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## Emissivity of Blackbody Cavities

Technical Guide 35

Blackbodies are commonly used as a transfer medium for the calibration of radiation thermometers. While the emissivity of such cavities is generally close to 1, for high-accuracy calibrations it is important that the actual emissivity is known. This emissivity will depend on the geometry of the cavity, the emissivity of the material it is made from, and the field of view of the radiation thermometer viewing the cavity. Additionally, the emissivity will vary with any temperature non-uniformities that exist along the cavity length. Methods to calculate the cavity emissivity are highly mathematical and require sophisticated computer programs to implement them. This guide provides a brief overview on how to calculate cavity emissivity and is accompanied by a software application (Microsoft Windows compatible) that implements the method and illustrates the concepts graphically.

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