A group of researchers have developed LEDs that can emit redder, warmer colours when dimmed. To create shades that are more favourable to the human eye, the bulbs are coated with composite material that absorbs different wavelengths of light as the temperature of the LED changes. It is hoped that the invention could speed up the widespread use of LEDs, especially in household and hospitality markets where a 'warm and cosy’ atmosphere is desired.
The scientists from the Eindhoven University of Technology, Netherlands described the LEDs in a paper published in the journal Optics Express.
According to Hugo Cornelissen, a principal scientist at the Optics Research Department, Philips Research Eindhoven, a human’s preference for redder colours in low-light conditions may have developed back in time, when they ‘experienced the daily rhythm of sunrise, bright daylight at noon, and sunset, each with their corresponding colour temperatures.’
Incandescent lamps naturally emit warmer colours when dimmed, however, LEDs do not normally change colour at different light intensities.
To cause the colours to change, the Eindhoven research team first produced white LEDs by coating blue LEDs with phosphor material. Part of the blue light is absorbed by the phosphor and re-emitted at a different wavelength, and the multiple colours then combine 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. To form a temperature-dependent technique to create this shift, a coating from a composite of liquid crystal and polymeric material was produced. The composite normally scatters light, but if it is heated above 48°C, the liquid crystal molecules rearrange and the composite becomes transparent.
When white LEDs were covered with the coating and the power was turned up, 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.
Image 1: At low current the coated LEDs emit a cozy, warm glow compared to uncoated reference LEDs. Credit: Hugo J Cornelissen
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 colours.
The new technique has advantages over previous methods, which included using multiple colour LEDs and complex control circuitry. As this technique is more complex and required a higher number of components, meaning that the cost and the risk of failure are increased.
The researchers from Eindhoven University are currently carrying out further tests before the LEDs are ready for commercialisation, Cornelissen explained: ‘We might see products on the market in two years, but first we'll have to prove reliability over time.’