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Prediction Model of Optical Characteristics for Barrel Vault Skylights

The topic of this paper is part of a project to develop software to analyze the optical characteristics and daylighting performance of conventional and tubular skylights. Skylights are found in many building types, such as commercial and institutional buildings, houses, shopping malls, hotels, etc. However, skylight manufacturers lack design tools to predict the optical performance of skylight products. Prediction of skylight optical characteristics is a difficult task, due to the complexity of skylight shapes that change with design requirements. This paper presents an analytical model to compute the overall optical characteristics of barrel vault skylights under direct beam light. The model is based on a ray-tracing technique, and can handle skylights with different glazing types (gables opaque, or glazed with different glazing types from the top surface of the skylight), different shapes (short/long with high/low-rise profiles) and different orientation (north-south, west-east, or any direction). Applications of the model showed that barrel vault skylights transmit much more light at high incidence angle on a horizontal surface than similar flat skylights. The skylight length-to-radius ratio (L/R) has a significant impact on the optical characteristics. Short skylights (L/R = 2) with uniform glazing transmit approximately the same amount of light at any sun position. However, long skylights (L/R = 5) transmit about 64 percent more light when the sun is perpendicular to the skylight axis than when the sun is parallel to the skylight axis. Short skylights with clear gables and tinted top transmit substantially more light than flat skylights with similar tinted glazing, particularly when the sun is parallel to the skylight axis.