It is impossible to calibrate radiometers, radiation thermometers, optical radiation detectors etc. without knowledge of radiation characteristics (primarily, effective emissivity) of blackbody calibration sources. Frequently, uncertainty in determination of effective emissivity is the predominant component in the uncertainty budget. Obviously, the best way to determine effective emissivity is measurements.

According to its definition, direct measurement of effective emissivity should be performed by comparison of radiation emitted by the blackbody calibration source and radiation emitted by a perfect blackbody having the same temperature. However, the perfect blackbody is the physical abstraction that doesn't exist in the real world. There are two methods for resolving this problem:

1) To perform absolute measurements of artificial blackbody radiation characteristics (e. g., spectral radiance in W·m-3·sr-1), assign a certain temperature to this radiator (real-world blackbody sources are always nonisothermal), then divide the measured quantity by homonymous quantity calculated for the perfect blackbody at the same temperature (e. g., using Planck's law for the case of spectral radiance).
2) To perform relative measurements by comparison of radiation emitted by the artificial blackbody and the better-quality artificial blackbody (such as fixed-point blackbodies).

Realization of both methods is extremely complicated. It requires special instrumentation and highest qualification of experimenter. Now, such measurements can be conducted only in several world's leading metrological centers.

Another approach implies indirect measurement when the cavity reflectance is measured instead of its emissivity. Indirect measurements can be implemented easier, for instance, using integrating sphere with the laser radiation source. However, this approach has serious disadvantages:

1) Usually, cavity reflectance is measured for the cavity temperature that significantly differs from the temperature at which the blackbody operates.
2) Often, it is difficult to provide geometry of cavity irradiation and collecting reflected radiation which correspond to geometrical conditions of collecting radiation emitted by the cavity when it is used for calibration. This violates the reciprocity principle and raises a query about correctness of effective emissivity determination.
3) Sometimes, such measurements are impossible at all, for example, due to the difference in dimensions of the blackbody and reflectometric device.

At the design stage, calculation is the only way to evaluate the quality of the blackbody, to determine its optimal parameters, and to choose suitable material. But if you purchased a blackbody calibration source and are going to use the value of effective emissivity specified by the manufacturer, you should be doubly careful because:

1) Values of effective emissivity might be correct only for a part of operational spectral and temperature ranges.
2) You may not know how manufacturer determined the effective emissivity, which measured equipment was used and what are the  measurement uncertainties.
3) Finally, none of blackbody manufacturers never purchased our blackbody emissivity modeling software. Other commercial software for this purpose simply does not exist.
Measurements performed by independent researchers are not always confirm manufacturer's values. The right-hand picture shows how manufacturer's emissivity of a flat-plate blackbody agrees with that measured at NIST (adopted from: S. N. Mekhontsev, V. B. Khromchenko, L. M. Hanssen, "NIST Radiance Temperature and Infrared Spectral Radiance Scales at Near-Ambient Temperatures," Int. J. Thermophys. 29, 1026-1040 (2008) - Fig. 14. Effective spectral directional emissivity of a high-temperature flat-plate BB. Manufacturer specification is 0.95 (+0.00, -0.05)).

If such disagreement takes place for flat-plate blackbody (metallic plate coated with a black paint) for which emissivity measurement is not too difficult, one can imagine the situation with the cavity radiators.

The list of some publications concerning the use of our blackbody emissivity modeling programs which can be found here shows that
serious researchers compute effective emissivities using Virial's software. We invite our visitors to explore our STEEP320, STEEP321, and STEEP323 programs for axisymmetric cavities, INCA333 program for a cylindrical cavity with an inclined bottom, and PyramidA for pyramid array radiators in order to choose  the most suitable software for solution of your tasks.
Why Should Compute Effective Emissivities