(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:
18.3 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 (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;1
Figure 18.2 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 ap-
plication 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 reflec-
tions so only an infinitesimal 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
generates 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 cali-
88 Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004
18.3 – Blackbody radiation
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