Experimental and simulation-based approaches to assess bidirectional properties of complex fenestration systems

A serious effort has been made in developing accurate and efficient bidirectional goniophotometric devices for detailed studies of Complex Fenestration Systems, capable of measuring BTDFs and/or BRDFs in an appropriate way. The existing instruments are almost all based on a scanning process, i.e. on relative individual movements of the detector and of the sample and/or the source to monitor all incoming and outgoing light flux directions for which BT(R)DF data are needed, except for two that rely on a flux-based investigation combining the use of digital imaging for detection with the collection of the light emerging from the sample on a projection surface (diffusing screen or mirrored ellipsoid). The second part of the paper focuses on virtual goniophotometers that have been developed, mainly based on commercial forward ray-tracing simulation tools and allowing one to complement experimental assessment in a very efficient way.