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Theory of thermography 37
Figure 37.9 Spectral emissivity of three types of radiators. 1: Spectral emissivity; 2: Wavelength; 3: Black-
body; 4: Graybody; 5: Selective radiator.
37.4 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 sur-
face, through which most of it escapes; part of it is reflected back again. Although the pro-
gressive 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 ef-
fective 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 re-
flectance than to measure emissivity directly.
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