20
Theory of thermography
20.1
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 ther-
mography will be given.
20.2
The electromagnetic spectrum
The electromagnetic spectrum is divided arbitrarily into a number of wavelength re-
gions, 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 electromagnetic spectrum. They are all governed by the same laws and the
only differences are those due to differences in wavelength.
10067803;a1
Figure 20.1 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-wave-
length 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
20
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