Abstract: Directional emittance plays an important role in the calculation of radiative heat exchange. It partly determines the thermal insulation of single and multiple glazing and the efficiency of solar collectors. An emissiometer has been designed and built, capable for measurements of the directional emittance at angles up to 85$DGR@. The emissiometer can be used for absolute measurements, with a black body radiator as reference, and for relative measurements using a known reference sample. In the case of low-emissivity materials the directional emittance usually has a maximum at an angle between 80$DGR and 90$DGR (pseudo Brewster angle). The emissiometer is, therefore, especially useful for the accurate characterization of low-emissivity materials like spectral selective coated glazing and solar absorbers. The paper gives a description of the instrument and results are discussed for three coated glass samples with low, medium, and high emissivity, respectively, which were measured for a temperature of 283 K. Results are given of the calculation of the thermal transmittance (or U-value) and of three types of double glazing in which the low, medium, and high emissivity coating was applied. !18
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