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  • New Material Warms White LEDs

Photonics.com
Jan 2013
ATHENS, Ga., Jan. 21, 2013 — The cold, bluish light of current white LEDs has precluded their widespread use for indoor lighting, but a new LED prototype uses a single light-emitting material, or phosphor, with a single emitting center to produce a warm white glow that could bring hope for extensive use of LEDs in the home.

The University of Georgia prototype is thought to be the world’s first single-phosphor, single-emitting-center-converted LED. The new phosphor combines minute quantities of europium oxide with aluminum oxide, barium oxide and graphite powders heated to 1450 °C in a tube furnace. The vacuum of the furnace pulls the vaporized materials onto a substrate, where they are deposited as a yellow luminescent compound, which is encapsulated in a bulb and illuminated by a blue LED chip to emit a warm-white light.

“Right now, white LEDs are mainly used in flashlights and in automotive lamps, but they give off a bluish, cool light that people tend to dislike, especially in indoor lighting,” said Zhengwei Pan, an associate professor of physics at the university’s Franklin College of Arts and Sciences and the College of Engineering. “Our material achieves a warm color temperature while at the same time giving highly accurate color rendition, which is something no single-phosphor-converted LED has ever been shown to do.”


The University of Georgia’s Zhengwei Pan, center, an associate professor of physics and engineering, holds a prototype of what is thought to be the world’s first single-phosphor, single-emitting-center-converted LED that emits a warm white light. Postdoctoral researchers Feng Liu, left, and Xufan Li look on. Courtesy of University of Georgia.

The quality of artificial light is assessed by two main variables, Pan said. Correlated color temperature measures the light’s coolness or warmth; temperatures of less than 4000 K are ideal for indoor use, while temperatures of about 5000 K give off a bluish color that white LEDs are known for. The other important measure, color rendition, is the ability of a light source to replicate natural light. A value of more than 80 is ideal for indoor lighting, while lower values result in colors that don’t seem true to life.

The material developed by Pan and colleagues has a correlated color temperature of less than 4000 K and a color rendering index of 85, making it ideal for indoor lighting, they said.

It is possible to create warm-white light by using a blue LED chip coated with phosphors of different emitting colors to create what are called phosphor-based white LEDs. Producing them, however, can be difficult and expensive. In addition, the resulting color is unreliable because each source material responds differently to temperature variations.

UGA’s single phosphor “solves the problem of color stability because the color quality doesn't change with increasing temperatures," said doctoral student Xufan Li.

However, there’s still a ways to go before the single-phosphor LED can be used in the home. The new material is less efficient than today’s bluish white LEDs, and scaling production to an industrial scale will be a challenge, Pan said.

"We still have more work to do," Pan said, "but the color temperature and rendition that we have achieved gives us a very good starting point."

Findings were reported in Light: Science and Applications (doi: 10.1038/lsa.2013.6).  

For more information, visit: www.uga.edu


GLOSSARY
correlated color temperature
Temperature of the blackbody having chromaticity nearest to that of the test source on a specified chromaticity diagram.
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