Light distribution through advanced fenestration systems

Most energy-saving applications of advanced fenestration systems, e.g. solar blinds, novel types of glazing and daylight redirecting devices, require a precise knowledge of their directional light-transmission features. These photometric properties are described by a Bi-directional Transmission Distribution Function (BTDF), which is experimentally assessed by a bi-directional photogoniometer. As such a function represents a heavy amount of data, there is a need for a synthetic and intuitive visualization of a system's transmission behaviour. For this purpose, four kinds of graphical representations have been created and are presented in this paper. These are based on bi-directional data assessed by a novel digital imaging-based photogoniometer, whose measurement principle allows a continuous knowledge of the whole transmission space, and therefore an appreciable liberty in data processing. The geometric properties of the different representations are described, together with the corresponding image operations. The information extraction from these graphical visualizations is given through a comparison example between a conventional venetian blind and an optimized prototype.