Goniophotometry and assessment of bidirectional photometric properties of complex fenestration systems

Abstract

This paper seeks to provide an overview of the currently available assessment tools for Bidirectional Transmission or Reflection Distribution Functions (BTDFs, BRDFs) of complex fenestration systems (CFS). In the first part of the paper, the existing experimental devices (goniophotometers) developed specifically for CFS measurement are described. All but two are based on a scanning process to investigate the emerging light flux distribution, the alternative approach being based on digital imaging techniques. A critical analysis of their advantages and shortcomings is proposed to provide both researchers interested in replicating them and more generally potential users of BTDF or BRDF data with a lucid idea of the available options.The second part presents an alternative to physical measurements, made possible by using computer simulations based on ray-tracing techniques. In this case, every component of the modelled system must be of well-known geometric and material properties. Three virtual goniophotometer models are described and their validation results are analyzed.Whether they have been generated experimentally or through simulation, BTDF and BRDF data need to be processed into transmission or reflection functions that are directly related to sky and room conditions for them to be usable. The third part of the paper introduces a new BT(R)DF database management system whose aim is to become a reference resource for viewing and interpreting these complex data. The main visualization features of its interface are presented, that include a special focus on flexibility and on providing intuitive graphical information, in a similar way as for luminaires selection in electric lighting design.

Experimental validation of simulation methods for bi-directional transmission properties at the daylighting performance level

“…The BTDF data of the tested samples were obtained by means of three different methods: one experimental approach based on digital imaging techniques, and two numerical methods based on ray-tracing techniques. A short description of these methods is given in this section while detailed information can be found in [10].…”

“…The FHG-IBP Numerical Goniophotometer represents an automated environment allowing a virtual test set up to be configured, CFS samples to be parameterized and combined, and includes a post-processing of data for further use in daylight simulation [10,23]. The environment is based on the commercial forward ray-tracing tool OptiCad™ and uses a fluxbased method [24].…”

“…Predicting the performance of CFS in buildings remains however a critical issue because the amount of publicly available BTDF data is still relatively small [10] and reference experimental BTDF data are not yet available for lack of standards in validation protocols. The reliability of existing BTDF data is therefore strongly dependent on the assessment method and errors have been found to vary between a few percent and 50% or more [10]. In addition, BTDF data must be combined with an accurate description of the sky luminance distribution for indoor illuminance maps to be deduced [11,12,13,14,15,16].…”

“…Many efforts are being investigated internationally to propose new experimental methods for assessing the Bi-directional Transmission Distribution Function (BTDF) of CFS as well as alternative simulation approaches based on ray-tracing methods, reviewed in [10]. Predicting the performance of CFS in buildings remains however a critical issue because the amount of publicly available BTDF data is still relatively small [10] and reference experimental BTDF data are not yet available for lack of standards in validation protocols. The reliability of existing BTDF data is therefore strongly dependent on the assessment method and errors have been found to vary between a few percent and 50% or more [10].…”

“…This is done by assessing the Bidirectional Transmission (or Reflection) Distribution Function (BTDF, BRDF) of these CFSs, which is defined by the Commission Internationale de l'Eclairage as the "quotient of the luminance of the medium by the illuminance on the medium" [9]. Many efforts are being investigated internationally to propose new experimental methods for assessing the Bi-directional Transmission Distribution Function (BTDF) of CFS as well as alternative simulation approaches based on ray-tracing methods, reviewed in [10]. Predicting the performance of CFS in buildings remains however a critical issue because the amount of publicly available BTDF data is still relatively small [10] and reference experimental BTDF data are not yet available for lack of standards in validation protocols.…”