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Theory of thermography32
Figure 32.2 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
cavity. Such cavity radiators are commonly used as sources of radiation in temperature
reference 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
temperature 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.
32.3.1 Planck’s law
Figure 32.3 Max Planck (1858–1947)
Max Planck (1858–1947) was able to describe the spectral distribution of the radiation
from a blackbody by means of the following formula:
where:
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