Fraunhofer Institute for Building Physics IBP
When it comes to planning and evaluating needs-based and energy-efficient visual environments, it is essential to accurately characterize the photometric properties of transparent façade elements. Today’s façade systems have both protective and supply functions as far as lighting is concerned. On the one hand, indoor areas where people work and spend time must be protected from excessive luminance; on the other hand, the façade itself can contribute to efficient natural illumination in these areas. In offices, for example, sufficient daylight inside administrative buildings can be provided for more than 80 percent of the annual operating time.
At Fraunhofer IBP, an existing numerical method for characterizing the photometric properties of transparent façade elements has now been supplemented by a testing facility that enables elements and systems to be measured quickly with directional resolution. The new testing facility also means that façade systems can now be comprehensively evaluated in terms of luminance in addition to the standard tests (U-value, g-value) that have been possible for a long time. The systems can also be classified, e.g. according to DIN V 18599-4.
The testing facility is based on the principle of measuring luminous flux. A sample holder with adjustable angle of elevation and azimuth is illuminated by a spotlight (HMI lamp). The light transmitted through the sample falls on a screen covering one-sixth of the half-space above the sample, which is moved in six steps around the sample to fully detect the transmission of light. Luminance on the screen is recorded by a moving luminance camera and calculated into luminance coefficients by determining the luminous flux. By directly evaluating one sixth of the hemisphere at a time, measurements can be made much faster than with goniophotometers, which are based on point measurements of illuminance.
The testing facility is operated via control and evaluation software that is directly connected to a database containing the photometric characteristic values of façade systems.
The test equipment was calibrated and validated. Samples that had already been measured with other test equipment in earlier projects served as a reference. A close match was ascertained.
The goniophotometer complements Fraunhofer IBP’s existing measurement equipment for evaluating the thermal energy of façade elements (U value, g value). By integrating control software and a database system for the façade parameters, the data can be further used directly to assess interior lighting conditions numerically and visually. This is made possible thanks to light simulation methods such as RADIANCE as well as normative approaches, e.g. according to DIN V 18599-4, for determining the impact on the energy requirements of buildings.
