Standards for Infrared Optical Properties (Emittance)

standard ISO 10292
revised 1994
title Glass in building -- Calculation of steady-state U values (thermal transmittance) of multiple glazing
committee ISO TC160/SC2/WG2

ISO 10292 is primarily about thermal properties of glazing systems; determination of emittance is an annex. A new TG has been formed under this WG2 to create a standard dedicated to emittance.
  1. Spectral weighting function. At least there is no controversy over the choice of blackbody spectrum as there is for the solar source spectrum. The implementation of the spectral average however is given in a tabular selected-ordinate form that was more useful in the days when calculators were used to perform this operation.

  2. FTIRs and Thermes. The discontinuation of dispersive spectrometers and the forced adoption of FTIRs is nearly complete in the industry. A thorough study of the experimental considerations was done by the Thermes Project and a new standard should be written based on the recommendations of this study. 

  3. Normal to Hemispherical Emittance. ISO 10292 and NFRC 301 use empirical correlations between the measured normal emittance and the desired hemispherical emittance. These correlations are very similar, but not identical, and world standards should be harmonized to one or the other.

standard EN 673
revised 2002
title Glass in building - Determination of thermal transmittance (U value) - Calculation method
committee CEN TC129/WG9

This 2002 version of this standard is virtually identical to ISO 10292. A new version is nearing approval in 2010. The annex on emissivity has been removed and reference is made to EN 12898, a new standard for emissivity only.

standard EN 12898
title Glass in building - Determination of the emissivity
committee CEN TC129/WG9
A stand alone emissivity standard that will, when complete, supersede the portion of EN673 on emissivity. Discussion continues at the WG meeting in October 2010.

standard NFRC 301
committee NFRC Optical Properties Subcommittee

Like all other standards in this area, which stem from the era prior to the common use of FTIR spectrometers, this standard is outdated. A new ISO TG has been formed under

Emissometers

Many manufacturers and test labs use simple emissometers. There are several ASTM standards related to emissometers but their accuracy is not clearly known. It would be nice if this inexpensive option was available but an interlaboratory comparison should be performed to evaluate the reliability of the method first. This is a possible task for the ICG TC 10 which has overlapping membership with the CEN and ISO committees.

NFRC 101 allows the use of ASTM E1933 or ASTM C1371 for all window materials other than glazing. This provision could be extended for diffuse materials used in shading. Such materials cannot be measured by FTIR with the possible exception of an InfraGold integrating sphere, another option which must be tested first. ASTM E1933 requires the specimen to be at a temperature at least 10 K warmer or cooler than the ambient temperature while ASTM C1371 provides a comparative means of quantifying the emittance of opaque, highly thermally conductive materials near room temperature. The long wave range is considered from 2.5 μm to 40 μm. Emissivity for non-metallic materials is defaulted to 0.90 without actual measurements.



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