Btdf Measurements and Ray-tracing Simulations Comparisons for Prismatic Glazing

Evaluation of solar heat gain and daylight distribution through complex window and shading systems requires the determination of the Bi-directional Transmission Distribution Function (BTDF). In this paper, measurements and ray-tracing-based calculations are compared for the specific case of prismatic daylight-redirecting panels. The latter have been chosen as a validation example because they consist of a material of well-known refraction coefficients, thus easily handled by the software applying Snell-Descartes’ law, whereas at the same time they present complex transmission features because of the multiple inside reflections and interactions between the gratings. Measurements were performed in a photogoniometer equipped with a digital-imaging detection system (Andersen et al., 2001), developed at the Swiss Federal Institute of Technology (EPFL), which allows to considerably reduce time-consumption when characterizing a fenestration system (2 to 4 minutes per incident direction) and to get a continuous knowledge of the whole transmission space, averaged into zones, instead of discrete transmission assessments that need to be interpolated. A virtual copy of the photogoniometer was constructed with the commercial forward ray-tracing software TRACEPRO® (v. 2.3 & 2.4, Lambda Research Corporation) based on MonteCarlo calculations. For the first time, an attempt is made to validate detailed bi-directional properties for a complex system. The results generally agree under a range of input and output angles to a degree adequate for evaluation of glazing systems. Calculations were also performed using a more realistic model of the source and ideal model of the detector. Deviations from the photogoniometer model were small and the results were similar in form. Despite the lack of an absolute measurement standard, the good agreement in results promotes confidence in both the photogoniometer and in the calculation method.