Assessment of Bidirectional Transmission (BTDF) / Reflection (BRDF) Distribution Functions

Student: Courtney Browne (Mechanical Engineering), Advisor: Prof. Marilyne Andersen

Not only need optical properties of window-shade and glazing systems be determined for normal incidence, but also for varying impinging directions, as an angle dependent characterization is essential to accurately predict their performances for variable climatic conditions and sun courses. On the other hand, many applications, and in particular complex or heterogeneous fenestration systems, also require a precise characterization of the spatial distribution of emerging light, in a similar way as for lighting fixtures. Having access to this detailed information will help manufacturers to develop and optimize such products, and provide guidelines to architects in their judicious selection already at the project's level. Such a characterization is also necessary for daylighting simulation programs to improve their performances and achieve a reliable modeling of light propagation in rooms using advanced fenestration systems.

The quantity used to describe these photometric properties, that are angle-dependent at both the incidence and the emerging levels, is called Bidirectional Transmission (or Reflection) Distribution Function, abbreviated BTDF (or BRDF), illustrated in the figure below. This function is defined as the quotient of the luminance of a surface element in a given direction by the illuminance incident on the material, and hence expresses the emerging light distribution for a given incident direction and is experimentally assessed with a bidirectional goniophotometer.

The "HelioDome" project aims at developing an original, leading-edge and time-efficient measurement device for bidirectional distribution functions: the measurements will be performed by collecting the light flux emitted by the analyzed sample on a semi-transparent hemi-ellipsoid (to allow the incident flux to reach the sample in the first place, the sample being positioned at one of the focal points) that redirects it towards the second focal point, where a CCD camera is placed for detection, equipped with a fish-eye lens (see figure).

The assessed bi-directional distribution functions will be those of innovating materials or coatings used for solar shading or daylight redirecting systems, or of energy-efficient artificial lighting components like luminaires reflectors e.g. The instrument is expected to be able to provide both the bidirectional transmission (BTDF) and reflection (BRDF) functions of the considered materials, and to allow a wavelength-dependent investigation over the solar spectrum.

Today, no experimental equipment suitable for daylighting applications is capable of answering requirements both in time-efficiency and completeness of information (transmission, reflection, spectral), which places this research in a leading position. It is expected to be complemented by ray-tracing simulations for the characterisation of whole daylighting systems or luminaires, restricting the measurements to their components and allowing flexibility and adjustment possibilities by integrating the geometry and arrangement options to the computer-based approach. The spectral information will allow going further in the system's investigation by integrating the thermal aspects of solar radiation.

As a first stage in this large project, the construction and calibration of the light detection system is being undertaken, including the selection and adjustment of light source(s), the construction of the ellipsoid, and the calibration of the camera as a multiple-points luminance-meter.