HB LEDs eye LCD back-lit displays

Despite a dramatic growth slowdown in the market for high brightness light emitting devices (HB LEDs), attendance at the annual Strategies in Light conference, held in downtown San Francisco, CA, this year from February 15-17 climbed to a new high, with 500 attendees, 36 exhibitors and a strongly upbeat mood.

SAN FRANCISCO, CA - Despite a dramatic growth slowdown in the market for high brightness light emitting devices (HB LEDs), attendance at the annual Strategies in Light conference, held in downtown San Francisco, CA, this year from February 15-17 climbed to a new high, with 500 attendees, 36 exhibitors and a strongly upbeat mood.

In 2005, the worldwide HB LED market grew by only 8%, as compared to the 46% per year average from 2001 to 2004, according to remarks by Robert Steele, director of optoelectronics at Strategies Unlimited (Mountain View, CA). Unit sales continued double-digit growth in 2005, with camera phones continuing as the fastest growing market segment (despite a slackening of cell phone sales), but declining prices stunted overall market growth. Future prospects remain bright, however, in all three of the primary application areas for HB LED technology, automotive, general lighting and display.

Among display applications for HB LEDs, LCD TV has become the Holy Grail, according to Eran Fine, CEO and founder of Oree (Natanya, Israel). Fine said that the large-screen LCD backlighting market for HB LED technology was expected to reach $3 million last year, and is expected to grow to $160 million by 2008. The market reach of plasma display panel (PDP) technology will ultimately be limited by its maximum performance capabilities and LCD technology has the potential to beat PDP in both performance and price, he said. But that potential has yet to be realized.

Cold cathode fluorescent lamp (CCFL) technology can offer efficiency advantages over PDP technology for LCD backlighting and is coming down in price, but LED technology can outperform CCFL in terms of screen size, brightness, color gamut and service life. Technological hurdles to be overcome by LED technology (in addition to luminous flux efficiency, power consumption, heat management and price) include size. Device size is increased by the need for optical path length to perform RGB mixing and to maintain uniform illumination. In addition, LED brightness tends to improve with size.

Oree proposes to address these issues using a planar flexible light guide pFLG based on fiber-optic wave guide technology and made from a highly-transmissive, multi-layer, polymer-based transparent sheet (less than 300 µm thick with elasticity in excess of 500%). The combination of thinness, flexibility and transmissivity along with improved output coupling over conventional LEDs enable the pFLG material to yield improved brightness while reducing power consumption, device thickness and cost, Fine said.

Another LED display application with major market potential, electronic billboards, is less dependent upon the development of new technology than upon engineering solutions to application-specific problems, according to Karl Boldt, founder and executive vice president of Visioneered Image Systems (VIS; Lake Forest, CA). For a billboard placed on the side of a freeway and intended to be viewed by passing motorists, for instance, the vertical viewing arc from initial viewing distance of 1500 feet to the nearest viewing point of 350 feet subtends an angle of less than 6 degrees, Boldt said. So VIS shapes and diffuses its solid-state billboard imagery using a diffractive optic film to improve overall system efficacy by increasing luminous energy delivered to the target viewing area.

Lee Soo Ghee, vice president and general manager of the optoelectronic products division of Avago (formerly Agilent) Technologies (San Jose, CA and Penang, Malaysia) described a feedback system developed at Avago to address what he described as the top five challenges to using RGB LEDs for backlighting. The list included impossibility of maintaining color due to different degradation rates for different LEDs; shifting of emission wavelength with temperature variation; panel-to-panel variation in LED color; high LED cost caused by tight binning procedures to address part-to-part variability; and lack of plug and play solutions.

In evaluating potential monitoring methods, engineers at Avago found that only color sensing enabled monitoring of intensity change with temperature, intensity change with age and wavelength shift with temperature. They have developed a user-friendly, closed loop feedback system for illumination and color management that allows more design freedom and user-control of color palette.

- Hassaun A. Jones-Bey

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