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Theory of thermography 37
temperature 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 ra-
diant emittance occurring within the invisible ultraviolet spectrum, at wavelength 0.27 μm.
Figure 37.5 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 37.6 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).
37.3.3 Stefan-Boltzmann's law
By integrating Planck’s formula from λ = 0 to λ = ∞, we obtain the total radiant emittance
(W
b
) of a blackbody:
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