9 – Infrared Primer
A6600/A6650 User’s Manual
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Figure 8-12: Spectral radiant emittance of three types of radiators. 1: Spectral radiant
emittance; 2: avelength; 3: Blackbody; 4: Selective radiator; 5: Graybody.
Figure 8-13: Spectral emissivity of three types of radiators. 1: Spectral emissivity; 2:
Wavelength; 3: blackbody; 4: Graybody; 5: Selective radiator.
9.2.4 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:
