Theory of thermography
19
19.3
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 ex-
plained 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 emis-
sion of radiation.
Figure 19.2
Gustav Robert Kirchhoff (1824–1887)
The construction of a blackbody source is, in principle, very simple. The radiation charac-
teristics of an aperture in an isotherm cavity made of an opaque absorbing material repre-
sents 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 ex-
cept for an aperture in one of the sides. Any radiation which then enters the hole is scat-
tered and absorbed by repeated reflections 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 cav-
ity. Such cavity radiators are commonly used as sources of radiation in temperature refer-
ence standards in the laboratory for calibrating thermographic instruments, such as a
FLIR Systems camera for example.
If the temperature of blackbody radiation increases to more than 525°C (977°F), the
source begins to be visible so that it appears to the eye no longer black. This is the incipi-
ent red heat temperature of the radiator, which then becomes orange or yellow as the tem-
perature increases further. In fact, the definition of the so-called
color temperature
of an
object is the temperature to which a blackbody would have to be heated to have the same
appearance.
Now consider three expressions that describe the radiation emitted from a blackbody.
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