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19
Theory of thermography
For highly polished materials ε
approaches zero, so that for a perfectly reflecting material
λ
(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.
Figure 19.8 Spectral radiant emittance of three types of radiators. 1: Spectral radiant emittance; 2: Wave-
length; 3: Blackbody; 4: Selective radiator; 5: Graybody.
Figure 19.9 Spectral emissivity of three types of radiators. 1: Spectral emissivity; 2: Wavelength; 3: Black-
body; 4: Graybody; 5: Selective radiator.
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