Bilateral Intercomparison of Spectral Directional Emissivity Measurement Between CENAM and PTB

Abstract

Both Centro Nacional de Metrologia (CENAM, Mexico) and Physikalisch-Technische Bundesanstalt (PTB, Germany) are national metrology institutes and provide the dissemination of the spectral directional emissivity as a calibration service. CENAM started this service recently. The emissivity measurement capability of PTB took part in two international comparisons performed in the past among other national institutes. The measurement instrumentation and techniques used for emissivity measurements at CENAM and PTB are both based on Fourier transform infrared spectrometers. Both setups are based on the principle of a spectral comparison of the radiances of the sample and blackbody radiator. In detail, the setups differ: CENAM has the capability of measuring the directional spectral emissivity normal to the sample surface, while PTB measures the directional spectral emissivity at angles of observation ranging from \(5^{\circ }\) to \(80^{\circ }\), and provides the hemispherical spectral and total emissivity of samples as well. For this comparison, it was agreed to compare the value of the directional spectral emissivity normal to the sample surface obtained by CENAM with the one determined at an angle of \(5^{\circ }\) by PTB. Four samples of different spectral directional emissivities were measured by the two institutes. For the samples, four copper disks with a diameter of 50 mm and a thickness of 5 mm were used. Three of them were coated with Comex 1402470 (white), 1402471 (aluminum), and 1402474 (black) paints, respectively, and the other one with Nextel 811-21 paint. Measurements were obtained for each sample at a temperature of about \(150\,^{\circ }\)C, and in the spectral range from \(400\,\mathrm{cm}^{-1}\) to \(2500\,\mathrm{cm}^{-1}\) (\(25\,\upmu \mathrm{m}\) to \(4\,\upmu \mathrm{m}\)). The description of the experimental setups used and the comparison results are presented in this paper. It was found that the results obtained at CENAM and at PTB agree well within the declared standard uncertainties.