FLIR ThermaCAM PM575, Operator'S Manual, Page: 64 / 68

[ 9.3.4 — Non-blackbody emitters ]
ThermaCAM™ PM575/595 Operator’s Manual 56
• The spectral transmittance
τ
λ
= the ratio of the spectral radiant power transmit-
ted through an object to that incident upon it.
The sum of these three factors must always add up to the whole at any wave-
length, so we have the relation:
For opaque materials
τ
λ
= 0, and the relation simplifies to
Another factor, called the emissivity, is required to describe the fraction
ε
of the
radiant emittance of a blackbody produced by an object at a specific temperature.
Thus, we have the definition:
The spectral emissivity
ε
λ
= the ratio of the spectral radiant power from an object
to that from a blackbody at the same temperature and wavelength.
Expressed mathematically, this can be written as the ratio of the spectral emit-
tance of the object to that of a blackbody as follows:
Generally speaking, there are three types of radiation source, distinguished by the
ways in which the spectral emittance of each varies with wavelength.
• A blackbody, for which
ε
1
= ε
= 1.
• A greybody, for which
ε
1
= ε
= constant less than 1.
• A selective radiator, for which e varies with wavelength.
According to Kirchhoff’s Law, for any material the spectral emissivity and spectral
absorptance of a body are equal to any specified temperature and wavelength.
That is:
From this we obtain, for an opaque material (since
α
λ
+ ρ
λ
= 1):
For highly polished materials
ε
λ
approaches zero, so that for a perfectly reflecting
material (i.e. a perfect mirror) we have
For a greybody radiator, the Stefan-Boltzmann formula becomes
α
λ
ρ
λ
τ
λ
1 = + +
α
λ
ρ
λ
1 = +
ε
λ
W
λ
o
W
λ
b
------------- =
ε
λ
α
λ
+
ε
λ
ρ
λ
1 = +
ρ
λ
1 =