Radiation, Emissivity, Absorbance and Reflectance
Thermal radiation is the made of electromagnetic waves emitted due to agitation of the molecules in the substance. The electromagnetic waves behave in a similar way to light, in that they travel linearly and at the speed of light
Black Body RadiationWhen radiation hits a body, it can be absorbed, reflected or transmitted through the body. In order to conserve energy, the following must then be true
Where:
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For most materials, transmissivity is 0, therefore it is a common assumption that the following is true:
A Black Body is a body with absorbance of 1 and therefore reflectivity and transmissivity of 0.
EmissivityThe emissive power of a body is defined as being the absorbance of the material over the equivalent black body materials emissive power:
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Where: 
By rearranging the above equation, we can see that Absorbance is in effect the emissivity of the grey body, or ε.
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Grey Body Radiation
A grey body is a body where some of the energy is reflected, and some of the energy is absorbed. As reflectivity and absorbance are linked they will always add up to or close to 1 and therefore can be thought of as a ratio.
Emissivity
Emissivity, Absorbance, reflectance and radiation in general, are all dependant on surface finish, temperature and wavelength. The following table shows some example materials and their emissivity:
Surface Material/Finish |
Emissivity |
White Paint |
0.95 |
Black Glossy Paint |
0.95 |
Brick |
0.97 |
Concrete |
0.85 |
Rough Machined Steel |
0.87 |
Well Finished Steel |
0.39 |
Polished Steel |
0.07 |
Gold |
0.05-0.2 |
Silver |
0.01-0.1 |
Aluminium Foil |
0.07 |
Carbon |
0.96 |
Cast Iron |
0.3 |
Copper (Polished) |
0.02 |
Copper (Oxidised) |
0.6 |
Wood |
0.95 |
Asbestos |
0.95 |
Asphalt |
0.95 |
*Unless otherwise stated, Emissivity vaules given for room temperature (293K)