TestLab Solar Façades

Information on the spatial distribution of transmitted or reflected light is required to evaluate the daylighting and glare control performance of solar-shading systems and other building envelope technology.

Light scattering is characterized by measurements of the bidirectional scattering distribution function (BSDF). For these measurements, we use a 3D scanning photogoniometer (pab-pgII, pab advanced technologies Ltd.), which offers several combinations of detectors, light sources and sample holders for different purposes.

Spectral or polarization-dependent BSDF measurements are available on request.

We use spectrophotometers of different types to determine the transmittance and reflectance of glazing, solar-shading products and other materials. For many years, we have focused on measuring light-scattering and light-deflecting materials such as screen-printed or etched glass, solar-shading textiles and slats for blinds.

Our laboratory is equipped with large integrating spheres with large sample apertures, TAUWIN and RHOWIN, with which spectra can also be measured for obliquely incident radiation. For non-scattering samples, a high-end laboratory benchtop spectrophotometer with a customized integrating sphere completes the suite of spectrometric equipment.

From the measured spectra of the single components, we calculate many other parameters such as the light and solar transmittance and reflectance, the g-value (or SHGC: solar heat gain coefficient) and the color rendering index CRI of multi-pane glazing units according to EN 410 and ISO 9050, parameters for combinations of solar-shading elements and glazing according to EN 14500 and ISO 15099, color coordinates according to CIE 15 and the Solar Reflectance Index (SRI) according to ASTM E1980-2011.

We use a calorimeter facility with a solar simulator to determine the g value (or total solar energy transmittance TSET or Solar Heat Gain Coefficient SHGC). This test facility is suitable for measuring individual components, such as glazing units, as well as for fenestration systems combining a glazing unit and a shading device (external or internal) and for entire façade elements such as double façades and closed-cavity façades. This test facility is also used to characterize façade-integrated PV or solar thermal technology.

Our calorimeter also enables angle-dependent g-value measurements, as the angle of incidence of the irradiation can be varied both by adjusting the height of the solar simulator and by rotating the measurement setup and absorber around a vertical axis. The spectrum of our solar simulator generated with the help of metal halogen lamps is almost identical to the standard solar spectrum according to EN 410.

For the metrological determination of the heat transfer coefficient or U-value, a guarded hot-plate apparatus is used to determine the thermal conductivity of the component under test.

The guarded hot-plate apparatus, of the type TLP 800 S from the manufacturer Netzsch Taurus Instruments GmbH, measures according to the two-plate method specified in the ISO 8302. The test specimens are placed in the system between an electrically heated plate with a surrounding protective heating ring and two external cooling plates. The measurement stack is integrated into an insulated measurement chamber. The measurement chamber with the samples can be tilted for testing in both horizontal and vertical orientations.

The U-value of glazing units is determined with the guarded hot-plate apparatus in accordance with EN 674. In this case, the dimensions of the glazing samples are larger than those of the heating plate and protective heating ring. Hence, the results obtained with this method apply to the central area of ​​the glazing without edge effects, the so-called COG value (center of glazing).

The evaluation of luminance and color under real operating conditions is used for daylighting, glare protection and material characterization.

We use a scientific-grade luminance camera to generate luminance images and determine color coordinates (TechnoTeam LMK 98-4 color). Our luminance camera is equipped with a filter wheel for color measurement, which is adapted to the CIE color sensitivity functions of the 2° standard observer (CIE 1931).

The filter wheel allows the installation of a total of 6 filters, whereby 4 filters are required for color measurement. In addition, the measuring system can also be equipped with filters for scotopic luminance V' (λ), the circadian effect function C (λ), part of the NIR spectral range (only 780 nm - 1000 nm), BLH (blue light hazard) or with a clear glass filter.

High resolution and the evaluation of luminance and color images are available in combination with the LMK LabSoft software package. 

The effects of façade technologies and their control strategies on thermal and visual comfort must be evaluated in laboratories with typical room dimensions.

For thermal and visual comfort investigations, we use an office-like facility consisting of two identical rooms. This test facility is mounted on a rotatable platform that allows different orientations to be analyzed. The room dimensions are 3.65 m x 4.60 m x 3 m. Each room has a glazed façade (3.65 m x 3.00 m), which can be removed and replaced entirely.

Our rotatable two-room test facility is equipped with sensors to quantify the thermal and lighting conditions in the rooms. In addition, comfort conditions can be evaluated subjectively on the basis of user surveys. The Daylight Glare Probability metric, which is used for glare evaluation according to EN 17037, was developed using this facility.

EN 14500 defines the direct-direct, direct-diffuse and direct-hemispherical reflectance as the ratio of the directly or specularly, diffusely and hemispherically reflected radiant flux to the directly incident radiant flux. Likewise, the standard defines the direct-direct, direct-diffuse and direct-hemispherical transmittance as the ratio of the directly, diffusely and hemispherically transmitted radiant flux to the directly incident radiant flux. For energy and daylighting applications, the spectrally resolved reflectance and transmittance of fenestration and façade materials are aggregated into broadband optical characteristics, representing properties for the solar spectral range (wavelengths from 300 nm to 2500 nm according to EN 410) and the visible spectral range (wavelengths from 380 nm to 780 nm).

BSDFs describe the spatial distribution of light scattered by a sample in reflection and/or transmission. A BSDF value (BSDF_j) at a certain position with respect to a sample is given by the sample radiance as seen by a sensor located at that position and directed towards the center point of the sample (L_j), divided by the incident irradiance on the sample (E) for a uniformly illuminated and uniform sample:

BSDF_j = (L_j / E) [sr-1]

The Solar Reflectance Index (SRI) is the relative temperature of a surface with respect to standard white (SRI = 100) and standard black (SRI = 0) surfaces under standard solar and ambient conditions [ASTM E1980-11(2019)].  It is determined from an energy balance between the surface and its surroundings under steady-state conditions, taking the radiative and convective heat exchange into account.

To determine it, initially the spectral reflectance of material samples in the solar and IR spectral ranges is measured in the laboratory.  Then, the solar reflectance, the solar absorptance, the thermal emissivity and finally the Solar Reflectance Index (SRI) are determined according to the method specified in ASTM E1980-11(2019).

In the building sector, the U value (or thermal transmittance) describes the heat transfer across a component (e.g. through a wall or glazing) due to a temperature difference between the adjacent air layers. As a result, the thermal transmittance not only takes the heat conduction within the solid body into account, but also includes the heat transfer at the interfaces between the body and the adjacent gases. It thus represents a measure of the insulating effect of the component (SI unit is W / (m² · K)). The larger the U-value, the worse the insulating effect. Conversely, a small U value indicates that the component is a good thermal insulator. 

The g value or total solar energy transmittance (TSET) or Solar Heat Gain Coefficient (SHGC) is a measure of the proportion of the solar energy incident on a building envelope that is transferred into the interior of the building. The g value is a ratio with a value between 0 and 1. A g value of 0.3 means, for example, that 30% of the incident solar energy enters the interior of the building, either as short-wavelength radiation or as heat. The total solar energy transmittance thus provides the basis for calculating solar heat gains in buildings, especially in the case of transparent façades and building envelopes. These solar gains contribute to the heating of the building, which leads to a reduction in the heating energy required, especially in winter, but often also to unwanted overheating in summer.

For simple cases, there are calculation methods for determining the g value, for example according to DIN EN 410, but these can be used only for optically simple systems such as clear glazing. For more complex geometries, such as venetian blinds or light redirecting systems, experimental determination is required.