26 Nov 2014
Human preference for red low-light may have developed long ago, when we experienced "daily rhythm of sunlight".
“We have demonstrated a seemingly simple – but sophisticated – way to create LED lights that change brightness 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, Netherlands.
Incandescent lamps naturally emit warmer (redder) wavelengths when dimmed, and Cornelissen comments that people’s preference for redder colors in low-light situations possibly 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.”
Conventional LEDs, however, do not normally change color at different light intensities. Other R&D 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.
They began with cold white LEDs, which can be made from blue LEDs surrounded by 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.
Cornelissen and his colleagues knew that 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 scientists made a coating from a composite of liquid crystal and polymeric material. The composite 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 color. When the power was turned down, the coating reorganized into a scattering material that bounced back more of the blue light into the phosphor, generating a warmer glow.
Materials and experimental procedures
LEDs: The cold and warm white light LEDs used were models NSSW156 and NSSW157 from Nichia, respectively. The coating applied to the LED comprised: A 50% solution by weight of a 1:2 mixture by weight of compound 3 and ethoxylated bisphenol A diacrylate (Sigma-Aldrich) containing 1% Irgacure 184 (Ciba Specialty Chemicals) as photoinitiator in toluene was stirred at room temperature for 3 hours.
CCT measurements: The CCT and the chromaticities of the emitted light at different currents was measured using a Konica Minolta CL500A spectrometer in a confined black space. The current was applied using a Keithley SMU2400 power source.
The scientists 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.”
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,” Cornelissen said.
About the Author
Matthew Peach is contributing editor to optics.org.