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Theory of thermography 18
This is Wien’s formula (after Wilhelm Wien , 1864–1928), which expresses mathematically
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 tem-
perature is obtained by applying the rule-of-thumb 3 000/T μm. Thus, a very hot star such
as Sirius (11 000 K), emitting bluish-white light, radiates with the peak of spectral radiant
emittance occurring within the invisible ultraviolet spectrum, at wavelength 0.27 μm.
Figure 18.5 Wilhelm Wien (1864–1928)
The sun (approx. 6 000 K) emits yellow light, peaking at about 0.5 μm in the middle of the
visible light spectrum.
At room temperature (300 K) the peak of radiant emittance lies at 9.7 μm, in the far infra-
red, while at the temperature of liquid nitrogen (77 K) the maximum of the almost insignifi-
cant amount of radiant emittance occurs at 38 μm, in the extreme infrared wavelengths.
Figure 18.6 Planckian curves plotted on semi-log scales from 100 K to 1000 K. The dotted line represents
the locus of maximum radiant emittance at each temperature as described by Wien's displacement law. 1:
Spectral radiant emittance (W/cm
2 (μm)); 2: Wavelength (μm).
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