On the sensitivity of daylight simulations to the resolution of the hemispherical basis used to define bidirectional scattering distribution functions

TitleOn the sensitivity of daylight simulations to the resolution of the hemispherical basis used to define bidirectional scattering distribution functions
Publication TypeReport
Year of Publication2011
AuthorsAndrew McNeil
Pagination19
Date Published09/2011
TypeDOE / CEC PIER Technical Memorandum
Abstract

The Radiance simulation program includes new tools that enable daylight modeling of complex fenestration systems (CFS) using bi-directional transmission distribution functions (BTDF). The tools use the Klems angle basis to define the number of paired incoming and outgoing data values. However, the Klems angle basis was developed for thermal simulations and may be too low of a resolution for some types of daylight systems, particularly those that exhibit peaky, specular transmission. This study evaluates the sensitivity of the angle basis resolution by comparing simulation results for the Klems angle basis against results using two higher resolution angle bases. The first evaluation compares results for specific points in time. The second evaluation compares simulation results using annual performance metrics. Annual lighting energy data were found to agree to within 1%. Annual assessments of discomfort glare were found to disagree by 7% because high resolution basis resolved glare sources into smaller, more intense sources. We concluded that high resolution bases are appropriate for specific types of CFS and performance metrics.

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