Marilyne Andersen, Christian Roecker, Jean-Louis Scartezzini

Solar Energy and Building Physics Laboratory (LESO-PB), Swiss Federal Institute of Tech­nology (EPFL), CH — 1015 Lausanne, Switzerland

This paper describes the design process and setting up of a novel bidirectional go — niophotometer, relying on digital imaging and allowing the combination of transmis­sion and reflection measurements. As its measurement principle is based on the projection of the emerging light flux on a rotating diffusing screen towards which a calibrated CCD camera is pointed (used as a multiple-points luminance-meter), sev­eral strong constraints appear in reflection mode due to the conflict of incident and emerging light flux: for five out of the six screen positions (unless incidence is nor­mal), the incident beam must penetrate in a way that it is restricted to the sample area only; in addition to this, when the screen obstructs the incoming light flux, a special opening in the latter is required as well to let the beam reach the sample. The practical answer to these constraints, detailed in this paper, proved to be reliable, appropriate and efficient.

Introduction

To allow the integration of advanced daylighting systems in buildings and benefit from their potential as energy-efficient strategies, a detailed knowledge of their directional optical prop­erties is necessary. These properties are accurately described by the Bidirectional Transmis­sion (or Reflection) Distribution Function, abbreviated BT(or R)DF, that expresses the emerg­ing light flux distribution for a given incident beam direction (Commission Internationale de l’Eclairage, 1977). An original experimental method for their assessment, illustrated in Fig­ure 1 was first developed for transmission measurements (Andersen et al., 2001): the light emerging from the sample is reflected by a diffusing triangular panel towards a CCD camera, which provides a picture of the screen in its entirety. Within six positions of the screen and camera around the sample (each separated by a 60° rotation from the next one), a complete investigation of the transmitted or reflected light is achieved.

This innovative approach brought several major advantages when compared to characteriza­tion techniques requiring a sensor to be moved from one position to the other (Papamichael et al., 1988; Bakker and van Dijk, 1995; Aydinli, 1996; Breitenbach and Rosenfeld, 1998; Apian-Bennewitz and von der Hardt, 1998): a significant reduction of the BT(R)DF data assessment time (a few minutes instead of hours per incident direction) and a continuous information about the transmission (reflection) hemisphere, whose resolution is only limited by the pixellisation of the images.

The camera is used as a multiple-points luminance-meter and calibrated accordingly. A luminance mapping of the projection screen is carried out by capturing images of it at differ­ent integration intervals, thus avoiding over and under-exposure effects, and appropriately combining the latter to extract BT(R)DF data at a pixel level resolution.

Material samples showing large range of luminances can thus be handled without any loss of accuracy, while an appreciable flexibility is allowed in the data processing (Andersen, 2004).

For BRDF measurements (reflection mode), however, additional constraints appear due to the conflict of incident and emerging light flux.

For five out of the six screen positions (unless incidence is normal), the detection princi­ple can be kept identical as in transmission mode (Figure 2(a)), except that light flux must penetrate the measurement space in a way that the beam is restricted to the sample area only. As there is one position (all six for normal incidence) where the screen obstructs the incoming light flux, a special opening in the latter is required to let the beam reach the sam­ple, producing a blind spot at that specific screen position (and only in reflection mode), as illustrated in Figure 2(b).

The design process of the instrument combining BTDF and BRDF measurements is pre­sented in this paper, and the mechanical components specifically developed to answer to these constraints in a practical and efficient way are described.