brating thermographic instruments, such as a FLIR Systems AB 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 incipient 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.
18.3.1 Planck’s law
10399203;1
Figure 18.3 Max Planck (1858–1947)
Max Planck (1858–1947) was able to describe the spectral distribution of the radi-
ation from a blackbody by means of the following formula:
where:
Blackbody spectral radiant emittance at wavelength λ.
W
λb
Velocity of light = 3 × 10
8
m/s
c
Planck’s constant = 6.6 × 10
-34
Joule sec.
h
Boltzmann’s constant = 1.4 × 10
-23
Joule/K.
k
Absolute temperature (K) of a blackbody.
T
18.3 – Blackbody radiation
Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004 89
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