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What does Emissivity mean?

Answer

The term "Emissivity" is used to describe the efficiency of a surface for radiating Electromagnetic energy at a given wavelength as compared to a perfect, theoretical "Blackbody” radiator.  It is a value expressed as a percentage from 0% to 100% where 0% would be no emission of infrared energy and 100% would imply the perfect infrared radiator. In the real world, there is no perfect radiator so emissivity value of an object will always be a fraction of 100%, such as .98, or .45. 

The flip side of Emissivity is called "Reflectivity". Emissivity and reflectivity are two sides of the same coin; you cannot have one without the other. All surfaces have both an emissive component and a reflective component, at the same time. For example, a surface with an emissivity value .82 also has a reflectivity value of .18 for a total of 100%.

The higher the reflectivity value is the more "shiny" the object appears in visible light and infrared. For an infrared imager viewing a surface, which has an “E” value of .5 it, is also .5 reflective. This means that 50% of the energy seen by the infrared imager may be coming from another source being reflected off of the target surface. The reflected energy may be from source colder than the target’s surface or hotter than the target’s surface. Unfortunately, the infrared imager cannot tell the difference between reflected and emitted energy, that determination is left to the trained thermographer to sort out.

Wood, concrete, dirt, water, brick, human skin, most organic materials all have relativity high emissivity values and are good emitters and absorbers (Kirchhoff) of infrared radiation. These materials can easily be viewed through an infrared imager and measured with accurate results by adjusting the imager's emissivity value to match the "E" value of the objects surface.

Low emissivity surfaces such as polished steel, shiny copper, aluminum foil, and in general metals with various degrees of light oxidation do not emit infrared well. With these materials the infrared imager will “see” relatively little of the objects heat and much more of what the shiny surface is reflecting from the background of the room it is in. Low emissivity surfaces are more prone to measurement and observational errors because they do not radiate infrared well.

Surface emissivity and reflectivity are a function of the micro texture of a surface relative to the incident wavelength. When the texture becomes so fine that the wavelengths can no longer fit into the texture, it is reflected away from the surface. In addition, because of the mismatch between wavelength size and surface texture size the surface cannot emit infrared energy.  Simply put, reflective surfaces are bad antennas for broadcasting and receiving electromagnetic energy at IR frequencies.

When the EM wavelengths are able to fit into the surface micro texture, not only can the infrared energy be absorbed, but it can also be broadcasted or emitted.  Surfaces with high emissivity values are very good at broadcasting and receiving electromagnetic energy at IR frequencies.

 

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