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:
21.3 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 radia-
tion is explained by Kirchhoff’s Law (after Gustav Robert Kirchhoff, 1824–1887), which
states that a body capable of absorbing all radiation at any wavelength is equally
capable in the emission of radiation.
10398803;a1
Figure 21.2 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 reflections so only an infinites-
imal fraction can possibly escape. The blackness which is obtained at the aperture
is nearly equal to a blackbody and almost perfect for all wavelengths.
By providing such an isothermal cavity with a suitable heater it becomes what is
termed a cavity radiator. An isothermal cavity heated to a uniform temperature gener-
ates blackbody radiation, the characteristics of which are determined solely by the
temperature of the cavity. Such cavity radiators are commonly used as sources of
radiation in temperature reference standards in the laboratory for calibrating thermo-
graphic instruments, such as a FLIR Systems camera for example.
Publ. No. T559498 Rev. a461 – ENGLISH (EN) – August 19, 2010 69
21 – Theory of thermography
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