FLIR Ranger HRC MS - Infrared Semi-Transparent Materials

Ranger HRC™ operator´s manual – Theory of thermal imaging
Publ. No. TM G007971 Rev. A1 – ENGLISH (EN) – Sept 09. 2008 245
17.4 Infrared semi-transparent materials
Consider now a non-metallic, semi-transparent body – let us say, in the
form of a thick at plate of plastic material. When the plate is heated,
radiation generated within its volume must work its way toward the sur-
faces through the material in which it is partially absorbed. Moreover,
when it arrives at the surface, some of it is reected back into the interior.
The back-reected radiation is again partially absorbed, but some of it
arrives at the other surface, through which most of it escapes; part of it is
reected back again. Although the progressive reections become weaker
and weaker they must all be added up when the total exitance of the plate
is sought.
When the resulting geometrical series is summed, the effective emissivity
of a semitransparent plate is obtained as:
λλ
λλ
λ
τρ
τρ
ε
−
−−
=
1
)1)(1(
When the plate becomes opaque this formula is reduced to the single for-
mula:
λλ
ρε −=1
This last relation is a particularly convenient one, because it is often easier
to measure reectance than to measure emissivity directly.