Paper Conference

Proceedings of SimBuild Conference 2010: 4th conference of IBPSA-USA


Dynamic radiance: predicting annual daylighting with variable fenestration optics using BSDFs

Mudit Saxena, Greg Ward, Timothy Perry, Lisa Heschong, Randall Higa

Abstract: Existing annual daylight simulation software fall short with respect to variable fenestration optics that change interior daylight distribution with sun position and/or operating schedule, thus limiting the ability to compare the performance of advanced fenestration systems. Many of these window or skylight systems can be described efficiently as a bidirectional scattering distribution function (BSDF), which characterizes their flux output as a function of input for a particular configuration. In this paper, we describe a new method that employs measured or simulated BSDFs to permit fast, matrix-based annual daylighting calculations. The matrices themselves are precomputed by Monte Carlo ray-tracing in a modified daylight coefficient approach we call Dynamic Radiance. The inner time-step loop then consists of multiplying the desired sky luminance vectors against three matrices in the general case, where a separate BSDF matrix permits dynamic fenestration control strategies. In this paper, the authors describe their implementation of the Dynamic Radiance method and demonstrate its application to a set of 61 real spaces modeled for a research project to determine new daylight metrics. We present results from these simulations and discuss advantages and limitations of the new approach.
Pages: 402 - 409