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Figure 20.8 Spectral radiant emittance of three types of radiators. 1: Spectral radiant emittance; 2:
Wavelength; 3: Blackbody; 4: Selective radiator; 5: Graybody.
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Figure 20.9 Spectral emissivity of three types of radiators. 1: Spectral emissivity; 2: Wavelength; 3:
Blackbody; 4: Graybody; 5: Selective radiator.
20.4 Infrared semi-transparent materials
Consider now a non-metallic, semi-transparent body – let us say, in the form of a thick
flat plate of plastic material. When the plate is heated, radiation generated within its
volume must work its way toward the surfaces through the material in which it is
partially absorbed. Moreover, when it arrives at the surface, some of it is reflected
back into the interior. The back-reflected radiation is again partially absorbed, but
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Publ. No. T559733_en-US Rev. a571 – ENGLISH (EN) – November 4, 2011
20 – Theory of thermography