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The heat emitted by radiation depends essentially on the emissivity of the surface of the material; polished metals have a very low emissivity and therefore emit little radiation, black surfaces have a high emissivity. Emissivity is thus fundamental in all temperature measurements by optical means since the amount of radiation recorded by a pyrometer can only be converted into temperature through the Planck law if one knows the emissivity of the surface. This is a major problem that makes it almost impossible to measure accurately temperature by optical means if the material surface is non-uniform.

The project involves the comparison of spectral emissivity measurements at 500 C on a black-body with those taken normal to the surfaces of low and medium emissivity materials (platinum, stainless steel and silicon carbide or graphite) at 3 wavelengths (2.26 3.79 and 5.06 um). Instead of using a black-body reference, one laboratory (IMGC) used an integrating sphere reflectometer and an infrared thermometer.


In a first phase that did not involve IMGC, the agreement between the laboratories was not good, and the normal spectral emissivity values obtained by the different laboratories varied by more than 0.2 in some cases. The agreement was improved when the results from 2 laboratories were corrected as a result of re-calibrations of their commercial black-body cavities against an NPL standard.

In a second phase, involving only IMGC and NPL, very good agreement was obtained, even though the techniques used by the two laboratories were very different. All the normal spectral emissivity values agreed to within 0.02 and there appeared to be no systematic differences.


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National Physical Laboratory (NPL)
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Queen's Road
TW11 0LW Teddington
Vereinigtes Königreich

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Beteiligte (3)