Exprimental part 2. Goniophotometric measurements

The parameters that quantify the redirecting or diffusing properties of a daylighting glazing system can be measured using a goniophotometer with a light beam perpendicularly incident on the sample. The luminance coefficient q, defined as the ratio between the luminance L of the sample surface and the incident illuminance on the sample as a function of the observation angle є, with 0° < e< 90°, is the main parameter. The observation angle is 0°, when the observation is normal to the sample. The goniophotometer available at Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino [2, 3], was used to perform investigations on the samples 12 and 13.

If the following conditions are satisfied, an absolute (a reference standard is not required) and accurate measurement is performed:

• all the half-plane (or half-space if the sample is not isotropic) in the transmission configuration is analysed with short steps for the observation angle;

• the detector exhibits such a linearity that a sufficiently accurate ratio is obtained between the measured luminance and illuminance.

As a consequence of the rotational symmetry of the samples, they were supposed to be isotropic and measurements were performed on the horizontal half-plane (only one angular coordinate, e, is needed to establish the observation angle). The collimated light
beam was in the opposite half-plane (transmittance configuration) perpendicularly incident on the sample.

Due to the samples characteristics, the observation angle є step was varied as follows:

• between -2° a +2° to the sample normal the measurements have 0.5° as resolution step;

• between (+)2° and (+)10° the step is 1°;

• between І0° and 25° the step is 2.5°;

• between 25° and 60° the step is 5°;

• between 60° and 90° the step is 10°.

These intervals were chosen considering the low scattering properties of the selected samples. It was, hence, expected to collect the highest amount of transmitted energy, close to the normal direction, typical of materials with regular behaviour. The resolution then increase with the increase of the observation angle.

Figure 4. Angular light transmittance of the selected samples

In figure 4 the goniophtometric measurements on the samples 12 (continuous line) and 13 (dotted line) are presented. In particular the graph reports the luminance coefficient as a function of the observation angle. No experimental data are available for sample 6, since it was not available during the test campaign. As expected, no redirecting components, nor diffusing behaviour of the samples, are evident in figure 4 and the two glazings confirmed their mostly regular properties.