Wien's Displacement Law - FLIR Exx series User Manual

30 – Theory of thermography
-6
➲ The factor 10 is used since spectral emittance in the curves is expressed in
2
Watt/m , 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 and after passing
max
it approaches zero again at very long wavelengths. The higher the temperature, the
shorter the wavelength at which maximum occurs.
10327103;a4
Figure 30.4 Blackbody spectral radiant emittance according to Planck's law, plotted for various absolute
2 3
temperatures. 1: Spectral radiant emittance (W/cm × 10 (μm)); 2: Wavelength (μm)
30.3.2

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 . A good approximation of the value of λ for
max max
a given blackbody temperature is obtained by applying the rule-of-thumb 3 000/T
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Publ. No. T559597 Rev. a500 – ENGLISH (EN) – December 10, 2010