Comparison of measured and computed BSDF of a daylight redirecting component

Abstract

ABSTRACTTheBidirectionalScatterDistributionFunction(BSDF)ofaselectedDaylightRedirectingComponent (DRC) is computed by a virtual goniophotometer using the enhanced photonmap extension in Radiance, and compared to measured BSDF data.The DRC comprises a stack of tilted aluminum louvers with configurable inclination angle.The profile of the louvers is designed to control transmission depending on sun altitude,and to redirect light up towards the ceiling.The measured BSDF of the DRC is obtained from a scanning goniophotometer. For asparse set of three source directions, the distribution is recorded at ≃ 250,000 receiver di-rections. The asymmetric angular resolution allows detailed observation of characteristicfeatures in the distribution, which are assumed to persist over a range of source direc-tions. For each pair of source and receiver directions in the measurement, the computedBSDF is generated from a model of the DRC, replicating the measurement with a vir-tual goniophotometer. The simulation relies only on the enhanced photon map extensionfor Radiance. The BSDF from measurement and simulation are compared qualitativelyand quantitatively to discuss the degree of accordance. The presence of characteristic fea-tures and their topology is evaluated by comparing polar surface plots of the distributionsand profiles of the scatter plane. The direct-hemispherical transmission is compared foreach measurement and simulation. The RMSE of each computed distribution against thecorresponding measurements is calculated to quantify the directionally resolved deviation.A high degree of qualitative accordance between the computed and the measured BSDFis achieved. Prominent features in the BSDF are represented by the model. A deviationof −6% to +15% is observed in a quantitative comparison of direct-hemispherical trans-mission by integration of computed and measured BSDF. The RMSE indicates higherdeviations for lower source altitudes, where a direct transmission peak in the distributionis underestimated by the model. The method is proposed as a means to validate thecapability of the enhanced photon map to predict transmission through DRC.Keywords: daylight simulation, modeling, validation, daylight redirection, BSDFINTRODUCTIONDaylight Redirecting Components (DRC) reduce solar gains and energy demands forelectrical lighting in buildings, and improve visual comfort for occupants. Planners aimat finding the minimum required transmission providing sufficient and evenly distributedilluminance during occupancy hours. Daylight simulation supports this optimization byproviding illuminance and luminance data for assessments based on metrics such as SpatialDaylight Autonomy and Daylight Glare Probability.