on . Posted in Thermodynamics

Emissivity, abbreviated as \(\epsilon\) (Greek symbol epsilon), a dimensionless number, is a measure of a material's ability to emit thermal radiation.  It is defined as the ratio of the thermal radiation emitted by a material to the thermal radiation emitted by a blackbody at the same temperature.

The value of emissivity ranges from 0 to 1, where a value of 0 represents a material that does not emit any thermal radiation and a value of 1 represents a perfect blackbody that emits thermal radiation at a maximum rate.  In practice, the emissivity of a material depends on many factors, such as temperature, surface finish, and wavelength of the emitted radiation.  The emissivity of common materials is often available in reference tables or can be measured experimentally using techniques such as infrared thermography or spectrometry.


Emissivity formula

\( \epsilon \;=\; E \;/\; Eb \)     (Emissivity)

\( E \;=\; \epsilon \; Eb   \)

\( Eb \;=\; E \;/\; \epsilon \)

Symbol English Metric
\( \epsilon \)  (Greek symbol epsilon) = emissivity of the surface   \( dimensionless \)
\( E \) = thermal radiation emitted by the material   \(BTU\;/\;hr-ft^2\)    \(W\;/\;m^2\) 
\( Eb \) = thermal radiation emitted by a blackbody at the same temperature   \(BTU\;/\;hr-ft^2\)     \(W\;/\;m^2\)  


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Tags: Thermal Energy