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Blackbodies, Emissivity and Real Surfaces

where: ε = emissivity of the radiating surface

Real Surfaces:

Stefan-Boltzmann Law

The maximum radiation rate at which energy can be emitted
from a surface is given by:

where:

= Surface area perpendicular to heat transfer (m2)

= Stefan-Boltzmann Constant = 5.67 x 10-8 W/(m2-K4)

= Absolute temperature of the surface (K)

The emissivity of a surface, ε, quantifies its ability to radiate.

A blackbody has an emissivity of one (ε = 1) and real substances have an emissivity between zero and one (0 <ε< 1).

Lesson
4B Blog

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Lesson 4B: page 22 of 26

Emissivity is a property of a surface that quantifies how closely a real surface approaches a blackbody in terms of emitted thermal radiation.
A blackbody, or perfect emitter, has an emissivity of 1 and any substance at absolute zero has an emissivity of zero.
All real substances at temperatures greater than absolute zero have emissivities that lie between 0 and 1.
The emissivity is just the ratio of the amount of energy emitted by a real substance to the amount of energy that would be emitted by a blackbody at the same temperature.
So, the heat transfer rate from a real surface is just the emissivity multiplied by the blackbody heat transfer rate.
This is not a big deal. Emissivity is basically a fudge factor that corrects for the characteristics of the surface of an object.
It is almost impossible to accurately PREDICT the emissivity of a surface.
It is generally necessary to measure the radiation heat transfer rate and then calculate the emissivity.
As a result, emissivity is generally a number that you look up in a reference book for any given surface.
There is one more little quirk about radiation heat transfer before we conclude this lesson.

Blackbodies, Emissivity and Real Surfaces

Stefan-Boltzmann Law

Ch 4, Lesson B, Page 22 - Blackbodies, Emissivity and Real Surfaces