High-brightness LEDs continue to make inroads into lighting applications that were traditionally dominated by incandescents and other light sources. Their use in traffic signals, automotive brake lights, architectural lighting, and full-color displays keeps growing. Designers also are finding new ways to apply these versatile semiconductor components as they improve their performance.
LEDs promise significant reductions in power consumption compared to incandescents, as well as longer life and greater reliability. Moreover, improvements in LED semiconductors and packaging are increasing their light output, making them brighter and reducing their cost versus the replaced sources. Consequently, LEDs are becoming viable in more applications.
For example, improvements in LED technology will soon make it possible for LEDs to supplant cold-cathode fluorescent lamps (CCFLs) as backlighting sources in notebook computers. Backlighting is becoming a very important application for high-brightness (HB) LEDs due to the availability of brighter white LEDs. As handheld devices move to full-color LCD displays, LEDs are being used to provide the white sources of backlighting required by color displays in cell phones, PDAs, digital cameras, and camcorders.
Beyond backlighting, though, are innumerable other lighting applications for LEDs, if their performance can be improved and their cost lowered sufficiently. As the global lighting market accounted for $12 billion in sales last year, LED developers have ample motivation to push their technology further.
But members of the LED industry aren't alone in this quest to advance the technology. An effort is now under way to establish a government and industry partnership to speed LED and organic LED-based (OLED) lighting development. Collectively, they're called solid-state lighting.
Last July, legislation to fund the Next-Generation Lighting Initiative (NGLI) was introduced into Congress as part of the Senate Energy Bill S.1766, Sec.1213. This act would create a 10-year program through which the Department of Energy and an industry-led consortium would jointly conduct the R&D needed to make solid-state lighting a primary general-lighting source. The Optoelectronics Industry Development Association (OIDA) is facilitating the industry consortium.
Solid-state lighting adoption is expected to dramatically affect energy consumption and pollution. In the U.S., it could reduce the demand for electrical energy by 10% by 2020. The new light sources would consist of flat arrays of LEDs or laminates of OLEDs, assembled in any pattern or form, then mounted to floors, walls, ceilings, or even furniture.
But before such futuristic lamps can be manufactured, the LED industry must further improve the performance of HB devices. That means higher values for power conversion efficiency (lm/W), operating life, color rendering index, and light output per device. At the same time, it's critical that cost, measured in dollars per thousand-lumen, be reduced dramatically.
One of the NGLI activities is developing a roadmap of technical performance goals for each of these parameters (see the table). Along the way, the roadmap establishes the criteria necessary for white LEDs to replace incandescent lamps, and later fluorescent lamps.1, 2 The table's data is only tentative. Final numbers are expected later this spring.
Parameters like those shown in the table are useful tools for describing the state-of-the-art for solid-state lighting and drawing comparisons among existing lighting sources. For instance, a comparison of the lumens per watt efficiencies of red and green LEDs with their incandescent and fluorescent counterparts, which require filtering, shows higher efficiencies for the LEDs. Viewing these same sources in terms of cost measured in dollars per lumen may also give LEDs the edge, when life-cost factors, like lamp replacement and energy savings, are considered. This is why LEDs are now commonplace in traffic signals and other long-life applications.
For white LEDs, the situation isn't that advanced yet, particularly when using them in general lighting applications. In terms of efficiency, white LEDs are already competitive with incandescents in terms of lumens per watt. Still, the LEDs' cost per lumen is much higher. Efficiency comparisons with fluorescents are even less favorable.
But numbers only tell part of the story. Because LEDs are so different from incandescents and fluorescents, they can't be viewed simply on a lamp versus lamp basis. As Makarand Chipalkatti, director of lamp module development at Osram Opto-Semiconductors, explains, when changing to solid-state lighting, "You're not replacing the lamp, you're replacing the lighting system." For example, before viewing different light sources in terms of an illumination specification like candellas/m2, designers must consider how much light generated by the incandescent or fluorescent is actually blocked by the fixture.
Beyond those comparisons, solid-state lighting has profound implications for both lighting and architectural design. Solid-state lights will offer other advantages besides reducing power consumption and extending lamp life ("cost of use" factors). It will be possible to dim them without affecting their color, efficiency, or operating life, and to create electronically controlled full-color light sources.
Furthermore, the low profile of tile-shaped LED light sources could call into question the need for drop ceilings that currently hide fluorescent lighting fixtures. Although not the only elements overhead, lighting fixtures may be the critical ones. So consider that in a multistory building, replacing overhead fixtures and their associated drop ceilings could add up. Space savings may be equivalent to an additional story.
Even now, LEDs are enabling the development of lighting designs that were previously impractical. A striking example of such LED lighting illuminates the text frieze that encircles the interior of the Jefferson Memorial in Washington, D.C. (Fig. 1). In this application, a series of 17,000 surface-mount LEDs are assembled on 17-in. linear strips, mounted on a high ledge just below the frieze. The entire 250-ft. long fixture mixes white and yellow LED strips to produce a hue of light that nicely matches the marble wall. Before the LED lighting was installed, the frieze re-mained unlit because the ledge holding the LED strips was too shallow to sit a conventional light fixture on.