9 – Infrared Primer
A6700sc/A6750sc User’s Manual
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9.2 Theory of Thermography
9.2.1 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.
9.2.2 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 8-5 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 longwavelength 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:
9.2.3 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 explained by Kirchhoff’s Law
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