While the cool white XR-E produces 80 lm typical at 350 mA, the luminous flux available from warm white devices on the market is considerably lower, typically around 30 lm for a correlated color temperature (CCT) of 3500K. This of course affects the economic argument for using warm white LEDs in place of incumbent lighting technologies.
There are some exceptions such as Nichia's NS6L083 1W product, which is rated at a luminous flux of 52 lm (the median value in the distribution) at 300 mA for 3500K, and 48 lumens at 2800K. This LED contains six small chips, resulting in high efficacy, while Nichia's Power Rigel, a single-chip 1.2W power LED, is rated at 32 lm at 350 mA at 2800K, and has a maximum operating current of 650 mA.
In contrast, Cree limits its warm white XLamp 7090 XR series LEDs (the version preceding the XR-E) to 350mA, while the rest of its product line is rated at 700mA and higher. The reason is to ensure that customers can achieve the stated datasheet performance values with reliability measured at standard conditions of 85°C and 85% relative humidity.
The main issue with warm white LEDs, according to Cree's Mark McClear, has been with the stability of the phosphor systems in the presence of high current densities and high junction temperatures. "These conditions cause the red components of the [warm white] phosphor to degrade and stop emitting light at these wavelengths," he says. "This manifests itself as a pronounced shift towards the blue (cooler color temperature) end of the CCT range." When the LED's chromaticity shifts, the device can quickly change from warm white to cool white. This is particularly unacceptable for indoor applications in which warm-white LEDs are seeking to replace incandescent lamps.
McClear says that Cree's qualification requirements for white LEDs are that the (x,y) chromaticity shift should remain within one bin. "To meet this requirement, we have maintained the same rigorous 85°C/85% conditions, but have had to limit the drive current of this product to 350 mA," he says. "Others have solved this problem by reducing the conditions on their testing to 85/60 or 60/90."
Chromaticity shift is not limited to warm white LEDs. Seoul's new P4 power LED package has a nominal CCT of 6500 K, and a typical output of 85 lm at 350 mA, equivalent to 76 lm/W. Elsewhere on the same P4 datasheet, there is a footnote to say that "the chromaticity coordinate of the LEDs can shift approximately x = 0.02, y = 0.03 in the direction of blue 1000 hours later." So during that time period the CCT could shift to approximately 8200 K. These figures suggest that Seoul's phosphor is very efficient, hence the excellent luminous flux and efficacy, but is not particularly stable over time.
Interestingly Nichia has a slightly different view of the warm white issue. Nichia America's Dan Doxsee agrees that color shift for warm white is greater than for cool white. "It is not well understood why, and our researchers in Japan are investigating this," he says. "It appears to be related to packaging materials and heat, more than phosphors." One factor with warm-white LEDs is that they contain a higher mass of phosphor, which has an insulating effect and causes the local temperature in the package to be slightly higher.
Doxsee believes that the lower efficacy of warm white is the main issue today. "The efficacy of red phosphor(s) varies dramatically from company to company," he says. "The Nichia warm white efficacy is the industry's highest since we have a very good red phosphor; Nichia has been a phosphor company for decades. In the incandescent (warmest) rank of 2800K, which is the lowest efficacy rank, the efficacy is 80% of the cool white value."
Even so, Doxsee says that Nichia is continuing to work on phosphors and packaging materials to improve the quality of its warm white devices. The next generation of the NS6L083, launching in the second half of 2007, will be 40% brighter than the current version for both cool and warm white color ranks.
Cree expects to launch a warm white version of the XR-E in the first half of 2007. McClear says the new product will address the issues of limited drive current and chromaticity shift. Not before time, it appears that warm-white LEDs are poised to make significant improvements during 2007.