FLIR ThermaCAM PM575, Operator'S Manual

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[ 9.4 — Infrared semi-transparent materials ]
ThermaCAM™ PM575/595 Operator’s Manual 58
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 effec-
tive emissivity of a semi-transparent plate is obtained as
This formula represents a generalisation of Kirchhoff’s law, which is reduced when
the plate becomes opaque (
τ
λ
= 0) to the single formula:
This last relation is a particularly convenient one, because it is often easier to mea-
sure reflectance than to measure emissivity directly.
ε
λ
I
ρ
λ
–
( )
I
τ
λ
–
( )
I
ρ
λ
τ
λ
–
------------------------------------ =
ε
λ
1
ρ
λ
– =