Without swapping current between a warm LED and a cool LED?
It is, according to researchers from Philips.
They coated a cool white LED with a layer of thermally-responsive liquid crystal.
When it is hot, because the LEDs is being driven hard, the coating is transparent and cool white light leaves the device uninterrupted.
When the coating is cooler, because the LED is not being driven so hard, it turns from transparent to scattering (milky).
Because the coating is now scattering, it reflects some of the cool white back into the LED’s phosphor, where the phosphor gets a second chance to convert the blue content to longer wavelengths, causing the LED emit warmer white.
And there is a subtlety in the phosphor used.
It is a mixture of blue-to-yellow phosphor, and a phosphor that converts both blue and yellow to red.
This combination improves the shift to warmer whites as temperature falls, achieving the black body curve within five MacAdam ellipses between 3,200 and 4,150K.
The change was repeatable with little variation over many cycles.
An Optical Society paper, ‘Thermoresponsive scattering coating for smart white LEDs‘, describes the work.
The team also included scientists from Eindhoven University of Technology and Queen Mary, University of London.
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