Stefan-Boltzmann's Law; Non-Blackbody Emitters - FLIR ThermaCAM B2 User Manual

18 – Theory of thermography
18.3.3
By integrating Planck's formula from λ = 0 to λ = ∞, we obtain the total radiant
emittance (W
b
This is the Stefan-Boltzmann formula (after Josef Stefan, 1835–1893, and Ludwig
18
Boltzmann, 1844–1906), which states that the total emissive power of a blackbody is
proportional to the fourth power of its absolute temperature. Graphically, W
the area below the Planck curve for a particular temperature. It can be shown that the
radiant emittance in the interval λ = 0 to λ
about the amount of the sun's radiation which lies inside the visible light spectrum.
10399303;a1
Figure 18.7 Josef Stefan (1835–1893), and Ludwig Boltzmann (1844–1906)
Using the Stefan-Boltzmann formula to calculate the power radiated by the human
body, at a temperature of 300 K and an external surface area of approx. 2 m
obtain 1 kW. This power loss could not be sustained if it were not for the compensating
absorption of radiation from surrounding surfaces, at room temperatures which do
not vary too drastically from the temperature of the body – or, of course, the addition
of clothing.
18.3.4
So far, only blackbody radiators and blackbody radiation have been discussed.
However, real objects almost never comply with these laws over an extended wave-
length region – although they may approach the blackbody behavior in certain
spectral intervals. For example, a certain type of white paint may appear perfectly
white in the visible light spectrum, but becomes distinctly gray at about 2 μm, and
beyond 3 μm it is almost black.
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Stefan-Boltzmann's law

) of a blackbody:

Non-blackbody emitters

is only 25 % of the total, which represents
max
Publ. No. 1557882 Rev. a156 – ENGLISH (EN) – February 28, 2006
represents
b
2
, we