Validation of the performance of a new bidirectional video-goniophotometer
An accurate knowledge of the directional optical properties of advanced fenestration materials is necessary for them to be adequately integrated in buildings. These properties are expressed by the Bidirectional Transmission (or Reflection) Distribution Functions (BTDF, BRDF) of such elements, which are measured by specifically designed measuring equipment: an innovative, time-efficient bidirectional goniophotometer, based on digital imaging techniques, was designed and set up for that purpose. In this paper, the in-depth validation used for the bidirectional measurements performed with this apparatus is presented. It is based on different approaches including experimental error estimation, comparisons to analytical or ray-tracing based models and to other measured data, and calculation of the directional-hemispherical transmittance (reflectance) gauged against measurements of the same systems with Ulbricht (integrating) spheres. The high accuracy and reliability of this novel device were confirmed by this detailed investigation, and led to a maximum error for BT(R)DF data of only 10%.