Influencing factors

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Emissivity is not easy to measure accurately because it depends significantly on many physical and chemical properties, such as temperature, wavelength, angle, oxidation, roughness, heat treatment and so on. This is why looking up emissivity values in a data book for even such a common material as stainless steel can be a confusing experience (see the graph on the right).    

The emissivity of stainless steel as presented in the TPRC data book (eds Touloukian & DeWitt), showing that the emissivity of steel can range from about 0.1 to 0.9, a very large spread.

The key factors to bear in mind concerning what determines emissivity:

- the emissivity of metals is strongly dependent on surface condition, the main influences being oxidation and roughness

- clean, polished and oxide-free metals have low emissivity
 
- roughened, oxidised metals have high emissivity

- non-metals are less strongly influenced by surface condition

- spectral and total emissivity generally increase with temperature for metals
 
- emissivity either increases or decreases with temperature for non-metals
 
- emissivity of oxide ceramics is highly influenced by the structure within the sample, as oxide ceramics are semitransparent to infrared radiation

- spectral transmissivity and reflectivity of semi-conducting coatings depends on the band gap and the doping of the material

- spectral transmissivity and reflectivity of selective metal coatings depends on the thickness of the coating

- emissivity of powders depends on the refractive index of the material and the particle size distribution

- a layer of water adsorbed by redistribution from other parts of the measuring apparatus can greatly influence the emissivity (Musilová et al., 2010)

Factors that influence emissivity values

Influencing factor Degree of dependence Notes
Wavelength strong The emissivity of polished metals, ceramics, polymers, films and other semitransparent materials exhibit strong wavelength dependence
weak Thickly oxidised metals and carbides
Temperature weak to moderate Weak for spectral emissivity; moderate for total emissivity
Surface conditions strong Strong dependence with most materials. Dielectrics are generally less affected
Emission angle moderate Greater influence for metals, lesser for dielectrics
Degree of polarisation weak to moderate Greater influence for polished metals

 

Reference

Musilová V. et al., 11th Cryogenics Conference of the IIR, Bratislava, 26 - 29 April 2010.

     
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