For highly polished materials ε
λ
approaches zero, so that for a perfectly reflecting
material (i.e. a perfect mirror) we have:
For a graybody radiator, the Stefan-Boltzmann formula becomes:
This states that the total emissive power of a graybody is the same as a blackbody
at the same temperature reduced in proportion to the value of ε from the graybody.
10401203;a1
Figure 20.8 Spectral radiant emittance of three types of radiators. 1: Spectral radiant emittance; 2:
Wavelength; 3: Blackbody; 4: Selective radiator; 5: Graybody.
20
162 Publ. No. 1557966 Rev. a155 – ENGLISH (EN) – February 6, 2006
20 – Theory of thermography
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