In well under a decade, LEDs have expanded their role from little red spots which tell you the HiFi is on, to include brilliant white light sources capable of illuminating a room or lighting the road ahead.
Production quality LEDs are now capable of efficiencies of around 70 lm/W, compared with 12 lm/W for incandescent light bulbs, and single die devices are producing over 100 lm.
Here, we present a round up - where the latest relevant stories are dynamically updated - of information related to White LEDs.
History of LEDs
Practical white LEDs were made possible by work with GaN semiconductor materials in the 1990s by Dr Shuji Nakamura at Nichia Corporation in Japan.
There has been heated argument as to where the actual credit lies, but it is certain that: the company, the man, and the previous research were all necessary.
Nichia initially concentrated on making white LEDs in the classic 5mm and 3mm LED packages, as well as small surface-mount variants.
The Philips-owned Californian company Lumileds introduced the first successful high-power white LEDs, using larger die (1x1mm) for intensity and complex packages to extract the heat generated.
Cree of North Carolina has joined, or even surpassed, Lumileds as the producer of the brightest and most efficient LED die, and Far Eastern companies are not far behind.
How LEDs work
The 'white' of white LEDs comes from the narrow-band blue naturally emitted by GaN LEDs, plus a broad spectrum yellow generated by a phosphor coating on the die which absorbs a proportion of the blue and converts it to yellow.
'GaN' die are actually InGaN heterostructures, which can produce operational wavelengths from green to ultra-violet by varying the relative amounts of indium and gallium during production.
Although this blue die + yellow phosphor approach yields light which appears white, it has little green and almost no red content leading to inferior colour rendering compared with incandescent bulbs and even 'tri-phosphor' florescent tubes. 'Warm white' LEDs, which include a red-producing phosphor, are an attempt to improve this situation as well as make LEDs illumination more acceptable in living spaces.
LEDs for lighting?
White LED versions of traditional luminaires are already available from several manufacturers, particularly in the MR11 spotlight style.
LED headlights for cars are in the pipeline, notably through the work of Germany's Osram, which is producing a range of multi-die packages that are close to producing enough light for road illumination. LED bicycle headlights are already available.
Reality checking
Firstly, firms which claim 90 per cent efficiency from LED light sources are making it up. Even the latest ones convert far more electricity to heat than they do to light.
Next, according lighting industry experts, LEDs will remain expensive compared with light bulbs and florescent tubes. And whilst they will increasingly appear in homes and offices, they will almost certainly will not replace florescent tubes in office lighting.
Florescent tubes, at 100 lm/W for the best fittings, are equal in electrical efficiency to the best LEDs.
In the future, large area sheet emitters based on organic LED or AC electroluminescent technology are likely match LEDs in efficiency and to cost far less per lumen to manufacture.
One of these two will probably end up replacing florescent tubes in office lighting and eventually most light bulbs in the home.
In their favour, LEDs are the only real alterative if colour-tuneable accent lighting or colour-tuneable whole-room lighting is required.
And lastly, at the time of writing, large arrays of 5mm LEDs compete well with fewer high-power LEDs in terms of efficiency, cost and heat dissipation.
In the following, we bring together resources from Electronics Weekly and UK and EU governmental bodies to provide detailed reference information about LED technology, specifically white LEDs.
Related Links:
LED technology - Coloured LEDs
