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White LEDs may not be too far away, as an Indian scientist has shown. T.V. Jayan reports.

LEDs are brighter, longer-lasting and more energy efficient than conventional light

The utterly inefficient light bulb that ruled interior lighting for more than a century may soon be extinguished if a Calcutta researcher has his way. Dipankar Das Sarma, a materials scientist at the Centre for Advanced Materials at the Indian Association for the Cultivation of Science has created a material which — when exposed to ultra violet light — produces pure white light that can illuminate homes and offices.

The work by Sarma, also a professor at the Bangalore-based Indian Institute of Science, and his student Angshuman Nag, is being hailed as “an advance” towards obtaining the Holy Grail of the illumination industry — it has the potential to produce new generation, white light light-emitting diodes (LEDs) which are brighter, longer-lasting and more energy efficient than conventional light sources such as incandescent and fluorescent lamps.

Phosphors are materials that that give off distinct hues (depending on their composition and condition) when irradiated with LEDs. Such LEDs are the mainstay of what researchers call solid-state lighting. Modern-day traffic lights are some of the most commonly used single-colour LEDs.

Making an LED that produces white colour is an onerous task because the white colour that we perceive is not a single colour but a mixture of different colours. Still, researchers in the past did produce white LEDs by coating a blend of materials that give off red, green and blue hues, but they all have many inherent weaknesses. First, getting the blend right is tricky. The second and more important problem is that of self-absorption. Phosphors that emit colours that have longer wave lengths will absorb colours of shorter wave lengths produced by other materials. For instance, blue emission from nanocrystals that give out a blue hue will be absorbed by those that emit green and red colours. Over a period of time, the quality of whiteness — or “chromaticity” — deteriorates, resulting in unstable shades of white light that mar the performance of these LEDs.

“We have been able to tackle this fundamental issue of self-absorption,” Sarma told KnowHow. Sarma and Nag achieved this by doping cadmium sulphide nanocrystals with the right number of manganese ions.

The material developed by the Indian researchers is far away from being exploited commercially as only two per cent of the energy that goes into it was coming out as white light. “But, it still is a good beginning,” says P.K. Khanna, who works on LEDs at the Centre for Materials for Electronics Technology, Pune.

It is estimated that if half of world’s lighting switches to LEDs by 2025, the world would save 120 gigawatts of electricity, worth more than $ 100 billion annually.

Source:The Telegraph (Kolkata, India)