Design of a time-efficient video-goniophotometer combining bidirectional functions assessment for transmission and reflection

Abstract

A detailed knowledge of the light distribution characteristics through advanced window systems is required to improve the visual comfort of the building's occupants while controlling the propagation of daylight in rooms and the solar gains. An innovative bidirectional goniophotometer has recently been set up for this purpose, using digital image capture and the projection of the emerging light on a diffusing screen.It therefore provides a continuous investigation of the bidirectional transmission figures in a time-efficient way. This instrument was converted into a double-purpose device, allowing both transmission and reflection measurements, which induced several strong constraints due to the conflict of incident and emerging light flux in reflection mode: on one hand the incident beam had to be restricted to the sample area only; on the other hand, as the screen obstructed the incoming light flux in some positions, a special opening in the latter was required. The practical answer to these constraints, detailed in this paper, proved to be reliable, appropriate and efficient.

Goniophotometry and assessment of bidirectional photometric properties of complex fenestration systems

“…, a different design was realized using a CCD camera with a wide-angle lens aimed at a triangular and diffusing projection screen [56,57]. The latter collects the light emerging from the sample and rotates around it to cover the full 2π steradian hemisphere.…”

“…The experimental conditions for the LESO-PB/EPFL instrument [57] were reproduced virtually with the commercial forward ray-tracer TracePro 5 that is based on Monte Carlo calculations. Computer simulation results were then compared to BTDF data assessed with the experimental goniophotometer.…”

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

“…To cover all possible emerging directions (2π steradian), the camera and the screen perform rotations of a 60° angle magnitude. [19,20] 3.2 Numerical goniophotometers LESO-PB / EPFL The experimental conditions described above were reproduced virtually with the commercial forward ray-tracer TracePro® based on Monte Carlo calculations [21,22]. The simulation model included a detection screen (made out of 6 panels covering 360°) and a model of each sample as close as possible to the physical elements.…”

Development and validation of a Radiance model for a translucent panel

“…As a matter of fact, a hexagonal symmetry could be discerned when BTDF values start being lower than 0.04 cd m -2 lux -1 , as in Figures 2(b), 2(d) and 2(e). This is a sign that detected light levels become close to the limit (minimal) luminances still accurately captured by the CCD camera (estimated at 0.015 cd m -2 emitted from the screen 21 ): indeed, the assessment method itself, based on a combination of 6 adjacent projection screen positions 18 , then becomes faintly apparent, which means that the correction and calibration factors applied to the detected screen luminances are not able to compensate fully for the device's particular geometry anymore. This would have been exacerbated with the smaller (150 mm diameter) sample area, which, again, confirms the necessity for the chosen, larger illuminated area and therefore coarser measurement set.…”

“…Goniophotometer measurements were carried out for this purpose at the Solar Energy and Building Physics Laboratory (LESO-PB) of the Swiss Federal Institute of Technology in Lausanne (EPFL) with an instrument based on digital imaging techniques, comprising a rotating diffusing screen on which the emerging light flux is collected and reflected towards a digital video-camera (CCD), used as a multiple-points luminance-meter 18 .…”