Ranger HRC™ operator´s manual – Theory of thermal imaging
238 Publ. No. TM G007971 Rev. A1 – ENGLISH (EN) – Sept 09. 2008
By providing such an isothermal cavity with a suitable heater it becomes
what is termed a cavity radiator. An isothermal cavity heated to a uni-
form temperature generates blackbody radiation, the characteristics of
which are determined solely by the temperature of the cavity.
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 incipient red heat temperature of the radia-
tor, which then becomes orange or yellow as the temperature increases
further. In fact, the denition 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.
17.3.1 Planck’s law
Figure 17.3 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:
M
e,l
Blackbody spectral radiance (radiant exitance)
c Velocity of light = 3 × 10
8
m/s
h Planck’s constant = 6.6 × 10
-34
Joule sec
k Boltzmann’s constant = 1.4 × 10
-23
Joule/K
T Absolute temperature (K) of a blackbody
l
Wavelength (m)
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