➲ The factor 10
-6
is used since spectral emittance in the curves is expressed in
Watt/m
2
, μm.
Planck’s formula, when plotted graphically for various temperatures, produces a
family of curves. Following any particular Planck curve, the spectral emittance is zero
at λ = 0, then increases rapidly to a maximum at a wavelength λ
max
and after passing
it approaches zero again at very long wavelengths. The higher the temperature, the
shorter the wavelength at which maximum occurs.
10327103;a4
Figure 21.4 Blackbody spectral radiant emittance according to Planck’s law, plotted for various absolute
temperatures. 1: Spectral radiant emittance (W/cm
2
× 10
3
(μm)); 2: Wavelength (μm)
21.3.2 Wien’s displacement law
By differentiating Planck’s formula with respect to λ, and finding the maximum, we
have:
This is Wien’s formula (after Wilhelm Wien, 1864–1928), which expresses mathemati-
cally the common observation that colors vary from red to orange or yellow as the
temperature of a thermal radiator increases. The wavelength of the color is the same
as the wavelength calculated for λ
max
. A good approximation of the value of λ
max
for
a given blackbody temperature is obtained by applying the rule-of-thumb 3 000/T
Publ. No. T559498 Rev. a461 – ENGLISH (EN) – August 19, 2010 71
21 – Theory of thermography
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