Cree smashes white LED record – again

09 May 2011

The pioneering LED chip and component manufacturer demonstrates world-record laboratory performance of 231lm/W with a 'neutral white' emitter.

XLamp MC-E

The US-headquartered LED maker Cree has once again set a new benchmark for the efficacy of white-light production, with a laboratory demonstration of 231 lumens per watt – approximately a 10% improvement on the previous record.

The result, achieved in standard conditions with a single-die component at room temperature and an operating current of 350 mA, beats the company’s previous best of 208 lm/W, which Cree announced in February 2010. Both emitters delivered so-called “neutral-white” light, with a correlated color temperature (CCT) of around 4500 K.

John Edmond, one of Cree’s original founding team and now director of advanced optoelectronics at the Durham, NC, firm, said: “It wasn’t long ago when 200 lumens per watt was considered the theoretical maximum efficiency [sic] for a lighting-class LED. We broke that barrier in 2010.”

“The innovation from our labs is the foundation for our industry-leading XLamp LED family, and an invention that continues our leadership of the LED lighting revolution,” he added.

While the new record represents a research-level breakthrough, likely based on the highest-performing chip from a wafer showing a range of chip performances, Cree has a strong track record of turning those laboratory results into full production – typically after around two years of further development.

The improved conversion efficiency of electrical energy into photons has an additional benefit, since the reduction in heat that is produced by the chip eases thermal management issues and luminaire design.

Among Cree’s best-performing commercial offerings currently – in terms of output efficiacy - are its cool-white (6500 K) XLamp XM-L LEDs, which are used in high-flux applications such as streetlights and industrial settings. They deliver a maximum efficacy of 160 lm/W at 350 mA, which falls to 150 lm/W at 700 mA due to the “droop” effect that impacts LEDs when they are driven at high currents.

Warm-white light
XM-L LEDs are also available in neutral white (4000 K), delivering up to 138 lm/W at 350 mA. Here, the impact of droop if more noticeable, with efficacy falling to 84 lm/W at a drive current of 3 A. In warm-white (3000 K), the preferred color for many indoor residential applications because of its similarity to the traditional incandescent glow, the XM-L emitters provide 117 lm/W at 350 mA.

Cree’s rival Osram Opto Semiconductors is regarded as the industry’s leader in terms of warm-white color, and it recently announced breakthrough performance of 142 lm/W for a laboratory LED with a CCT of 2755 K.

That and Cree’s latest breakthrough will need to be replicated on a volume production basis to drive down the cost-per-lumen measure of solid-state lighting, in line with the rule of “Haitz’ law”, the LED industry’s equivalent to Moore’s law in the conventional semiconductor world.

Named after Roland Haitz when he worked at Agilent Technologies, the observation was first established in 1999, when Haitz and colleagues estimated the cost of LED light at about $100 per kilolumen – about 200 times as expensive as a conventional light bulb.

Using red LED performance as a guide, Haitz’ law predicted that the dollar-per-lumen cost of semiconductor light would decrease by a factor of ten each decade, while the flux per lamp would improve by a factor of 20 over the same period.

The improvement in performance of cool-white LEDs over the past decade has in fact beaten that prediction, and now exceeds that of conventional incandescent lamps. In a paper published late last year, co-authors Haitz and Jeff Tsao from Sandia National Laboratories said that to disrupt conventional lighting, SSL power lamps delivering up to 10,000 lumens with a conversion efficacy of 150-200 lm/W were needed.

The efficacy figures compare with a maximum theoretical limit of 400 lm/W that can be seen by the human eye. Cree’s existing XM-L emitters yield 1000 lumens at 100 lm/W.