FLIR i5 User Manual page 153

For opaque materials τ
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
that from a blackbody at the same temperature and wavelength.
Expressed mathematically, this can be written as the ratio of the spectral emittance
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
■
A graybody, for which ε
■
A selective radiator, for which ε varies with wavelength
■
According to Kirchhoff's law, for any material the spectral emissivity and spectral ab-
sorptance of a body are equal at any specified temperature and wavelength. That is:
From this we obtain, for an opaque material (since α
For highly polished materials ε
material (i.e. a perfect mirror) we have:
For a graybody radiator, the Stefan-Boltzmann formula becomes:
This states that the total emissive power of a graybody is the same as a blackbody
at the same temperature reduced in proportion to the value of ε from the graybody.
Publ. No. T559382 Rev. a358 – ENGLISH (EN) – June 23, 2009
= 0 and the relation simplifies to:
λ
= the ratio of the spectral radiant power from an object to
λ
= ε = 1
λ
= ε = constant less than 1
λ
approaches zero, so that for a perfectly reflecting
λ
22 – Theory of thermography
+ ρ = 1):
λ λ
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