Nichia's LEDs hit 249 lm/W
By Richard Stevenson in San Jose, California
Researchers at Nichia have grabbed the attention of their LED industry peers by cranking up the efficacy of their white emitters to 249 lm/W at 20 mA.
However, the results reported at Photonics West on January 29 show that this falls to 145 lm/W at 350 mA, the drive current often used to compare the output of today s high brightness LEDs.
This decline in efficacy with current is incredibly steep, and Nichia s Takashi Mukai told delegates that it might be due to a mistake in the process.
Mukai initially introduced the result by stating that the theoretical efficacy limit for white LEDs is 263 lm/W. Yet, when questioned by the audience he explained that Nichia has pushed this ceiling to over 300 lm/W thanks to the introduction of a new phosphor technology.
Further questions revealed that the LED shared two features associated with the company s best devices from 2006 "“ a hexagonally patterned sapphire substrate and a transparent indium tin oxide contact.
In comparison to Nichia s latest chips, Cree and Osram claimed 161 lm/W and 136 lm/W laboratory demonstration efficacies respectively at 350 mA in 2008 (see related stories).
Mukai s colleague, Atsuo Michiue, detailed the performance of a buried-stripe 445 nm laser with a double quantum well active region and a 15 µm by 2 µm emitting area. No catastrophic optical damage - the common failure mode for high-power semiconductor lasers - was observed at output powers of up to 3W.
This edge-emitter produced a threshold current of 185 mA, a wall plug efficiency that peaked at 24.3 percent, and had an estimated lifetime of over 30,000 hours. Far-field beam divergence measurements were 38.2° in the perpendicular direction, and 11.7° in the parallel direction.
One member of the audience asked if Nichia had managed to push the wavelength of its lasers beyond 488 nm, only to be met by the terse reply “top secret”.
