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Figure 18.9 Spectral emissivity of three types of radiators. 1: Spectral emissivity; 2: Wavelength; 3:
Blackbody; 4: Graybody; 5: Selective radiator.
18.4 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
some of it arrives at the other surface, through which most of it escapes; part of it is
reflected back again. Although the progressive reflections become weaker and
weaker they must all be added up when the total emittance of the plate is sought.
When the resulting geometrical series is summed, the effective emissivity of a semi-
transparent plate is obtained as:
When the plate becomes opaque this formula is reduced to the single formula:
This last relation is a particularly convenient one, because it is often easier to measure
reflectance than to measure emissivity directly.
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Publ. No. 1557882 Rev. a156 – ENGLISH (EN) – February 28, 2006
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18 – Theory of thermography