9 – Infrared Primer
A6700sc/A6750sc User’s Manual
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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 8-9: 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 infrared, while at
the temperature of liquid nitrogen (77 K) the maximum of the almost insignificant amount of radiant
emittance occurs at 38
μm, in the extreme infrared wavelengths.
Figure 8-10: 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/cm2 (
μm)); 2: Wavelength (μm).
Stefan-Boltzmann's Law
By integrating Planck’s formula from
λ = 0 to λ = ∞, we obtain the total radiant emittance (Wb) of a
blackbody:
