December 17, 2014
LED lighting has numerous benefits, but dimming is one area that has presented significant challenges. One problem is the fact that LEDs do not normally change colour as their intensity is varied – turning the lights down in an LED lit room does not produce the warm glow associated with traditional incandescent bulbs.
A possible solution to this problem is to use multiple coloured LEDs and control circuitry to achieve redder output at lower powers. However, the added complexity of mixing light entails additional cost and potential technical pitfalls.
Now a Dutch research group has demonstrated a new approach that may circumvent these problems. They base their system on cold white LEDs made from phosphor-coated blue LEDs (this is standard technology – the phosphor absorbs and re-emits the blue light at different wavelengths, which combine to produce white light).
Physics dictates that a warmer white light will be emitted if a higher proportion of the blue light is converted. The researchers discovered a way to do this in a temperature dependent way, by coating the LEDs with a layer of specially designed material.
The material coatings consist of a liquid crystal and polymer composite that is transparent at high current and temperature, but scatters at lower powers. Therefore, as the LED is dimmed, the coating starts redirecting the emitted light back to the phosphor where it is absorbed and re-emitted at another wavelength, producing the ‘warm glow’ effect.
Using multiple phosphors in the white light LED allowed for the emission of warm white light in the range between 2900 K and 4150 K, and with a chromaticity complying with the ANSI standards.
Lead scientist, Hugo Cornelissen of Philips Research Eindhoven, comments: “We might see products on the market in two years, but first we’ll have to prove reliability over time.”
Thermoresponsive scattering coating for smart white LEDs J. Bauer et al., Optics Express, Vol. 22 Issue S7, pp. A1868-A1879 (2014)