Warm LED without blue

Warm white LED prototype reduces rather than masks blue light

Image credit: Jakoah Brgoch

Researchers from the University of Houston have developed a prototype LED which produces a warm white light. Unlike existing warm LEDs, it manages this by reducing rather than masking blue light.

In a move towards energy efficiency, many people have replaced their incandescent lights (which convert less than 5 per cent of the energy they consume into visible light) and fluorescent bulbs with LED bulbs. LED bulbs are not just more energy efficient; they also have a long lifespan and can be switched on and off more quickly than alternatives.

However, LED bulbs currently on the market emit a lot of cool blue light, which has been connected with eye problems (including cataract formation) and poor sleep when used in the evening, due to disruption of sleep-inducing hormones like melatonin.

The Houston researchers have created a prototype of an entirely new type of LED which reduces, rather than masks, the blue frequencies of light, while also making colours appear as they do under natural sunlight.

Inside an LED bulb, an LED chip converts electrical current into high-energy light, including high-frequency blue, violet, and (invisible) UV light. A cap placed on the chip contains multiple phosphors, which convert this light to visible light of a lower frequency. Each phosphor emits light of a different frequency (colour), and when combined these colours produce broad-spectrum white light. In commercial LED bulbs, blue LEDs and yellow-emitting phosphors are used to create a cold white light similar to daylight.

While the researchers had previously considered creating a warmer white light for evening use with red-emitting phosphors, they found that this only masked the blue hue rather than getting rid of it. Instead, they set out to develop a phosphor which, when used in a violet LED device, would produce a warm white light without the problematic frequency range.

They identified and synthesised a new luminescent crystalline phosphor containing europium. Testing showed that this emitted a light with a colour consistent between “room temperature” lighting and higher operating temperature of commercial LED lighting. The compound appeared to perform well in long-term moisture experiments, showing no change in colour of intensity of light.

Next, they created a prototype LED to see how the new phosphor may perform in a light bulb. They used a violet-light LED covered with a silicone cap containing their new phosphor and red-emitting and green-emitting phosphors. The LED produced the perfect warm light colour while minimising intensity across high-frequency wavelengths, unlike commercial LEDs, and revealing the colour of objects nearly as well as natural sunlight.

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