SOLID-STATE ILLUMINATION: High-brightness LEDs move into backlit displays
Despite a dramatic slowdown in growth in the market for high-brightness light-emitting devices (HB LEDs), attendance at the annual Strategies in Light conference (Feb. 15-17; San Francisco, CA) climbed to a new high this year, with 500 attendees, 36 exhibitors, and a strongly upbeat mood.
In 2005 the worldwide HB LED market grew by only 8%, 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). The camera-phone segment (LED flash) was the fastest growing segment within the mobile phone market in 2005, which actually declined overall because of falling LED prices, despite continuing handset growth (20%). For the overall HB LED market, lighting was the fastest growing segment.
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, liquid-crystal-display (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 performance capabilities; 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 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 because of the need for a long optical path length to perform red-green-blue (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 fiberoptic-waveguide technology and made from a highly-transmissive multilayer 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; Garden Grove, 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 an initial viewing distance of 1500 ft to the nearest viewing point of 350 ft subtends an angle of less than 6°, 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. The company has also developed a self-calibration system to monitor and maintain color uniformity.
Correcting color by PWM
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 pulse-width-modulation (PWM)-based feedback system developed at Avago to address what he described as the top five challenges to using RGB LEDs for backlighting. These challenges included the impossibility of maintaining color due to different degradation rates for different LEDs; the shifting of emission wavelength with temperature variation; panel-to-panel variations in LED color; high LED cost caused by tight binning procedures to address part-to-part variability; and the lack of plug-and-play solutions.
In evaluating temperature sensing, intensity sensing, and color sensing as 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. Based upon those findings they developed a user-friendly, closed-loop feedback system for illumination and color management that not only maintains color uniformity but also greatly simplifies RGB LED implementation and allows user control of the color palette (see figure).