Going underground

A subterranean home for contemporary art has been added to Frankfurt’s Städel Museum. Angeline Albert reports on how Licht Kunst Licht combined daylighting techniques with LED fittings to produce an a typical low-energy museum scheme.

The Städel Museum’s new contemporary-art space is an underground extension opened in February 2012. While to this observer at least, its circular skylights bring to mind the subterranean home of the Teletubbies, they perform a rather more serious function in reality.

Keeping it natural

Right from the start of the project, the architect and museum were clear they wanted daylight to play a large role in lighting the space. “This was a make-or-break part of the client’s lighting criteria,” says Tanja Baum, project leader at Licht Kunst Licht. But to maintain consistent light levels over the course of the day and the seasons, it would be important to have an automated system that would allow artificial lighting to step in when necessary. At the start of the process in 2008, Baum says, the greatest hurdle they faced was making a complex lighting system look simple. This was when it was decided to make the architectural detail multifunctional.

After many “mock-ups, measurements and calculations,” [of the underground space], as well as meetings with the museum and architects Schneider+Schumacher, Licht Kunst Licht decided to incorporate most of the lighting requirements within the 195 giant skylights fitted in the underground extension’s domed ceiling. These vary in size from 1.5m to 2.5m and increase in size as they get nearer to the centre of the exhibition space.

Baum says: “After calculating daylight levels during the year, we came up with illuminance levels and main daylight transmission levels. Four different layers of screen with varying light transmission levels were agreed upon for the skylights: namely total blackout (0 per cent), 9 per cent, 30 per cent and 100 per cent.”

Ambient lighting

As well as introducing daylight into the space, each skylight has a ring of LEDs to help deliver the required ambient lighting in the exhibition spaces. Half of these LEDs are warm white (2,700K) while the remaining 50 per cent are cool white (5,000K).

Baum explains that when daylight levels are very low – for example, on a cloudy day, in the evening and at night – this blend of LEDs (cool white and warm white) ensures the space is uniformly illuminated. Separately, the warm-white LEDs and the cool-white LEDs can each provide 350 lux on a wall; together they can offer a maximum of 700 lux, if necessary. Baum says: “The two types of LEDs in the perimeter are not switched separately. There is always a mixture of the two light colours at different dim levels”.

Although LEDs are central to the project, the decision to use them was taken at a late stage. “The problem at the beginning of the project was that LEDs were not powerful enough to provide sufficient ambient light for the space,” says Baum. “However,as the project progressed, LED development rapidly moved forward and therefore LEDs [in March 2009] became an attractive alternative to the previously-preferred compact fluorescent lights”.

“Daylight can be reduced with screens with varying transmission levels, right down to complete darkness if required”

Tanja Baum, Licht Kunst Licht

Cloud control

Artificial and natural light levels are controlled by an intelligent lighting management system.The Luxmate Professional lighting controls dim the LEDs positioned in the skylights according to the amount of available daylight and the lighting levels required for the exhibits.

A daylight sensor installed on an adjacent building’s roof, monitors outdoor brightness and sends data to the management system to control the LEDs.The controls system also manages the roller blinds in each skylight to protect exhibits from excessive light. As Baum explains: “The blinds can be moved depending on the position of the sun and outdoor brightness. Daylight can be reduced with screens with varying transmission levels, right down to complete darkness if required.”

Accent lighting

To provide accent lighting for specific exhibits and enhance wall surfaces, Arcos LED projection spotlights can be plugged in at 16 different connection points on the perimeter of each skylight. The exhibition space is often divided by partitions into smaller spaces – which can be lit by specific skylights. This means lighting conditions, including illuminance levels, can be adjusted for particular exhibits.

Licht Kunst Licht used 3,000K throughout the space and set the illuminance levels for its four ‘medium-specific’ art zones in collaboration with the museum’s exhibition manager. The luminaires have a colour rendering index exceeding 90. Any decisions made about illuminance levels were based on the materials used in each piece of art, its colours, age and the environment in which it is placed.

Higher lux levels were allowed for paintings than for graphic works as a result of the fact that the paintings are mostly oil on canvas and, as such, not as sensitive to radiation. Graphics are typically made with a graphite pencil or with water-based colours on paper; these materials are more sensitive to damage and must be protected by using a much lower light level. In the graphics-only zone, 50 lux was used.

Licht Kunst Licht’s decision to used LEDs was influenced by a scientific study that was conducted by Darmstadt University and Zumtobel. This study compared the impact of LED use on paintings with the impact of halogen use; the research concluded that LED light on paintings was not as harmful as that emitted by halogens.

Energy efficiency

Another advantage of installing LEDs, rather than tungsten halogen, is of course the lower energy consumption. Baum says 25-30W/m² is the lighting average benchmark for halogen lights used in a museum space that is the same size as the extension. Referring to the equivalent lighting average benchmark for LEDs in the 3,000m² extension, she says: “In a normal situation, with the dimming levels that apply, we would have 11W/m² which is pretty low for a museum.”

Some fine-tuning has taken place throughout the year, since the museum’s opening last February, says Baum. After one year, based on measurements taken throughout the seasons, it will shortly be possible to determine how further savings can be made.

Additional built-in sensors alert the operator to issues such as a bird blocking a skylight. Birds, however, aren’t the only things that are attracted to them – one observation the museum has already made is just how popular the skylights are with visitors, particularly children who like to jump on them when playing in the gardens. Perhaps that Teletubbies analogy isn’t so silly after all.

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