Glass in building – Determination of the emissivity
|Publication Date:||18 March 2019|
|ICS Code (Glass in building):||81.040.20|
This document specifies a procedure for determining the emissivity at room temperature of the surfaces of glass and coated glass. The emissivity is necessary for taking into account heat transfer by radiation from surfaces at the standard temperature of 283 K in the determination of the U value and of the total solar transmittance of glazing according to  to . The procedure, being based on spectrophotometric regular reflectance measurements at near normal incidence on materials that are non-transparent in the infrared region, is not applicable to glazing components with at least one of the following characteristics: a) with rough or structured surfaces where the incident radiation is diffusely reflected; b) with curved surfaces where the incident radiation is regularly reflected at angles unsuitable to reach the detector while using regular reflectance accessories; c) infrared transparent. However, it can be applied with caution to any glazing component provided its surfaces are flat and non-diffusing (see 3.6) and it is non-transparent in the infrared region (see 3.7). Although transmittance measurements are included in this document, they are only necessary to check if the sample is non-transparent in the infrared region in the context of this document (see 3.7). If the sample is transparent in the infrared region, this document is not applicable. The previous version of this document was based on the use of reflectance measurements using double beam dispersive infrared spectrophotometers capable of measuring over almost the entire spectral range of a black body at the standard reference temperature and determining the emissivity by the 30 ordinate method . This version takes account of Fourier Transform Infrared (FTIR) spectrophotometers where the spectral range is limited. It describes a method whereby spectrophotometers can be used to determine emissivity if they are able to measure up to the 24th ordinate point and if they satisfy a noise criterion for this spectral range. It allows the inclusion of data from the 25th ordinate point up to the 30th ordinate point. A new informative annex (Annex D) describing the principles of absolute reflection accessories has been added to this version. These accessories are intended to be used by qualified personnel. As FTIR spectrophotometers are single beam instruments as opposed to dispersive spectrophotometers which are double beam instruments (and thus able to correct for instrument drift), a procedure was developed by the European funded project, THERMES, to correct for drift. This procedure is described in  and . Other categories of ordinate errors using FTIR spectrophotometers are discussed in .