LED delivers warm white light
University of Georgia scientists have developed the world's first LED that emits a warm white light using a single light emitting material, or phosphor, with a single emitting center for illumination.
The move means that LEDs could finally become more suitable for normal home lighting, says the team.
"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," says Professor Zhengwei Pan.
"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 material has a correlated color temperature of less than 4,000 kelvins and a color rendering index of 85, making it ideal for indoor lighting, says the team..
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, though, can be difficult and expensive, and the resulting color is unreliable because each of the source materials responds differently to temperature variations.
"The use of a single phosphor solves the problem of color stability because the color quality doesn't change with increasing temperatures," says doctoral student Xufan Li.
To create the new phosphor, the team combined minute quantities of europium oxide with aluminum oxide, barium oxide and graphite powders, then heated the powdered materials 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. When the yellow luminescent compound is encapsulated in a bulb and illuminated by a blue LED chip, the result is a warm white light.
But there'ss till some way to go. The new LED is less efficient than today's bluish white LEDs, and scaling production to an industrial scale will be a challenge.
"We still have more work to do," says Pan, "but the color temperature and rendition that we have achieved gives us a very good starting point."