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PROS AND CONS OF LED LIGHTING

FOCUS ON: LIGHTING

Light emitting diodes (LEDs) are touted as the high-tech, low-maintenance light source for architectural lighting schemes. But are they all they are cracked up to be? And what are their benefits and pitfalls?

On paper, modern highoutput LEDs are a 'no-brainer' for architectural lighting, both inside and out. With a quoted '50,000-hour life' (compared with 10,000-15,000 for fluorescents or metal halide) they are a product that you can more or less fit and forget, which should cut maintenance costs to the bone.

Their energy efficiency is growing exponentially, year on year, and they provide a small, compact light source which is easy to focus, with little ugly spill light around the beam.

LED light beams produce virtually no heat or ultraviolet light, they come in a range of intense colours and white (including programmable RGB colour-change versions) and are easily switched and dimmed. They contain no toxic components, such as mercury or sodium, and have no filaments or cathodes, so are highly resistant to physical shock.

Their applications are diversifying all the time - for example, New York recently chose LEDs for the next generation of its city street lights, and I have successfully washed the 13m-high facade of a multi-storey car park in intense red, using a number of 18-watt LED burial fittings.

Go to any modern lighting show and you will be assailed by a baffling profusion of LED lighting products of all types, shapes and sizes, ranging from petite marker lights, through linear wall-washers, to substantial exterior projectors.

NOT ALL GOOD NEWS However, this mushrooming of the market is not all good news.

The performance of LED light fittings depends on two things:

? the quality of the LED component itself (Osram, Nichia and Lumileds are reliable names) ; and - how well that LED light source is incorporated into the luminaire (light fitting) housing - in other words, the engineering. Even the best LEDs will underperform and/or fail if the light fitting they are mounted in is badly engineered.

LED operation is highly sensitive to temperature - if LEDs overheat, their life and light output plummets in a very short time. The more LEDs you bundle together, for example in larger spotlights and projectors, the more internal heat they generate. Well-engineered LED light fittings have effective heat sinking to disperse the heat generated at the junction of the LED and the micro-chip on which it sits.

However, in their stampede into a booming market, many manufacturers are not building light fittings around the requirements of this light source, but are simply slinging good, bad or indifferent LEDs into any old available housing.

In many cases, the result will be over-heating, a reduction in light output and failure.

HISTORY LESSON For those of us old enough to remember the 1980s, there is an obvious precedent for this in the early history of low-voltage tungsten-halogen lighting.

Once again, the novelty of low voltage led to a charge into the market by every manufacturer - the result was a spate of transformer burn-outs (even resulting in some fires) and frequent lamp failure, which led to many designers losing faith in low-voltage technology - despite its evident advantages.

The actual material from which the LED housing is constructed is important too. Many smaller products (recessed LED marker lights, for example) boast 'stainless steel' construction - and we all trust stainless steel, don't we? Well in this case, we'd be wrong, because steel is a poor heat transmitter, compared to aluminium or brass, and so will disperse the LEDs' intrinsic heat less effectively.

For external architectural use, the ingress protection (IP) rating is also crucial. I would never specify anything lower than IP68 (ie. potentially submersible) because this ensures that the fittings won't suck in water vapour as they cool down - moisture and micro-chips simply don't mix.

Yet many otherwise reputable manufacturers still only rate their exterior LED fixtures as IP67.

RIGHT WHITE?

Colour quality and stability are still a problem, too, particularly with 'white' LEDs. While the quality of today's colour LEDs is superb - much more vibrant and intense than colour created by filters on conventional light sources - the quality of 'white' LEDs still leaves something to be desired.

There are two ways of producing 'white' LED light - by mixing the output of red-blue-green LEDs or through the new generation of dedicated single white LEDs. The former method can produce coloured striations in the beam and a rather pinky tint - and so-called 'white' LEDs are still rather 'cold' in appearance and definitely inferior to conventional light sources. This makes them unsuitable for applications such as stores or art galleries, where the true colour of the objects being lit is crucially important.

Many of these issues are rapidly being addressed, both by LED companies and by the more clued-up luminaire makers - and none of this is intended to dissuade specifiers from using this very versatile and exciting new light source.

But due care in specification - and some probing questions to suppliers - should be the order of the day. As ever, beware all those who come bearing gifts.

Lumileds publishes a list of approved luminaire manufacturers for its products at www. lumileds. com/ solutions/network. cfm

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