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Technological developments in LEDs:

More light and better colour rendering

FRANKFURT - The luminous efficacy of LEDs has been doubling every two years or so, and has now easily passed the 20 lm/W mark. However, it depends on the colour of the LED and on the design.

LEDs in series production currently achieve more than 30 lm/W for red and yellow LEDs as standard, and in the laboratory it has been shown that values in excess of 100 lm/W are possible. White LEDs based on luminescence conversion achieve 15 to 25 lm/W as standard, and as much as 50 lm/W in the laboratory.

These constant improvements in the luminous efficacy are the result of immense technological progress and special R&D know?how among LED manufacturers. In the past few years, tremendous advances have been made above all through breakthroughs in production techniques and product engineering, coupled with changes in the design of the chips and the cases. Increasing the size of the chip and developing cases with improved heat removal led to remarkable results. As series of experiments have shown, the larger Golden Dragon LEDs from the Top-LED family, which will be available soon from Osram Opto Semiconductors, will provide around seven times more light than the Power Top-LEDs. The trend will continue and will also benefit the LED modules as they become equipped with the latest generation of LEDs.

Improved homogeneity and colour rendering
Despite increases in brightness, there is still an impression, particularly with white LED modules, that the light in irregular in colour, even if all the LEDs used come from one colour group. The reason for this is the large tolerances that permit a relatively broad wavelength spectrum in each colour class. Improved testing and sorting procedures now enable the colour groups to be subdivided more finely and the relevant wavelength spectrum to be defined more closely. The individual LEDs in these colour groups hardly differ at all in their wavelengths, so the LED modules in which they are used produce homogeneous white light. The colour rendering index at present is Ra 80.

For area lighting there is another way of improving the colour rendering. If white LEDs are combined with coloured LEDs, any missing colour variants can be compensated for. By selecting the right LEDs and controlling individual colours it is now also possible to switch between different colour temperatures while retaining more or less the same colour rendering quality.

LEDs have come a long way from puny red dots of light to the powerful sources of light in different colours that are available today. In many areas they are the modern alternatives to conventional lamps. Red, orange, yellow, green, blue or even white - LEDs can now be found in hifi display panels, mobile phones, personal digital assistants (PDAs), in information displays at railway stations and bus stops, in illuminated advertising, in traffic signals and in car dashboards.