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“LED Update” Columnist Dr. Nisa Khan Comments on 231 Lumens/Watt LED Claim

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"It wasn t long ago when 200 lumens per watt was considered the theoretical maximum efficiency for an LED. We broke that barrier in 2010, and have now achieved 231 lumens per watt," said John Edmond, Cree co-founder and director of advanced optoelectronics. "The innovation from our labs is the foundation for our industry-leading XLamp(r) LED family and an invention that continues our leadership of the LED lighting revolution."

 

Nisa Khan: Although Cree s announcement on improving their white LED efficacy to 231 lumens per watt is noteworthy at CCT of 4500 Kelvin for a single die with a drive current of 350 mA, the quoted statement from Mr. John Edmond is incorrect on several accounts.

 

The lumens per watt figure is not a measure of efficiency as he states, but rather a measure of efficacy. Further, the maximum theoretical efficacy of 200 lumens per watt is for an electric white light for which the entire 1 watt of electrical input power is distributed uniformly with respect to wavelength in the visible region; this ideal white light is "perfect" in the sense that its color rendering index would be 100 by its own right. Cree s white LED, made with blue LED combined with certain phosphor(s), would be far from reaching the ideal CRI of 100 – hence their comparison or mention of the theoretical maximum number here is meaningless. Efficacy can be usually increased at the cost of CRI.

 

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For further reading and understanding of LED efficacy and CRI tradeoffs, please visit my ST article of February 2009 titled, "Understanding LED Energy Efficiency" at http://signweb.com/content/understanding-led-energy-efficiency?page=0%2C1.  

 

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