Simplified field measurement and verification of global solar transmittance for glazing samples under natural clear-sky conditions

This paper presents the results of an experimental study to measure and verify on-site global solar transmittance as a function of varying angles of incidence for glazing samples under natural clear-sky conditions, including: single-pane clear, double-pane clear, and double-pane low-e glazing. Field measurements were implemented using a silicon photodiode sensor and two thermopile-type pyranometers based on an easily-assembled test box with sample glazing. Measurement results were then compared to the published data (i.e., Tsol values in the WINDOW libraries). The results indicate that silicon photodiode sensors can be used to measure and verify direct solar transmittance within an acceptable range of accuracy. However, the global (i.e., direct and diffuse) solar transmittance measured by the thermopile-type sensors was significantly higher than the Tsol values from the WINDOW program. As a result, it is recommended that such field measurements could be used to verify the on-site direct and global solar transmittance of as-built glazing since the WINDOW program currently only accounts for direct (i.e., beam) solar radiation in the reference solar spectrum (i.e., ASTM E891).