Giving LEDs a cozy, warm glow

6 hours ago At low current the coated LEDs emit a cozy, warm glow compared to uncoated reference LEDs. Credit: Hugo J. Cornelissen

When the 2014 Nobel Prize in physics was awarded this October to three Japanese-born scientists for the invention of blue light emitting diodes (LEDs), the prize committee declared LED lamps would light the 21st century. Now researchers from the Netherlands have found a novel way to ensure the lights of the future not only are energy efficient but also emit a cozy warmth.

"We demonstrated a seemingly simple - but in fact sophisticated - way to create LED lights that change in a natural way to a cozy, warm white color when dimmed," said Hugo Cornelissen, a principal scientist in the Optics Research Department at Philips Research Eindhoven, a corporate scientific research entity owned by the company Royal Philips in the Netherlands.

Cornelissen and his colleagues from the Eindhoven University of Technology, Netherlands describe their new LEDs in a paper published today in The Optical Society's (OSA) open-access journal Optics Express.

Incandescent lamps naturally emit warmer colors when dimmed, and Cornelissen said our general preference for redder colors in low-light situations might even have developed far back in time, when humans "experienced the daily rhythm of sunrise, bright daylight at noon, and sunset, each with their corresponding color temperatures."

LEDs, however, don't normally change color at different light intensities. Other groups have used multiple color LEDs and complex control circuitry to make lights that turn redder as the power is turned down. The added complexity comes with its drawbacks: multiple components can increase the cost and the risk of failure, and mixing the light from multiple LEDs without creating color shadows and other light artifacts is a tricky business.

The Dutch research team tried an entirely different approach to creating cozy LEDs. The scientists had noticed that when they embedded LEDs in coated textiles or transparent materials, the color of the emitted light would sometimes change.

"After finding the root cause of these effects and quantitatively understanding the observed color shift, we thought of a way to turn the undesired color changes into a beneficial feature," said Cornelissen.

Starting with White LEDs

They began with cold white LEDs, which can be made from blue LEDs surrounded by a material known as a phosphor. Part of the blue light is absorbed by the phosphor and re-emitted at a different color. The multiple colors combine to form white light.

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Giving LEDs a cozy, warm glow