Dependence of emissivity on angle for coated and uncoated glass and the calculation of radiant energy exchange in double-glazed units

Abstract

The calculation of heat transfer by radiation between the inner surfaces of a double-glazed unit requires information on the variation of emissivity with angle for each surface. It is difficult and very time-consuming to obtain the information experimentally. Current practice is to obtain the normal emissivity indirectly by a reflectivity measurement and to multiply this by a factor to obtain the hemispherical emissivity. The factor increases with decreasing normal emissivity. Although there is some theoretical justification for the factors used, they need to be examined in some detail, especially when coated surfaces are involved. Using equations for the optics of multilayer thin films and published values for the optical constants of the layer materials, the angular dependence of emissivity has been calculated for glass coated in various ways, including some which simulate commercial coatings. It is shown that the currently used factors are likely to be satisfactory for the current commercial coatings. However, for sputtered coatings containing a silver layer, much higher factors are predicted if the silver layer were to have the same optical constants as carefully prepared, vacuum-evaporated silver. As new coatings and different methods of manufacture are developed, the current factors may need to be revised.