Successful commercialization of LED products for general illumination, however, will require a number of factors, including low cost, longer lifetimes, higher intensity levels, better luminous efficacy, and improved color quality. All of these issues are under investigation with promising results in the lab.
Cree Inc. achieved a record 161 lm/W of efficiency for white-power LEDs. An independent survey on the adoption, benefits, and deployment benefits of LEDs taken by Mindwave Research Inc. on behalf of Cree validates projected strong LED lighting applications. The survey also confirms the awareness of the energy-efficient benefits of LEDs and indicates that the adoption trend should continue through the end of the decade.
LEDs are competing with CCFLs, both of which are emerging in all types of energy-saving lighting applications. CCFLs have established a niche in mainstream applications, but their lifetimes are limited compared to LEDs. On the other hand, LEDs are still comparatively more costly to move into mainstream applications.
Expenses are changing as LED manufacturing methods improve and prices drop. However, it will take a few more years to reap mass benefits from LED displays. Many lighting experts predict that the breakthrough in LED general lighting will occur by 2012 or 2013.
High-brightness LEDs have already carved out a niche in lighting for signs. Industry experts expect LEDs to eclipse neon lighting this year and widen their share in the years to come. LEDs should also see expanded use in appliances and automobile headlights.
High heat levels are a sticking point for high-brightness LEDs, though. According to the U.S. Department of Energy (DoE), 75% to 80% of the energy used to drive LEDs is converted to heat, which can reduce light output and produce a color shift. A solution is out there via better materials and improving driver techniques, though.
One novel method for cooling LEDs comes from Nuventix Inc. The oscillation of a diaphragm and the pulling in of air produces synthetic jet cooling. The air is then expelled in a turbulent fashion, setting up a secondary flow (Fig. 2).
OLEDS ARE IMPROVING • OLEDs now offer higher efficiency and light output, lower operating voltages, longer lifetimes, and greater full-color light outputs. Developments in high-brightness white-light OLEDs continue, eagerly supported by the DoE, which hopes to lower overall lighting energy costs.
The agency has set an output level of about 150 lm/W and voltage operation around 2 V for white-light OLEDs to reach the 50% level of power-conversion efficiency by 2012. And according to NanoMarkets, the OLED lighting market will reach $4.5 billion by 2013.
Collaborative work funded by the DoE and Universal Display Corp. and developed at Princeton University and the University of Michigan has shown promising results for increasing white-light LED output efficiencies. University of Michigan researchers are embedding low-index grids into the OLED active organic layers to increase efficiency levels to 70 lm/W, compared with 15 lm/W for incandescent lamps.
Conversion efficiency levels near 100% can be achieved with monochromatic OLEDs. But this is more of a challenge for white light, which typically comprises red, green, and yellow light-emissive structures. Universal Display’s phosphorescent OLEDs (PHOLEDs) have achieved power efficacies of 102 lm/W at 1000 cd/m2, with operating lifetimes of 8000 hours to 50% of initial luminance. The phosphorescent material’s molecules allow conversion efficiencies of electrical current to light photons of close to 100%.
French startup MicroOLED and France’s CEA-Leti announced an OLED microdisplay they claim has the finest pixel pitch (more than 1.7 million sub-pixels, which is two to four times greater than other emissive technologies) and the lowest power consumption (four times more efficient) for a compact 0.38-in. wide video graphics array or WVGA microdisplay. It’s designed for camcorders, digital still-camera eyepieces, and video or interactive eyeglasses. The underlying technology marries the capabilities of CMOS processing and the flexibility of OLED capabilities.
ELECTRONIC INK • Electrophoretic displays (EPDs) are slowly emerging in more applications. Together with OLEDs, EPDs are being used as electronic-ink (e-ink) flexible displays on portables. EPDs are light, thin, and rugged. They also offer low power dissipation and design independence.
Market research firm iSuppli Corp. expects the flexible display market to reach $2.8 billion by 2013, a 35-fold increase from the paltry $80 million market for 2007. Prominent EPD applications include the Phosphor watch from Art Technology Ltd., a smart card from SmartDriver, electronic shelf labels from UPM, a Hitachi Mobile phone, and an automotive key fob from Delphi.
According to Philips spinoff iRex Technologies, it has come up with a third-generation e-ink display that’s the only technology to emulate all of the functions of paper. The iRex 1000 e-reader uses a 10.2-in. diagonal EPD from E Ink to show the entire printed page of letter-sized sheets with 1.25-in. margins (Fig. 3). The display is produced on a glass substrate.
Plastic Logic is planning to introduce an 8.5- by 11-in. e-reader this year. Like the iRex 1000, it uses reflective highcontrast gray-scale material from E Ink. However, this device is a read-only product, while the iRex unit is a read-write product. But the Plastic Logic device is lighter at 13 oz (versus 20 oz) with a larger diagonal at 10.7 in. It’s also thinner at 0.25 in. (versus 0.5 in.) and has integral memory (versus a memory card for the iRex 1000). Plastic Logic uses a plastic substrate in its technology.
E Ink is constantly improving its EPD materials. Most recently, it introduced the Vizplex next-generation material imaging film for segmented display cells. It is 20% brighter and has higher resolution than previous-generation materials to support smaller display segments and more detailed artwork.
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