Solid state sources have displaced T5 lamps in the world’s biggest LED office lighting installation in the City of London. But the rationale was not energy saving. Jill Entwistle reports

When Roger Dangell, director of Meit Associates, began considering the possibility of using LEDs as the main light source for the trading floor of the Nomura investment bank, he discovered there was no precedent. Which is not surprising – what was being proposed was the largest LED office lighting installation in the world.

Wila had to make a 400mm-wide, 100mm-deep LED luminaire

Aesthetics and maintenance were the driving decisions, with energy efficiency obviously being a key consideration. The 8,000-square-metre floor plate is long – about 100 metres at its longest point – and the chilled beams in the ceiling emphasise the linear appearance, drawing the eye south to the view of the Thames.

‘The design of the luminaire revolved around the chilled beams,’ says Dangell. ‘To enhance the linear arrangement, but at the same time provide a stronger feature in the flat ceiling to take the eye away from the chilled beams, the designer wanted to provide a continuous luminaire of greater proportions with a bright, evenly lit appearance. We decided to use LEDs because T5 could not do what we needed or achieve the aesthetic the architect wanted.’

The problem with T5 was that it overdelivered on lighting levels. Using overlapping lamps to achieve an even illumination created about 900 lux on the working plane, literally twice what was required. Dimming them down, however, jeopardised the lit appearance. ‘It was felt that this would cause the luminaire to appear lifeless and we wanted it to be quite bright,’ says Dangell. ‘We also wanted to see if we could achieve it with LEDs.’

That meant an evaluation, with Wila’s help – the company had been on board from an early stage when T5 was still a option – with the LED alternative having to fulfil three criteria: • Physical constraints. The luminaire had to be about 350-400mm wide, less than 100mm deep and mounted in continuous rows at three-metre centres. (The eventual fitting width was 400mm.) • Technical requirements. Glare and light levels had to be achieved at the fixed spacing to ensure screen reflections were not an issue. • Financial case. A convincing argument had to be made that the benefits of energy efficiency and savings on lamp replacement costs could offset the increased capital spend (50 per cent more than T5).

The luminaire was 400mm wide and mounted in rows

Client Nomura wanted the building to be green – rainwater harvesting, a moss roof and water-cooled desks are among the environmental measures that had been incorporated into the design. The aesthetic and maintenance arguments for LEDs were strong, so the go-ahead was given to develop special LED luminaires.

This, as Wila discovered, was easier said than done. ‘The main challenge was that we were using a microprismatic optic and if you shine LEDs directly through anything like that you end up with these awful striations,’ says Wila’s technical director Peter Le Manquais. ‘It took us a lot of time and experimentation to get the angle of the LED right so that we had a completely uniform front and no striations across the microprismatic panel.’

The tolerances that govern the relationship between the LEDs and the back reflector were extraordinarily exacting – if the back reflector was moved up by just 2mm, the appearance of the whole front face of the fitting completely changed. ‘It made a massive difference, so we had to be really accurate in manufacture as well,’ says Le Manquais. ‘We learnt a lot very quickly.’

The design of the LED luminaire revolved around the chilled beams

A further complication arose when it emerged that the fittings had been installed working from the centre outwards in two directions. This meant that special end products had to be made to just those measurements. ‘The complexity of delivering the right part to go in the right position in the building has been incredible,’ says Le Manquais.

The experience has not deterred Wila from pursuing the commercial development of the fitting. The company is currently working on a 600mm-square version that it hopes to launch in the first quarter of next year – but that has meant going back to the drawing board. ‘The different size needs a different solution again,’ says Le Manquais. ‘The principle worked on a luminaire 400mm wide but if you stretch it to 600mm it doesn’t work, even if you illuminate from four sides.’

Dangell was also convinced by the experiment. ‘The thing that surprised me most was that we couldn’t achieve even more energy savings with LEDs. That was probably the biggest disappointment. But the longevity and reliability are of huge operational cost benefit to the client. And I’m really pleased with the light quality – I think it’s one of the best illuminated offices spaces I’ve seen.’ 

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