A Correlation Between Normal and Hemispherical Emissivity of Low-Emittance Coatings on Glass

Transparent low-emissivity coatings are commonly used to reduce the thermal conductance of windows. It is important to be able to characterize and compare the wide variety of these coatings that are now available. The property that best describes the effectiveness of a coating in suppressing radiative heat transfer is the total hemispherical emissivity. It is much simpler, however, to measure the normal emissivity. This paper shows that a correspondence exists between these two properties which applies to most types of low-emissivity composite coatings. An empirical expression for this correspondence is provided.