As it is not possible in the present state of the art to determine the total emissivity in a practical way in routine laboratories, the problem of importance for industry is to know the correlation between hemispherical emissivity and normal emissivity. Industry actually determines normal emissivity by integrating spectral reflectivity measurements (in the infrared) by means of a spectrophotometer.
The project will start with a rather basic step in which the few laboratories which are able to make measurements a a wide angle from normal will compare results. In particular PTB does total emissivity measurement. NPL does absolute infrared spectral reflectivity measurement and should provide an interesting verification as both approaches should ultimately provide the same result.
Three types of coated flat glass, with normal emissivities of 0.14 0.32 and 0.62 were analyzed by measuring:
a) thermal radiation using broadband detectors (PTB, TNO and OU), b) spectral reflectance between 5 and 55 um (NPL, SG).
In spite of the different measuring procedures the agreement between single angular values for all three coatings was within .015 (NPL and PTB) and within 0.041 (all participants).
The agreement between the three laboratories who measured up to 80 degrees (NPL, TNO, OU) was within .028. After correcting for the initial differences between samples, the mean modulus of difference (averaged over all angles for each type of coating) was without .007 (NPL and PTB) and within .014 (all participants).
The significant extension of the angular range achieved with respect to previously published studies (from 70 to 85 degrees) allows a more accurate characterization of industrial architectural coatings, and can be used by the glass industry to improve the standards on thermal insulation.
Funding SchemeCSC - Cost-sharing contracts
TW11 0LW Teddington