FLIR EX series User Manual page 71

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Theory of thermography

• A selective radiator, for which ε varies with wavelength

According to Kirchhoff's law, for any material the spectral emissivity and spectral absorp-

tance of a body are equal at any specified temperature and wavelength. That is:

From this we obtain, for an opaque material (since α

+ ρ

= 1):

For highly polished materials ε

approaches zero, so that for a perfectly reflecting materi-

al (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.

Figure 17.8 Spectral radiant emittance of three types of radiators. 1: Spectral radiant emittance; 2: Wave-

length; 3: Blackbody; 4: Selective radiator; 5: Graybody.

#T559828; r.18043/22369; en-US

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