The Post Tower illuminates Bonn for the next century, both inside and out. Its color-changing façade is only one of the office tower's many offerings. Beautifully daylit offices and detailed integration of architecture and light are others.

» A view from the 'penthouse' of the Deutsche Post Tower in Bonn, Germany, emphasizes the building's distinctiveness. The 41-story structure is clearly the tallest building around, which at first makes it seem out of place, towering above an otherwise diminutive city. After experiencing the Post Tower, however, its stature seems appropriate, the privilege of a design that is uniquely beautiful, ecologically responsible and human-centric.

The building is also a shining example of collaborative effort. Design architect Helmut Jahn ensured that Parisian lighting artist Yann Kersalé and Berlin-based lighting design firm L-Plan were part of the Post Tower from its inception. This coordination produced a structure in which the lighting seems intrinsic to the architecture, be it the cafeteria, emergency stairwell, offices, elevator lobby or penthouse boardroom.

The most obvious synthesis-since it can be seen for miles-happens within the building's transparent double-glazed curtain wall, which begins to emit a unified and comprehensive lighting effect at sundown. Conceived by Kersalé and translated into a lighting scheme by L-Plan, the slow-loop color change is intended to express 'the life of the building.' After dark, the façade gently transitions through a series of colors at about one minute per color-'as if breathing,' says L-Plan principal Michael Rohde.

Sandwiched between the building's inner and outer glass layers, 1,000 custom fixtures fitted with red, yellow and blue neon tubes are horizontally oriented and recessed into the façade approximately every 5 feet. The lamps use 'filter glass'-real red, yellow and blue glass-chosen because it imparts a purer color when illuminated; turned off, the lamps maintain their color rather than reverting to plain white tubes.

Though enclosed inside a glass skin, the building is actually two separate towers linked by nine-story atria. Every ninth floor, a 'sky garden,' or open circulation space, joins the north and south tower halves. (A bridge with an elevator bank connects the other floors.) At the four sky-garden levels, Martin Professional 150W and 575W spotlights capable of changing color and distribution angle uplight the façade. A DMX system from Martin controls both the neon fixtures, as well as the spots, to create the color-changing effect.

If the building is 'alive,' it draws sustenance from what Kersalé refers to as racines de lumiere, or light roots. Two thousand custom-designed in-ground LED luminaires simulate the pattern of tree roots stretching across the plaza from the base of the tower. Seven white LEDs set inside a stainless-steel housing behind an opaque diffuser create the source; these are integrated into 10-millimeter-thick stainless-steel parallelogram-shaped plates. Six versions of the plates, each with a different number and placement of the LED luminaires, enabled the designers to create a random 'scattering' of the point sources. The plates, arranged in strips with a channel for wiring underneath, alternate with stone surfacing to create a striped pattern.

Given the tower's transparency, every building component had the opportunity to become an aesthetic element-even the emergency stairwell. According to the lighting artist's vision, only every second flight of the fire stairs is illuminated, visually emphasizing the architectural repetition of the 41 flights of stairs and the height of the tower. To prove Kersalé's idea would meet prescribed light levels of 100 lux, L-Plan prepared numerous calculations and a one-to-one mockup. ('We don't usually do a mockup for emergency stairs,' jokes Rohde.) A simple bare T5 fluorescent lamp in a brushed-aluminum fixture met both brightness and aesthetic requirements.

The top floors of the building are occupied by Deutsche Post executives, and while the office lighting is calculatedly democratic using the same direct/indirect scheme as the junior level employee spaces, the penthouse boardroom is an exclusive luxury. An arched structure folds over the tower's apex to form the walls and ceiling of the boardroom and several smaller meeting areas that share the floor. To accentuate the curvature, L-Plan integrated LED strips into the metal profiles, which for a change, the architect created to meet the lighting designer's specifications. 'It was one of the few times as a lighting designer that I have influenced the form of a metal profile,' comments Rohde, who required 40 millimeters in recessed depth and 60 millimeters in width to accommodate the LED technology available at that time. Like many of the luminaires on the project, the strips had to be custom fabricated. Lighting manufacturer Ansorg crafted the 800 meters of LED strips, as well as the 2,000 'light roots.' Double-focus downlights with both halogen and metal halide lamps supply the primary lighting for the meeting rooms. The halogen enables more comprehensive dimming in the space, which is occasionally used for events that require a personal atmosphere.

An outdoor space encircles the boardroom, providing access to the building's greatest amenity-a spectacular view. From the ground, the roof rendered in light becomes an important part of the architectural whole after dark: The 15 columns and horizontal elements that fortify the area around the deck are illuminated from their base with two narrow-beam uplights, creating a concluding glitter atop this stacked jewel. A high-performance projector accents the Deutsche Post logo.

Helmut Jahn's design, in its transparency, cannot help but be about daylight, which easily penetrates the many diaphanous glazed layers-the vast atrium spaces, the skylit conference center, the glass elevator shafts, and the airy, high-ceilinged ground-floor lobby. It is the window-walled offices, however, that bring the building's true character to the fore. Arranged around the perimeter of the building, all have floor-to-ceiling views of the Bonn landscape. And in addition to daylight and an inspirational scene, the 2,000 Post Tower occupants are granted unprecedented control over their environments. The shading system sandwiched between the two-layered curtain wall is user controlled, as are the operable windows. An indirect lighting system recessed into coves formed in the concrete ceiling (another example of the detailed integration of light and architecture) is completely adjustable by individual users; a task light at each desk provides still further control. This is architectural design that respects its end user.

It is also a design that considers its environment. The double-glazed façade dexterously compensates for heat gain, providing ventilation to the interior throughout the seasons without a central mechanical system. The south face of the building is shingled to enable natural airflow; the north face is flat. The shades and operable windows also facilitate climate control. Daylight sensors automatically adjust office light levels to 300 lux, which also helps reduce energy costs. Rohde notes that certain areas like the lobby were more extravagant in terms of energy usage. 'There is great awareness in Germany, since our energy costs are higher than in the United States, but in the lobby, we were allowed more freedom. The client wanted it to look nice,' he says. 'Where we did a lot of calculations was in the offices. We could not use more than 10 watts per square meter [about 11 square feet].'

Honored this year with an IALD Award of Merit, the Post Tower's stunning effects are catching the attention of more than just Bonn residents. Rohde credits the quality of the team, and he particularly appreciated Kersalé's uninhibited approach to lighting design. 'It is interesting to collaborate with someone who is used to working on light very 'freely,' without having codes or security in mind. It's one of the reasons this project is so special in terms of the lighting.' Another reason, he notes, was the quality of the client. Deutsche Post, formerly government-owned, wanted a headquarters that both demonstrated and facilitated the progressive mentality of this newly privatized organization. The environment it creates for employees and the statement it makes architecturally is indeed one for the future. emilie w. sommerhoff

project Deutsche Post Tower, Bonn, Germany
architect Murphy/Jahn, Chicago
lighting designer L-Plan, Berlin
lighting artist Yann Kersalé
photographers Atelier Altenkirch, Andreas Keller, Alan Toft
manufacturers Ansorg, Belux, Erco, Martin Professional, Sill, Siteco, Spectral, TIR Systems, WE-EF