Measuring the Sky

The daylight measuring site with its spectral sky scanner enables research to be conducted on the characteristics of daylight.

Perched commandingly on the roof of the engineering building is a unique piece of equipment in the form of a six-meter by four-meter aluminum construction. It houses a number of pieces of equipment, including a spectral sky scanner, a pyrheliometer for measuring the direct irradiance of the sun, as well as a daylight measurement head. Collectively, these and other instruments form the daylight measuring site. It might not exactly sound sexy, but it is something quite special.

A unique “open-air lab” at an unusual location

“A lot of people measure the sky and earth, but very few people’s work is concerned with the photometry and radiometry of daylight and the photometric characterization of nighttime conditions of perception in our cities and towns. Precisely these aspects are the focus of the work of the Chair of Lighting Technology at TU Berlin. Recent years have seen the development of measuring sites and test roads whose range of functions is unique in the world,” says Nils Weber, research associate at the Chair of Lighting Technology headed by Prof. Dr.-Ing. Stephan Völker. The location of the daylight measuring site is also unusual. “Normally,” Nils Weber explains, “such measuring sites are located in open fields, whereas ours in right in the middle of the city. The facilities, location and the field of research combine to make the daylight measuring site at TU Berlin unique.”

The team headed by Stephan Völker and deputy head of the chair and specialist in daylight, Dr. Martine Knoop, came up with the idea of an “open-air lab” in order to be able to research the exact characteristics of daylight. A range of studies shows that daylight has a positive effect on the wellbeing of people and is preferable to artificial daylight. But there is a great deal of uncertainty as to why this is.

A very special measuring instrument

Pride of place in the daylight measuring site goes to the spectral sky scanner, one of just four in the world! Since 2015, it has been taking measurements every two minutes in 145 different directions, day by day, week by week, month by month, in summer and winter and from sunrise to sunset. The researchers involved hope to be able to discover what daylight is, why it is beneficial to people and how it differs from artificial light. Ultimately, the knowledge gained will help design buildings to allow the optimal incidence of daylight. “This is something really significant,” say Nils Weber. Glass facades alone are not the answer as these tend to actually have a negative impact on a building’s energy footprint.

Test road in the heart of Berlin

The Chair of Lighting Technology’s second open-air lab is located on the grounds of the German Museum of Technology. A 100-meter-long test road is equipped with six masts on which continuously height-adjustable LED lights are mounted. The variability this provides makes it possible to recreate a very wide range of lighting scenarios in public spaces. “We use the test road to conduct experiments into visibility, for example cycling in potentially dangerous zones such as intersections, as well as to develop concepts for lighting streets and examining their reflectance characteristics - my special area of research,” say research associate Sandy Buschmann. “Berlin is somewhere that could really benefit from our research as the lighting in the city is not optimal, often too dark or erratic.” Next to the test road is a 1500-meter long demonstration stretch with variable distribution of lighting, which also forms part of the LED system. It enables TU researchers to demonstrate to the public how LED can be used to achieve lighting which significantly exceeds official requirements in terms of energy efficiency, traffic safety and emission protection.

In addition to these two off-site facilities, the Chair of Lighting Technology’s resources include a wide range of measuring devices which provide an outstanding basis for conducting excellent research. These include a spiral goniophotometer and an integrating sphere with a diameter of three meters which can be used to measure the luminous flux of a light, as well as a mirror goniophotometer for measuring the light distribution of lights, a 40-meter and a 120-meter photometer bench, a bidirectional goniophotometer for spatially resolved reflectance and transmittance measurements, a climate chamber and much more.

The team

Professor Stephan Völker, head of the Chair of Lighting Technology

The replacement of traditional light sources, such as thermal radiators and glow-discharge lamps, through LED presents us today with completely new options for distributing light according to our actual requirements. As such, lighting technology can actually contribute significantly to climate protection as well as traffic safety and a better quality of life. This requires new measuring procedures, such as in situ measurement of the reflectance properties of road surfaces as well as a completely new approach to light planning based on digital 3D modeling data. Interdisciplinary cooperation with psychologists, scientists from opthalmic optics, ophthalmologists, urban planners and architects will be decisive for our success.

Dr. Martine Knoop, deputy head and coordinator of teaching at the Chair of Lighting Technology

In order to be able to take account of the psychological and physiological effects of daylight on people in research, planning and product development, a lighting-technical and spectral description of this source of light is required. The Chair of Lighting Technology is world leader here. The spectral dataset and the models of the sky, planning aids and simplified sensors derived from this dataset can be used by researchers and graduates from a wide range of disciplines (electrical engineering, building energy engineering, and industrial engineering and management), planners, and engineers in the research and development of integrated daylight solutions.

Sandy Buschmann, research associate at the Chair of Lighting Technology

In my dissertation I developed an image-based measuring method for determining the reflectance characteristics of roads. This information is essential for my future research. The goal is to develop customized energy-efficient lighting for public spaces. I am using the research test road at the German Museum of Technology for my work. The facilities there enable me to examine which light distribution needs to be selected to achieve optimal lighting with minimal energy requirements for various features such as bus stops, intersections, cycle paths or the road surface itself.

Nils Weber, research associate at the Chair of Lighting Technology

The spectral sky scanner which we use to research the exact nature of daylight is a highly sophisticated measuring instrument. I am working on developing a simplified and less expensive daylight sensor. Such sensors could be installed all over cities to determine the daylight available at each location. This would enable us to plan buildings so as to optimize daylight for the benefit of the people using them and supplement this in a meaningful way with natural and artificial lighting.