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Theory of thermography 25
• A selective radiator, for which ε varies with wavelength
According to Kirchhoff’s law, for any material the spectral emissivity and spectral absorp-
tance of a body are equal at any specified temperature and wavelength. That is:
From this we obtain, for an opaque material (since α
λ
+ ρ
λ
= 1):
For highly polished materials ε
λ
approaches zero, so that for a perfectly reflecting materi-
al ( 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 25.8 Spectral radiant emittance of three types of radiators. 1: Spectral radiant emittance; 2: Wave-
length; 3: Blackbody; 4: Selective radiator; 5: Graybody.
Figure 25.9 Spectral emissivity of three types of radiators. 1: Spectral emissivity; 2: Wavelength; 3: Black-
body; 4: Graybody; 5: Selective radiator.
#T559950; r. AD/35720/35720; en-US 99