Intercomparison Of Normal Emissivity Measurements Of Coated Flat Glass. Report Of The International Commission On Glass, Technical Committee 10 "Optical Properties Of Glass"

This paper presents the results of an industrial intercomparison of emissivity measurements on coated flat glass performed by TC 10 of the ICG. Emissivity is important to quantify heat transfer by radiation. Standards EN 673 & ISO 10292 explain how it should be measured & incorporated in the evaluation of the thermal transmittance (U value) of glazing. The first part of the paper provides a wide background about the definition of emissivity & related quantities defined by the standards:IR spectral & total reflectance, normal & corrected emissivity at 10 deg C & radiation conductance across the gas space of multiple glazing. The second part describes the intercomparison carried out on five sets of samples whose emmisivity ranged between 0.015 & 0.4. Measurements were performed with 20 spectrophotometers, 7 dispersive & 13 interferometer-based FTIR spectrometers. Each participant used his own measurement procedure, reference mirror and reflectance accessory. The results are reported both as spectral curves and as calculated normal emissivity values. The single standard deviation of the normal emissivity results ranged between 0.003 and 0.11 and the worst-case discrepancies (max-min value) ranged between 0.010 and 0.025. Both values increased with sample emissivity. It was not possible to separate the various contributions of the instrument, accessory, reference material and measurement procedure to the overall measurement uncertainly, due to the empirical nature of the intercomparison. However, the results provide a useful estimate of the interlaboratory agreement presented reached. This value is particularly interesting for the standardization committees, being connected with the accuracy with which the standards can evaluate U values. An attempt to separate the various contributions and to improve accuracy will be the object of the European project THERMES.