At low current the coated LEDs emit a cosy, warm glow compared to the uncoated LEDs [Credit: Hugo J Cornelissen]

Scientists give LEDs a warm glow

Scientists working for lighting giant Philips have discovered a novel way of making the light from LEDs warmer, a longstanding challenge for designers.

The 2014 Nobel Prize in Physics was awarded this October to three Japanese-born scientists for the invention of blue LEDs, with the prize committee declaring that LED lamps would light the 21st century.

However a perennial problem for the energy-efficient light source has been recreating the cosy red glow found in incandescent bulbs as they are dimmed, rather than the harsh blue-tinged light commonly associated with LEDs.

Now researchers from Philips and the Eindhoven University of Technology have discovered a new coating for the LEDs that achieves the desired effect without the complexity of multiple colour LEDs and complex control circuitry used in other methods.

“We demonstrated a seemingly simple – but in fact sophisticated – way to create LED lights that change in a natural way to a cosy, warm white colour when dimmed,” said Hugo Cornelissen, a principal scientist in the Optics Research Department at Philips Research Eindhoven.

LEDs don’t normally change colour at different light intensities, but the Dutch research team had noticed 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,” Cornelissen explained.

The team started with cold white LEDs made from blue LEDs surrounded by a phosphor, which absorbs part of the blue light and re-emits it as a different colour that combines with the transmitted blue to form white light.

Cornelissen and his 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 is absorbed and re-emitted by the phosphor.

In a paper published in The Optical Society’s (OSA) open-access journal Optics Express, the scientists describe how they made a coating from a composite of liquid-crystal and polymeric material that normally scatters light, but if it is heated above 48°C 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, but as 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.

After fine-tuning their design they have created lights that comply with industry lighting standards across a range of currents and colours, which they say could help speed up acceptance of LED technology, especially in the household and hospitality markets, “where there is a need to create a warm and cosy atmosphere,” according to Cornelissen.

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

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