Bidirectional Scattering Distribution Function (BSDF) Resolutions for Solar Shading Devices

This paper investigates the influence of bidirectional scattering distribution function (BSDF) resolution on daylight simulations. Seven representative shading systems were selected and evaluated using a single-office model defined according to established recommendations. The tool genBSDF was used to generate BSDFs at different resolutions. Both annual climate-based and point-in-time daylight simulations were performed. The results show that BSDF resolution has only a minor impact on annual daylight metrics when compared with TT6, used here as the highest-resolution numerical reference, with maximum NRMSE values relative to TT6 of 0.04 for DA300, 0.01 for cDA300, and 0.02 for UDI100–3000 across the investigated systems. Lower-resolution BSDFs (e.g., TT3, TT4, or Klems) are therefore adequate for annual illuminance-based assessments under the conditions studied. In contrast, point-in-time simulations were substantially more sensitive to resolution when direct sunlight penetrated the shading system; for example, NRMSE for one investigated system decreased from 0.63 with TT3 and 0.25 with TT4 to 0.11 with TT5. Lower resolutions also showed greater deviations in the spatial distribution of illuminance, particularly at the transition between directly sunlit and shaded regions. TT5 generally showed close agreement with TT6 while requiring substantially less BSDF-generation time. The findings therefore support selecting BSDF resolution according to simulation purpose: lower resolutions can be used efficiently for annual daylight metrics, whereas higher resolutions, such as TT5 or TT6, are recommended for detailed point-in-time assessments involving strong directional or direct-solar transmission. These findings therefore describe the sensitivity of daylight simulation outcomes to BSDF resolution and their convergence towards TT6, used as the highest-resolution numerical reference, rather than the absolute accuracy of the simulations.