thermography makes use of the infrared spectral band. At the short-wavelength end the
boundary lies at the limit of visual perception, in the deep red. At the long-wavelength
end it merges with the microwave radio wavelengths, in the millimeter range.
The infrared band is often further subdivided into four smaller bands, the boundaries of
which are also arbitrarily chosen. They include: the
near infrared
(0.75–3 μm), the
middle
infrared
(3–6 μm), the
far infrared
(6–15 μm) and the
extreme infrared
(15–100 μm).
Although the wavelengths are given in μm (micrometers), other units are often still used
to measure wavelength in this spectral region,
e.g.
nanometer (nm) and Ångström (Å).
The relationships between the different wavelength measurements is:
18.3
Blackbody radiation
A blackbody is defined as an object which absorbs all radiation that impinges on it at any
wavelength. The apparent misnomer
black
relating to an object emitting radiation is ex-
plained by Kirchhoff’s Law (after
Gustav Robert Kirchhoff
, 1824–1887), which states that
a body capable of absorbing all radiation at any wavelength is equally capable in the
emission of radiation.
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