This study investigates the potential changes in Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT) ratings of vertical or tilted glazing systems that would result from a deliberate change in the reference spectrum used as Spectral Weighting Function (SWF). This SWF is necessary to evaluate broad band-average optical properties from their spectral values, and obtain the desired rating of such bulk properties. The SWFs currently specified by rating institutions in Europe and North America for SHGC and VT are now outdated, and their inadequacies are discussed. Six potential replacements, which have been recently adopted by ASTM are described, including three direct irradiance spectra and three global irradiance spectrum incident on tilted surfaces of various tilts (20 degrees, 37 degrees and 90 degrees). Some of these spectra have been tailored for use in building energy applications, including Building-integrated Photovoltaics (BIPV). The effect of tilt on the U-factor and hence SHGC of glazing systems used for skylights on roofs is discussed, using a representative dataset of 37 glazing system specimens. The spectral effects on SHGC induced by a change in the current North American SWF are also obtained for this dataset, and show small to moderate deviations from current ratings (-2% to +7% for windows, and -3% to +11% for skylights). The variations in VT are within +/- 2% for most glazing systems. To remove the current inconsistency in the SWFs used for SHGC and VT, it is recommended that a single SWF be used for both properties. For improved accuracy and reliance on active standards, it is also recommended that the SWF for SHGC and VT be either one of the two recent ASTM G197-08 global irradiance spectra, depending on application (incidence on a vertical surface for window applications, and incidence on a 20 degrees-tilted surface for skylight applications). No change in colorimetric calculations (based on the D65 illuminant) is recommended, however. (C) 2009 Elsevier Ltd. All rights reserved.