Ranger HRC™ operator´s manual – Theory of thermal imaging
Publ. No. TM G007971 Rev. A1 – ENGLISH (EN) – Sept 09. 2008 237
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 wave-
lengths 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).
17.3 Blackbody radiation
A blackbody is dened as an object which absorbs all radiation that im-
pinges on it at any wavelength. The apparent misnomer black relating to
an object emitting radiation is explained by Kirchhoff’s Law after Gustav
Robert Kirchhoff, 1824–1887), which states that a body capable of absorb-
ing all radiation at any wavelength is equally capable in the emission of
radiation.
Figure 17.2 Gustav Robert Kirchhoff (1824–1887).
The construction of a blackbody source is, in principle, very simple. The
radiation characteristics of an aperture in an isotherm cavity made of an
opaque absorbing material represents almost exactly the properties of a
blackbody. A practical application of the principle to the construction of
a perfect absorber of radiation consists of a box that is light tight except
for an aperture in one of the sides. Any radiation which then enters the
hole is scattered and absorbed by repeated reections so only an inni-
tesimal fraction can possibly escape. The blackness which is obtained at
the aperture is nearly equal to a blackbody and almost perfect for all wave-
lengths.
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