What is Emissivity?

All objects emit infrared radiation, but some emit more than others.  Emissivity is a measure of a material’s ability to radiate energy. The ability of a material to radiate infrared energy depends upon several factors: the type of material, the surface condition, the wavelength and the temperature all have an effect on the object’s emissivity to varying degrees.

Every object emits electromagnetic radiation.  The spectrum of the radiation (the relative amounts of energy at different wavelengths) depends on the temperature of the object.  A ‘blackbody’ object emits the full amount of energy at each wavelength in line with infrared theory established by Max Planck in 1900, but real surfaces emit less of their internal energy.

The emissivity of a material, usually denoted as ε, is the ratio of the energy radiated by the material to the maximum that could be radiated by a blackbody at the same temperature. A true blackbody would have an ε = 1 while any real object would have ε < 1. Emissivity is a numerical value and does not have units.
 Radiation Equation
40% Radiated Infrared Energy
Emissivity and Reflectivity

For real materials, less electromagnetic radiation is emitted than would be expected from a blackbody at the same temperature.  A proportion of the internal radiation is retained within the object.  When the internal radiation reaches the surface, a fraction R is reflected and the remaining fraction (1-R) is emitted.  The figure shows the effect for a surface with emissivity 0.4.

Emissivity is related to reflectivity by Kirchhoff’s Law:

  ε = 1 – R

If 'R' is the reflectivity of the surface. (1-R) then the emissivity of the surface = ε.  Thus, surfaces with high emissivity have low reflectivity; they are dark, matt surfaces.  Conversely surfaces with high reflectivity have low emissivity.  Bright shiny mirror-like surfaces emit very little radiation.  
 
Emissivity of real materials
Material Emissivity at 1 µm 
Unoxidised Steel 0.35 
Oxidised Steel 0.85 
Unoxidised Aluminium 0.13 
Oxidised Aluminium  0.40 
Unoxidised Copper 0.06 
Oxidised Copper 0.80
Brick 0.80 
Asphalt  0.85 
Asbestos  0.90

Emissivity of Real Materials

The table shows some typical, real-world values for emissivity for various materials. Observation of the table shows the non-metals such as brick tend to have high values of emissivity. Metals with un-oxidised surfaces tend to have quite low emissivities.

It is always worth remembering, that for an opaque object Emissivity + Reflectivity = 1.0. This means that a target surface which is quite non-reflective (such as asphalt) would have a high emissivity, and a highly reflective material (such as rolled aluminium) would have a low value of emissivity.

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