[9 — Theory of thermography]
© FLIR Systems AB – Publ. No. 557 369 – Ed. A
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9 Theory of thermography
9.1 Introduction
The subjects of infrared radiation and the related technique of thermography are
still new to many who will use an ThermaCAM™ 500 series camera. In this section
the theory behind thermography will be given.
9.2 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 electromagnetic spectrum. They are all governed by the
same laws and the only differences are those due to differences in wavelength.
Figure 9.1 The electromagnetic spectrum.
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 millimetre
range.
The infrared band is often further subdivided into four smaller bands, the bound-
aries 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 (micrometres),
other units are often still used to measure wavelength in this spectral region, e.g.
microns (µ), nanometres (nm) and Ångströms (Å).
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