BSDF Generation Procedures for Daylighting Systems

This white paper summarizes the current state of the art in the field of measurement and simulation characterization of daylighting systems by bidirectional scattering distribution functions (BSDFs) and provides recommendations broken down by classes of systems and use cases. It is the result of collaborative work conducted by members of the IEA SHC Task 61 / EBC Annex 77, Subtask C2. The document describes proposed procedures for: − measuring angle-dependent transmittance and reflectance properties of daylighting and shading systems, and − generating tabular BSDF data sets from these measurement data for use as input to simulation tools. The type of systems considered in this context include transparent systems, homogeneous or small pattern diffusing systems, diffuse or specular blinds and grids, macroscopic prismatic systems, and micro- or nano-structured systems. A complex fenestration system generally consists of a base material (“curtain”) to modify thermal and visual transmittance of the building envelope, and auxiliary elements such as mechanical fixing, lateral guides, and control systems. The procedures to determine the scattering properties described in this document apply to a representative central area of the façade system without edge effects. After an introduction, a description of the scope of the white paper, relevant definitions and a description of commonly used BSDF data resolutions, we describe empirically-based procedures for generating BSDF data sets for façade systems for later use in lighting simulation software. The overall procedure differs for microscopic and macroscopic systems and is therefore divided into these two areas. As different systems require different BSDF data resolutions for different applications, we provide tabulated recommendations for adequate characterization methods and BSDF resolutions for different classes of systems. The classes are clustered according to the optical properties of the systems and according to the resulting requirements for data resolutions. The document closes with a discussion section on still open issues in BSDF characterization of daylighting systems and their application in lighting simulation tools.