Measurements have been carried out on two low iron 4 mm “Eurowhite” glasses from Euroglas GmbH. One of the glasses is equipped with the commercial antireflection surfaces prepared by means of a liquid-phase etching by Sunarc Technology A/S.

The transmittances of the two clean glasses were first measured in an indoor goniospectrometer test facility [2]. A ray of light from a tungsten halogen lamp is reaching the surface of the glass, which is installed in the test facility in such a way that the transmitted radiation through the glass is measured in a half sphere with a diameter of 2 m for different wavelengths. Measurements of the irradiance in the sphere are carried out with and without the glass installed in the test facility. In this way the transmittance, that is the ratio between the transmitted radiation and the total radiation from the lamp, is determined. Since the wavelength distribution of the lamp is not identical to the wavelength distribution of solar radiation, a correction based on the wavelength distributions of the lamp and the sunlight (ISO 9050) is done in order to determine the solar transmittance.

The glass in the test facility can be rotated through a vertical axis. In this way the incidence angle can be changed. The transmittance is determined for five different incidence angles: 0°, 30°, 45°, 60° and 75°. The measured transmittances as functions of the wavelength and the incidence angle for the two glasses are shown in figures 1 and 2. The transmittance is reduced for increasing incidence angle. The maximum value for the transmittance is for increasing incidence angles moving towards small wavelengths, especially for the antireflection treated glass.

1.0

0.9

0.8

0.7

Fig. 1. Transmittance for the normal glass as functions of the wavelength and the incidence angle.

The measured solar transmittances for the two glasses are shown in table 1. The solar transmittance is increased by about 6-10 % point by the antireflection treatment. The increase is dependent on the incidence angle.

1.0

0.9

0.8

0.7

T1

0.6

0.5

Є

0.4

H

0.3

0.2

0.1

0.0

2200

Fig. 2. Transmittance for the antireflection treated glass as functions of the wavelength and the incidence

angle.

Table 1. Measured solar transmittance for the two glasses for different incidence angles in the goniospectrometer test facility. Incidence angle Normal glass Antireflection treated glass 0° 0.904 0.960 30° 0.888 0.955 45° 0.893 0.950 60° 0.822 0.909 75° 0.615 0.711

The solar transmittances for the two glasses as functions of the incidence angle are shown in figure

2. Best fits for the solar transmittance of the direct radiation are found to be:

t—— / 0.904 = 1 — tan4 31 — I for the normal glass

t—— / 0.960 = 1 — tan511 — I for the glass with antireflection surfaces where 0 is the incidence angle, °

t—h is the directional-hemispherical transmittance at the incidence angle 0, —

The diffuse-hemispherical transmittance xdlf h was measured in an outdoor laboratory test facility under weather conditions without direct solar radiation for the two clean glasses. The test facility is placed at the Technical University of Denmark, Kgs. Lyngby, Denmark. Tdif-h is the ratio between

The measurements were carried out in the test facility with four calibrated pyranometers, type CM 5 and CM11 from Kipp and Zonen. Before the tests started all 4 pyranometers were tested against each other after they have been put into position, but before the shadow ring and the glasses were installed to be sure they all gives the same output signal for the same solar irradiance. During the tests the pyranometers are measuring the total irradiance on the glasses, the diffuse irradiance on the glasses and the irradiances transmitted through the two glasses. The accuracies of the measured irradiances are estimated to be within 2%. The glasses are placed side-by-side on a 45° tilted surface facing 10° towards west from south, see figure 4.

Table 2 shows the measured results. The antireflection treatment increases the diffuse — hemispherical transmittance by 8 % points. Table 2. Measured diffuse-hemispherical transmittance for the two glasses. Glass Total irradiance Transmitted irradiance t dif-h Normal glass 167 W/m2 142 W/m2 0.85 Antireflection treated glass 167 W/m2 155 W/m2 0.93