# 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:
A
= Surface area perpendicular to heat transfer (m2)
σ
Ts
= 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).

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

• 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.

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