Dutch Team Develop ‘Warmer’ LEDs

Not all light is created equal. Fluorescent lamps generate a lot of light and little heat, but their blue glare can be harsh. Incandescent light is softer, with a hue that can shift from yellow toward red.

But LEDs are special because they generate light without the need for a lot of energy. It was for that reason that the 2014 Nobel Prize in Physics went to the three scientists “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources.”

So how do you improve on this?

“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 of the private Optics Research Department at Philips Research Eindhoven.

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Cornelissen noted that humans tend to prefer the warmer, redder colors that glow from dimmed incandescent lamps. He said that inclination probably is a vestige of the days before artificial lighting when humans “experienced the daily rhythm of sunrise, bright daylight at noon, and sunset, each with their corresponding color temperatures.”

Dimming an LED lamp, though, doesn’t change the color of its light. Some researchers have experimented with complex circuitry on arrays of LEDs, each with a different color, to achieve that effect. But that technique makes the LEDs not only more expensive but also more likely to fail. Besides, the different colors on the LEDs often create unsightly light shadows.

So Cornelissen teamed up with researchers from the Eindhoven University of Technology Netherlands and took a new approach. They observed that when LEDs were embedded in coated textiles, for example, their colors occasionally changed. Sometimes, though, the color changes weren’t attractive.

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

Here’s what they found: All LEDs emit blue light. But they can be converted to white LEDs if they’re surrounded by a phosphor, a material that becomes luminescent when it encounters light. In the case of Cornelissen’s research, the phosphors reflected enough different colors of light to create white light.

The research team was mindful that white light can be made warmer the more its phosphor absorbs and re-emits blue light. So they developed a coating made up of liquid crystal and a polymer and coated the white LEDs with this mixture.

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With more power, and therefore at higher heat – about 118 degrees Fahrenheit or 48 degrees Celsius – the LEDs glowed a cold white. But when the power was lowered, as with a common household rheostat, the LEDs emitted a warmer glow.

And what are the plans for this LED lamp capable of warm light? “We might see products on the market in two years,” Cornelissen said, “but first we’ll have to prove reliability over time.”

Their research is outlined in a paper published in the journal Optics Express.

By Andy Tully of Oilprice.com

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