Notes from the Front Lines of Backlights

I had the privilege of attending the LA Chapter one-day technical conference on LED technology for LCD backlights in early January. This was the third installment in this highly successful, sharply focused, technical program describing the current state of the art in LED backlighting for LCDs. The organizers of this event have found the ideal recipe: bringing the right mix of technology details and application overview in just the right balance for a full but manageable day of education. I commend them for their efforts.

In the opening remarks presented by Program Chair Bill Kennedy (Toyoda Gosei Co., Ltd.), we learned that the conversion efficiency in GaN LEDs has now substantially passed the similar metric for cold-cathode fluorescent lamps (CCFLs). Blue-emission LEDs using yellow bi-chromatic phosphors to make a white mix will dominate the backlighting applications for the near future. These white LEDs represent an efficient and low-cost method to make a white-light mix suitable for many LCD applications. As optical-power-efficiency and color-gamut demands increase, multi-spectrum phosphor versions will become more widely available as well. The most recent products from Toyoda are achieving as much as 70% external quantum efficiency, which puts them remarkably close to laser QE performance. Of course, getting the light out of the die and into the display efficiently is still a challenge.

The increasingly popular technique to use local-area dimming of direct-array LED backlights with matching image processing on the LCD for expanded dynamic range will continue to see investment and deployment in a number of applications. This approach, which has taken on the name Active Local Dimming (ALD), has been widely reported in ID magazine and demonstrated at numerous exhibitions in the past few years. While most embodiments being developed currently use white LEDs, there is renewed interest in using RGB arrays and achieving multiple axis of dynamic-range expansion, both in the color and luminance domain.

As the response-time metric of LCDs decreases, so does the renewed interest in color-field-sequential approaches, which eliminate the optical inefficiencies in the color filters. This is a controversial subject to many because of the known issues with color-frame breakup and demands for very high sequential field rates used in projec-tion systems, but the combined opportunities to increase luminous efficiencies, reduce power consumption, and inherently reduce the motion-blur problem are just too compelling to ignore.

The implementation of LEDs in backlights for cell phones and other portable devices is a mature and well-tested marketplace. The same pervasive penetration is just beginning to happen in the notebook-computer market and shows signs of moving rapidly into both the desktop monitor/panel market as well as the large-screen-television market. The penetration into notebooks is being driven at a dramatic pace by the combined demands for better battery life and more efficient use of available real estate.

Matt Knadler, Senior Display Development Engineer (Dell), gave a very candid overview of the challenges and driving factors for Dell's adoption of LEDs. On the one hand are the promising benefits of LED technology: thinner panels, lower power consumption for comparable light output, wider color gamut, reduced support electronics footprint (i.e., eliminating the inverter), absence of mercury, and low-voltage operation (i.e., intrinsic safety). On the other hand is the very real problem of slightly higher material cost (on the order of $10–$20 per display), the new supply-chain challenges in this very young component market, and the business/technical risks of using a new technology whose working lifetime is just now approaching that of CCFL. In Dell's case, this results in a very complex set of business-model tradeoffs that has resulted in their very careful but determined entry into this space, following closely behind Sony, Apple, and others. The consumer marketplace for notebook computers and its characteristic side-by-side display environment in retail channels results in the display being one of the most visible and obvious comparison factors between competing products. Dell's objective is to have "TV-like" displays that rival those of any other manufacturer, along with realizing other additional performance goals that together will overcome the near-term price penalty. Those additional goals are "all day" notebook run times; meeting all global "Green" requirements; improved color accuracy and control of color rendering; and the driving of long-term lower costs by enabling the supply chain.

Currently, Dell offers white-LED-backlit displays In 2008, it plans to introduce RGB-backlit displays and have a technology roadmap that shows color field-sequential RGB products available by mid 2009. Even if that last step is not realistic, it still shows a very aggressive ramp that will change the economics of everyone in their supply chain.

In order to enable the notebook-PC marketplace, panel manufacturers such as Chi Mei Optoelectronics, Samsung, and others are developing notebook LCD panels with LED backlights. The presence of these panels in the marketplace from the major manufacturers, along with the pull of notebook and TV manufacturers to build volume in the supply chain, now virtually ensures that the general display-applications community will soon have a variety of options to choose from at reasonable price points.

As the day wore on, it occurred to me that all the implementations being discussed used large arrays of very-small low-power LEDs. This is in contrast to the approaches involving a small number of high-power LEDs being presented just a few years ago. At that time, there was significant interest in the very-high -current-density offerings from Lumileds and Osram that promised to keep the system costs low due to low interconnection demands, low device count, low driver costs, etc. It appears from those I spoke to at this conference, those promises have not been fulfilled as the devices themselves remain somewhat expensive, their luminous efficiency is in some cases not as good as the low-power devices, and there is an increased system cost due to the complexity of the optical and thermal-management systems required for this approach. As a result, it looks like the low-power devices are going to be used in most new designs.

So, once again, as it did many years ago, the ubiquitous notebook-PC market has become the "killer application" that is driving a major technological evolution in the world of displays – first with the mass adoption of CCFL TFT-LCDs and now the coming mass adoption of LED TFT-LCD technology.

It is a privilege for me to introduce this month our Guest Editor Paul Drzaic, who is also our President-Elect of SID. Paul has provided us with a nice assortment of articles revealing important new advances in the field of Electronic Paper. Electronic-paper technology has been advancing rapidly of late with several new product launches and commercialization of some very unique developments. In his article discussing the role of Electro-chromics as an alternative to Electrophoretics, Chris Giacoponello from NTERA reveals their latest efforts to achieve full-color capabilities. Jacques Angelé from Nemoptic discusses the ins and outs of manufacturing e-paper displays with their liquid-crystal-based technology. And, for those of you not up on the overall state of the field, Minoru Koshimizu from Fuji Xerox Co. , Ltd., provides a very valuable survey of the past, present, and future outlooks for e-paper technology.

Last November, Guest Editor Bernie Lechtner gave us a great portfolio of information about High-Definition-Televison technology. In case you did not realize it, Bernie was also a very key contributor to the early developments of matrix addressing in LCD technology. I am very pleased to bring to you his personal account of those days and how some very crucial elements of that technology were developed and demonstrated. This is the first of a regular series of articles we're developing which will recount the history and pivotal moments in the development of various display technologies. Look for future installments later in this year.

As usual, we welcome your feedback on this or any other topic and look forward to serving your needs in 2008. Happy New Year!

Stephen P. Atwood