Applications Surge for High-Power LEDs

Doug Silkwood, Lumileds Lighting

Until recently, LEDs were best-known as the ubiquitous indicator lights on computers, printers and other electronic equipment. Their small footprint, energy efficiency and long life have posed intriguing possibilities for other applications for many years, but the development of new LED-based products has been stymied by their low light output.

Now brighter, high-flux emitters are changing the picture, giving engineers an illumination source that has implications for many industries. With anticipated advances in luminous output and other specifications over the next 12 to 24 months, LEDs are expected to show up everywhere from chandeliers to medical devices to liquid crystal display (LCD) backlights to aircraft.

Only a few years ago, the brightest LEDs could generate just 1 or 2 lm per emitter. Since then, Lumileds Lighting has released 25- and 120-lm LED packages in both white and color, and other manufacturers are working to boost the brightness of their products as well.

Lumileds has achieved higher brightness with its Luxeon LED product (Figure 1). The large metal “slug” dramatically improves heat transfer characteristics. This, in turn, allows higher current, a larger light-emitting surface and proportionally higher light output. As a result, the 1-W light source in the diagram has a 1-mm-sq area for light emission, operates at 350 mA and generates 25 lm. This is in sharp contrast to the ≤0.1-W, 0.25-mm-sq, 20- to 30-mA and 1- to 2-lm characteristics of a standard 5-mm LED.

The Luxeon LED product (a) has a metal “slug” that provides better heat-transfer characteristics, allowing higher current, a larger light-emitting surface and proportionally higher light output than conventional LEDs (b).

In another important advance, independent testing has validated the longevity of solid-state light sources, demonstrating that, with the proper engineering, high-power LEDs can retain 70 percent of their light output through 50,000 hours of operation. Given these positive developments, designers are expanding their horizons beyond established applications such as indicator lights, traffic signals, signage, automotive interior lights and taillights.

Last year, for example, one manufacturer announced a line of medical monitors with an LED-powered backlight system instead of cold-cathode fluorescent lamps. The new displays will provide tunable white points and improved color fidelity for better medical diagnoses as well as the advantages of longer life and mercury-free operation. Other vendors are now developing LED-based computer monitors for the consumer market.

Also on the drawing board are dental hardening systems using blue LEDs with a record-setting radiant output of 600 mW. The use of LEDs instead of conventional halogen lamps will dramatically reduce curing times while consuming only a fraction of the power, partly because no energy is wasted on filtering out unneeded colors. The LED’s small form factor also permits a lightweight, handheld design with important usability benefits for the dental community.

In the general lighting market, LEDs are already making their appearance in products ranging from accent and under-shelf lighting to flashlights, path lighting and architectural detail lights. In 2003 and 2004, because of increasing light output as well as color-mixing capabilities, they will be incorporated into directed lighting applications such as wall sconces and chandeliers; entertainment and studio lighting; and a new breed of intelligent color-changing light fixtures.

LEDs also are planned for near-term release in skin curing systems, where their cool light beam offers advantages over halogen lamps; in interior aircraft lighting, where their long life will reduce or eliminate bulb replacement and associated maintenance issues; and in automotive headlamps, where the higher light output of the newer LEDs is making it possible to meet the required 800 to 900 lm for the first time. The first LED-based vehicle headlamp was shown on a concept car at the Geneva Motor Show in Switzerland in March, and production models should be on the road within the next few years.

Applications promise to grow exponentially as technical advances continue and as falling prices make high-power LEDs more affordable.

By 2004, we can expect to see LEDs that deliver as much as 200 lm per emitter. The quality of white LEDs will also improve because of changes in the phosphors used to coat the emitters. The 5500-K “cool” phosphor-converted white emitters available today will be joined by “warmer” 3000- to 3500-K white products that will boost the appeal of LEDs for indoor lighting applications.

Energy efficiency will jump as well. In white, LEDs are 15 and 20 percent more efficient than halogen and incandescent lamps, respectively; in two years, that efficiency will double, bringing LEDs within striking distance of fluorescent lights’ efficiency. In color, efficiency gains of up to 75 percent can be expected.

The result will be a new generation of products powered by LEDs. The promise of solid-state illumination is finally being realized.

Meet the author

Doug Silkwood is director of marketing at Lumileds Lighting in San Jose, Calif.