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 deîš¿ned 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 reections so only an inni-
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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