Table of Contents
Advertisement
There are three processes which can occur that prevent a real object from acting like
a blackbody: a fraction of the incident radiation α may be absorbed, a fraction ρ may
be reflected, and a fraction τ may be transmitted. Since all of these factors are more
or less wavelength dependent, the subscript λ is used to imply the spectral depen-
dence of their definitions. Thus:
The spectral absorptance α
■
an object to that incident upon it.
The spectral reflectance ρ
■
an object to that incident upon it.
The spectral transmittance τ
■
through an object to that incident upon it.
The sum of these three factors must always add up to the whole at any wavelength,
so we have the relation:
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
Publ. No. 1558146 Rev. a156 – ENGLISH (EN) – February 28, 2006
= the ratio of the spectral radiant power absorbed by
λ
= the ratio of the spectral radiant power reflected by
λ
= the ratio of the spectral radiant power transmitted
λ
= 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
18 – Theory of thermography
+ ρ
= 1):
λ
λ
18
125
Advertisement
Table of Contents
