Purple-emitting LEDs Get Closer to the Sun

Seoul Semiconductor Co. Ltd., in partnership with Toshiba Materials Co. Ltd., has created an LED that it says more closely mimics the spectrum of natural daylight.

Called SunLike, the LEDs combine Seoul Semiconductor’s high-brightness purple LEDs with advanced red, green, and blue (RGB) phosphors developed at Toshiba Materials. Up to now, most approaches have mimicked daylight by combining blue-emitting LEDs with yellow and red phosphors to fill out the rest of the spectrum, but that method results in peaks in the blue spectrum.

Blue peaks are undesirable because the amount of blue light the human eye can accept is limited. Over-illumination with blue light results in scatter, which distorts the texture and color of illuminated objects. Research also suggests that exposure to excess blue light can have negative health effects related to interruption of the circadian rhythms.

The secret to SunLike’s performance is the phosphors, said Seoul Semiconductor CEO Chung Hoon Lee. “Other companies that have tried to use purple LEDs to mimic daylight still suffer from purple peaks,” he said. “Toshiba Materials’ new phosphor is perfect, and that’s the difference.”

(a) Seoul Semiconductor’s purple LED combines with Toshiba Materials’ RGB phosphors to create a daylight-like spectrum. (b) A traditional lighting LED uses a blue LED with YR phosphors, an approach that results in unnatural peaks in the blue spectrum.

The RGB phosphors used in the SunLike product underwent extensive engineering development at Toshiba Materials to produce a technology Toshiba calls TRI-R. A new phosphor paste coating process was also developed using a mixture of organic resin to minimize energy loss.

“As of today, there is no other manufacturer who can make daylight-like products in terms of spectrum,” said Katsuhiro Shinosawa, director and chief marketing executive at Toshiba Materials Japan. “Others are offering high CRI [color-rendering index] products, but our products have quite different properties in terms of spectrum—similar to the solar spectrum.”

Seoul Semiconductor’s Lee said SunLike’s luminous efficiency in lumens/watt is around 10 percent lower for than for existing lighting products. “We will work to make this equal within a year,” Lee said. “Color quality is the most important thing right now.”
Market trends in LED chips over the years have focused on efficiency, price, and added features for the luminaire, such as connectivity. “All these features have been given priority over the quality of light,” said Andreas Weisl, managing director and vice president of sales for Europe at Seoul Semiconductor.

Weisl said one product line will hit the European and U.S. markets immediately: a chip-on-board (COB) series intended for general residential, architectural, health-care, and retail lighting. A line of mid-power surface-mount products, targeting linear applications such as tubes, will launch in July. That series will use Seoul Semiconductor’s multijunction technology (MJT), which places more than one LED on a chip, doing away with wire bonds and allowing the junctions to be driven at higher voltages.

Seoul Semiconductor also has a “clear intention” to put SunLike LEDs into its package-free WICOP technology, Weisl said. Use of the package-free form factor will improve uniformity, system cost and efficiency and allow clustering of the chips, he said. The WICOP products should reach the market in 2018.