An Emissivity Primer
Emissivity is the ratio of radiation emitted by a blackbody or a surface and the theoretical radiation predicted by Planck’s law. Blackbody emissivity is frequently referred to as a single number. To be more scientifically correct one needs to be more explicit.
A material's surface emissivity is a measure of the
energy emitted when a surface is directly viewed. Surface emissivity is generally measured
indirectly by assuming that e = 1 - reflectivity.
A single energy bounce is measured and the reflected energy measured. 
A mirrored surface may reflect 98% of the energy, while absorbing 2% of the energy. A good blackbody surface will reverse the ratio, absorbing 98% of the energy and reflecting only 2%.
Effective emissivity is the ratio of the total amount of energy exiting a blackbody to that which is predicted by Planck’s law. This is the value most frequently referred to as "emissivity". Effective emissivity of a cavity type blackbody will normally be much higher than the surface emissivity due to the multiple energy bounces inside the body cavity.
Additional refinements to the term "emissivity" may be made by defining it in terms of the wavelength of interest, changes due to temperature affects, etc. The simple concept of emissivity can very quickly become a very complex topic!
Variables that Affect Emissivity
Effective emissivity is affected by several variables, the most important of which are the geometric shape of the blackbody, the uniformity of the blackbody temperature, the surface emissivity and wavelength dependence.
Just as important to the measured result can be the field of view of the device under test relative to the size of the blackbody.
Blackbody Geometry
Theoretical discussions of blackbodies generally use large spherical bodies with a small viewing opening. A sphere is conceptually easy to grasp and mathematically a special case which is simpler to analyze.
The geometric shape of a blackbody is normally a tradeoff between performance, total size and production costs. The practical design of a spherical shape is complicated by the need to place the viewing port off axis (so that it views the interior surface at an angle) and difficulty in obtaining uniform heating of the sphere. The effective emissivity of a sphere is dependant on the temperature uniformity, the interior surface emissivity and the ratio of the viewing port size to the sphere size.
Configurations such as cylindrical cavities, conical cavities and double cones provide practical solutions to these problems. These configurations promote multiple reflections of the energy similar to the spherical cavity. The effective emissivity of these configurations are well understood and approach the theoretical maximum when properly designed and fabricated.
Flat plates are the least desirable from a emissivity consideration, but are generally the only practical solution when large, uniform areas are required. Surface emissivity becomes the dominate factor when a flat plate design is used.
Field of View Considerations
The viewed area of the blackbody also affects the apparent emissivity. In general, it is preferred that the detector look at only the central portion of the cavity opening, preferably no more than 1/2 the viewing port or cavity opening diameter . (This is true regardless of the geometric shape of the body). This ensures that the detector is looking at a uniform energy field, with the highest possible emissivity.
A detector with a high hemispherical field of view may look directly at the side walls of the source (which is analogous to looking at a flat plate source) or perhaps even the background area surrounding the source. This may have a significant impact on the effective emissivity.
At low temperatures, special care must be taken to ensure energy from the surrounding area is not reflected into the detector.
Surface Emissivity Considerations
A number of materials and coatings may be used in
blackbody designs depending on the operating temperature of the body.
The emissivity of the cavity surface can vary significantly from material to material and will generally have variation over the spectral band.
The variation over 3 to 5 µm or 8 to 12 µm is frequently small enough to ignore. However, the variation can be significant to some testing conditions. Special attention may need to be taken when working in both bands or over the entire spectrum. If the emitting surface utilizes a high emissivity coating, it will probably need periodic re-coating to maintain its original properties.
EOI High Emissivity Coatings
EOI uses specially formulated blackbody coatings that provide
very high surface emissivity in the temperature bands of
interest.
Re-Coating Emissive Surfaces
To ensure maximum performance, EOI
includes a re-coating of the emissive surface as a part of each blackbody calibration.
Still Want to Learn More?
Read the EOI paper on calculation
of effective emissivity.
