November 20, 2014…Researchers from the Netherlands have found a novel method to efficiently emit warm white light from LEDs.“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 Royal Philips corporate research entity in the Netherlands. Cornelissen and his team from Eindhoven University of Technology, Netherlands detail their new LEDs in an article published in the Optical Society’s (OSA) open-access journal Optics Express.
Incandescent lamps naturally emit warmer colors when dimmed. Cornelissen pointed out that our general preference for redder colors in low-light situations might 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.”
Unlike incandescent lamps, LEDs don’t normally change color at different light intensities. Other groups have used multiple color LEDs and complex dimmer control circuitry to make lights turn redder as the power is reduced. The added complexity requires multiple components that can increase the cost and the risk of failure, and mixing light emission from multiple LEDs without creating light artifacts such as color shadows is tricky.
The scientists 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,”Cornelissen said.
They began with white LEDs made from a blue LED and a phosphor. Essentially, the phosphor absorbs and then re-emits the blue light as white.
Cornelissen and his team knew that if more blue light is absorbed and re-emitted then the color 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. The paper describes how they developed a novel, temperature-dependent way of creating a color shift towards the warmer white.
The group produced a coating that combined liquid crystal and polymeric material that scatters light when relatively cool but becomes transparent if it is heated above 48 degrees Celsius (approximately 118 degrees Fahrenheit). At the high temperature, the liquid crystal molecules rearrange and make the composite transparent.
After the team covered the LEDs with the material and increased the power enough to make the coating transparent, the LEDs emitted a cold white light. However, when they reduced the power, the coating restructured itself into a scattering material that bounced back more of the blue light into the phosphor, creating a warmer light. The group later fine-tuned the LED design and used multiple phosphors to create lights that comply with industry lighting standards across a range of currents and colors.
“We might see products on the market in two years, but first we’ll have to prove reliability over time,” Cornelissen said. “That is one of the important things to do next.”