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New research finds novel way to make LEDs warmer

Dutch researchers have found a novel way to give LEDs a warm glow when dimmed, which they hope will increase LED use in homes.

The research, published in Optics Express, describes embedding LEDs in coated textiles or transparent materials to change the colour of emitted light.

“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 colour when dimmed,” lead researcher Hugo Cornelissen said.

Incandescent lamps are naturally warmer when dimmed, and humans show a preference for warmer, redder lights in low-light situations, which Dr Cornelissen said was believed to have developed when humans “experienced the daily rhythm of sunrise, bright daylight at noon, and sunset, each with their corresponding colour temperatures”.

LEDs, however, don’t normally change colour at different light intensities.

Ways around this have included grouping multiple coloured LEDs and using controls to increase red light as the power is turned down. This added complexity, the researchers said, could lead to increased cost and failure rates – and mixing light from multiple LEDs without creating colour shadows and other light artefacts could be difficult.

The Dutch scientists took a different approach, noticing that when they embedded LEDs in coated textiles or transparent materials, the colour of the emitted light would sometimes change.

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

The researchers began with cold white LEDs, which can be made from blue LEDs surrounded by a a phosphor. Part of the blue light is absorbed by the phosphor and re-emitted at a different colour. The multiple colours combine to form white light.

Dr Cornelissen and colleagues knew that the colour of the white light could be shifted toward the warmer end of the spectrum if more of the blue light was absorbed and re-emitted by the phosphor.

A novel, temperature-dependent way to create this shift was created using a coating made from a composite of liquid crystal and polymeric material. The composite normally scatters light, but if it is heated above 48 degrees Celsius, the liquid crystal molecules rearrange and the composite becomes transparent.

When the team covered white LEDs with the coating and turned up the power, the temperature increased enough to make the coating transparent, and the LEDs emitted a cold white colour. When the power was turned down, the coating reorganised into a scattering material that bounced back more of the blue light into the phosphor, generating a warmer glow.

The scientists fine-tuned the LED design and used multiple phosphors to create lights that comply with industry lighting standards across a range of currents and colours.

“We might see products on the market in two years, but first we’ll have to prove reliability over time,” Dr Cornelissen said. “That is one of the important things to do next.”

The team believes the new lights could help speed up the acceptance and widespread use of LED technology, especially in the household and hospitality markets “where there is a need to create a warm and cozy atmosphere,” Dr Cornelissen said.

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