Thursday, Sep 6, 2012

The Elizabeth Bridge in the Hungarian capital of Budapest is a classical cable bridge. It spans over the Danube 380m between Buda and Pest.
The architectural accent lighting of a project like this was not a simple task. Lots of aspects have to be considered. Just think of the traffic on the road and the footpath and it has to be appealing for viewers from various points of the urban scape.

Early in the briefing it was agreed that the colour temperature of the light, highlighting the bridge had to be 4000 K. The bridge should appear as a uniform structure when accent lighted. The difficulty is that dark spots where the lighting is not covering would cause visual imperfection relating to the structure. This meant the lighting level and the light uniformity should be washing the two gates of the bridge, also on the load carrying cables and the suspension cables that are in a vertical position between the platform and the load carrying cables. During the trial demonstrations some designers proposed a few viable possibilities. All involved lots of obtrusive luminaires which made the sleek bridge rather unsightly. Finally the structural engineers made the verdict, no luminaires allowed to be fixed to the cables. The case remained open. The task was reiterated, with the emphatic need that the character of suspension bridge should be made visible. This meant that all the cables should be washed with white light. Following the abortive lighting trials a suggestion was made by one of the lighting designers to use WE-EF ETC inground luminaire range which could be built into the footpath of the bridge eliminating the need for cumbersome luminaires. The ball was back in the court of civil engineers. Their decision was favourable, since the bridge plates of the footpath are not part of the loadbearing structure of the bridge they can be used to build luminaires into them.

According to lighting design calculations the lighting level was decided at 300 lux. This was not easy to achieve considering the lighting distances. The longest was 34m and was achieved with a ETC 250W metal halide narrow beam luminaire. The closest luminaire was 1m away from the area of regard. This was achieved by a smaller power rated luminaire from the ETC inground luminaire family. Owing to the fact that between the footpath and the suspension cables there exists a gradually changing physical distance, the task of lighting distance became also variable. This required the whole WE-EF ETC inground family, so the 35W, 70W, 150W and 250W were all included with the exception of the 20W. Besides lighting levels the elimination of hot spots and dark gaps on the white painted bridge surface was a real challenge. In order to achieve uniformity the availability of a whole serious of WE-EF lenses were needed. These had to be inside the fitting without compromising the IP67 rating of the luminaire. By the use of various lenses the path of light elongated or widened on the vertical and horizontal plane perpendicular to the hard glass surface of the inground luminaire. This so called ovalisation of light beam had another beneficial effect - it increased the lighting accent on the thinner suspension cables. Sometimes 3 lenses were used. This caused no difficulty due to the standard practice of WE-EF placing all accessories inside the luminaire, without compromising its high IP rating sealed integrity. Besides lenses the luminaires contain glare shields inside to prevent disability glare to truck and bus drivers, driving through the bridge. To complete the task of illuminating the cables of the Elizabeth Bridge, the 176 ETC WE-EF luminaires were used. This was another challenge that could have not have been accomplished without such a professional accomplishment family of luminaires.   


text : Michael Toth M.I.E.S. Aust. NZ