Limits and artefacts of reflective imaging goniophotometers for complex solar façade systems

The design of systems for solar light collection, modulation and/or distribution requires a thorough knowledge of their optical properties. The angular distribution of the scattered incident light flux, described by the Bidirectional Scattering Distribution Function (BSDF), can be measured with a step-by-step scanning goniophotometer but requires considerable time, especially when aiming at high angular resolution over a wide range and numerous incidence angles like in typical solar applications. Considerably faster measurements can be achieved with a so-called imaging goniophotometer, which simultaneously measures light fluxes in all scattered directions by dispatching them over different portions of a two-dimensional sensor array. In this contribution, we revisit the widely accepted principle of a reflective imaging goniophotometer (RIG), which is based on a hemispherical (or ellipsoidal) mirror and a fisheye camera. Specifically developed ray-tracing tools allowed us to obtain accurate figures relative to the influence of key design parameters on angular resolution. Our calculations reveal that the measurement accuracy is too low for samples larger than a few tens of millimeters. Most importantly, we found significant limitations and artefacts in the angular-to-spatial mapping function inherent to the RIG principle, which generally severely bias BSDF measurements.