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Warmer whites top DOE's wish list

The research focus on high-performance LEDs will now shift towards more warm-white LED and high-efficacy luminaire development, says a US Department of Energy report.

The race between top LED manufacturers to deliver ever-increasing luminous efficacy will slow down in 2008 and 2009, as companies refocus on expanding their commercial reach.

That s the prediction coming out of the US Department of Energy (DOE) 2009-2014 multi-year program plan for solid state lighting (SSL), which suggests that efficacy improvements will soon start to tail off.

“There are new things to deal with now that we've got the efficacy up where we d like to see it,” said Fred Welsh of Radcliffe Advisors, who helped prepare the report.

“We ve seen efficient products coming onto the market and there's now more emphasis in getting warmer white light into the commercial product,” Welsh said. “There are markets, particularly residential lighting, where it s very difficult to sell cool white LEDs.”

Although the report is put together annually by the DOE with the help of many US-based SSL companies "“ including Philips Lumileds division, Osram s US unit and Cree - in 2007 efficacy improvement achieved in the industry exceeded its predictions.

For example, in September Cree delivered cold-white LED efficacy of 129 lm/W at a color temperature of 5813 K. However, it also pushed up the standard for the intrinsically lower-efficacy warm-white devices to 99 lm/W at a 2950 K color temperature (see related stories).

Off the pace
The DOE report s authors now predict that LED efficacy improvements will slow, before approaching a ceiling of 228 lm/W for cool white and 162 lm/W for warm white devices.

However, the 2007 acceleration has increased the overall milestones in the DOE s plan, which now call for 140 lm/W efficacy cool white and 90 lm/W warm white commercial chips by 2010. The report has also introduced a new LED goal that indicates the focus-shift away from device efficacy - to deliver a 126 lm/W efficacy, 1000 lm output luminaire by 2012.

According to Welsh, this represents a growing feeling amongst the report's authors that more attention should be paid to the specific challenges of designing LED luminaires.

“It s easy to ruin a good LED in a [poor] luminaire - and people are doing it,” Welsh said. “We re very concerned that will sour the market.”

Elsewhere in the report, epitaxy and chip fabrication research has been elevated up the list of research priorities while larger-diameter substrate development has been moved down.

The aim of this, Welsh says, is to improve final LED prices by developing more reproducible chip fabrication. Although larger substrates might also deliver lower costs, standardizing chip production is more central to the report s general focus on reliable manufacturing and long-lasting and desirable products.

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