An OEM's Perspective on the Impact of New Display Technologies on Mobile Display Devices
Which, if any, emerging display technology is ready to unseat AMLCD technology as the dominant force in mobile devices? The answer lies in which one can best address issues such as thickness and power management.
by Mike Neilio
IN THE PAST DECADE, active-matrix liquid-crystal display (AMLCD) technology has been established as the benchmark display technology, the one against which all others are measured. AMLCD technology has significantly enhanced the displays of many products over the past decade, from computer monitors and televisions to displays for mobile electronics, including MP3 players, digital cameras, portable navigation devices, and, of course, cell phones. As new products began to call for higher-resolution displays and faster response times, aggressive cost-erosion trends combined with technical improvements provided the catalyst for AMLCDs to displace color super-twisted nematic (CSTN) displays as the main choice for most display products today.
However, today mobile-device manufacturers face increasing challenges in differentiating the design of their products from the competition. As the feature set, size, and performance of new products continue to advance, new display technologies have emerged, including active-matrix organic light-emitting-diode (AMOLED) displays, microelectromechanical systems (MEMS) displays, electronic-paper (e-paper) displays, as well as other bistable displays such as electrowetting displays, just to name a few. Each promises improvements over AMLCD technology, much as AMLCD technology promised (and delivered) advances over previous technologies. These improvements focus on accepted industry parameters such as viewing angle, contrast ratio, response time, color gamut, and luminance, among others, to quantify the benefits of their technology.
These emerging technologies are positioning themselves to become the dominant force in the displays of tomorrow. But do any of these actually have what it takes to do so?
Only time will tell, but AMLCD technology is not going down without a fight. Constant advancements in optics and the transition from amorphous-silicon to low-temperature polysilicon (LTPS) technology offer opportunities for AMLCD technology to remain competitive versus these emerging challengers looking to take its crown. It would appear that for every marginal decrease in cost between AMLCD technology and a given technology such as AMOLED technology – which many suggest is the leading contender to assume AMLCD technology's hegemony – the performance of AMLCD technology improves by an equal amount, thus sustaining its competitive edge. So what other factors do mobile-device manufacturers need to consider when evaluating new display technologies?
Fig. 1: Shown is a cross section of the Nokia 8600 Luna. By following the red path, it is evident how this display is one of five major components in the critical path of the device stack-up that impacts thickness.
Device marketers often use industrial design as a key way to differentiate products, resulting in significant demands on design engineers to find the right balance between reliability, usability, and competitive packaging design. One parameter under close scrutiny is, of course, the resultant product dimensions. Taking cell phones as an example, when considering the main form factors – fold, mono-block, and slide – the display is a primary factor impacting the thickness, width, and length of a particular product (other than external displays on a fold, which, in general, mostly affect thickness.)
From an industrial-design perspective, product thickness commands the most scrutiny followed closely by width and length. Displays are almost always in the critical path of product thickness (Fig 1), and display manufacturers are responding to the demands of industrial designers and device marketers, as evidenced by their great efforts to use thinner glass substrates and more compact backlight systems. Among myriad other benefits, AMOLED displays, being emissive devices, are positioned to offer a very thin module construction because they do not require a backlight. If substrate thickness trends for AMOLED technology follow the same path as AMLCD technology, AMOLED displays may end up being a half-millimeter thinner than comparable AMLCDs. MEMS-based displays also provide significant thickness savings because the construction only requires one substrate on which the MEMS structure is assembled, and arguably no need for a lighting system, which results in one of the thinner display-thickness options.
Another important consideration for mobile-device manufacturers is display viewability in different ambient-light conditions. To this end, AMLCD manufacturers incorporated transflective-type panels – first found on earlier STN displays – into their designs. Transflective displays contain both transmissive and reflective properties such that, when the ambient light is brighter than the backlight of the display, ambient light is reflected, allowing for improved display viewing. These displays are exceptionally useful in bright sunlight where, without some reflective properties, the display would be washed out and not viewable in bright ambient light. However, thanks to improvements in backlight brightness achieved through optical efficiencies in light-emitting-diode (LED) technology, backlight construction, and AMLCD cell transmittance, less-expensive transmissive AMLCDs can now be viewed better in bright sunlight as well. The potential for a 5–10% cost savings over a transflective display translates into increased demand for transmissive AMLCDs.
In addition to transflective and transmissive displays, there is a third display type to be considered, i.e., reflective. More recent technologies such as MEMS, e-paper, and some bistable displays offer highly reflective properties that perform extremely well in bright sunlight. Unlike transmissive displays, the reflective properties of these displays mean that no lighting system is required except in the complete absence of ambient light. Additionally, important attributes such as contrast, viewing angle, and color gamut in reflective mode potentially offer better performance than transflective AMLCDs in reflective mode.
Depending on the specific use, these new display technologies may provide superior performance in future products for mobile-device manufacturers. In addition to improved readability in sunlight, these highly reflective displays provide other important benefits including reduced power consumption.
Power-Management Concerns and Solutions
Advancements in power management and battery capacity permit mobile-device manufacturers to add new functionality while further reducing overall product dimensions. New advanced features such as viewing video and other multimedia content and GPS navigation, combined with interface advancements such as QWERTY-style keymats and touch screens, offers benefits to the overall user experience, but at a cost to the power budget of the device, and consequently its operating time. However, operating time is not necessarily the attribute considered by consumers. Consumers are conditioned to look for terms such as talk time.
Talk time in its simplest form is a measure of the duration of a continuous voice call in nominal conditions that a cell phone can provide, based on the anticipated power consumption and the specified battery capacity. Talk-time trends have remained relatively flat as mobile-device manufacturers have focused on addressing consumer demands for thinner products, rather than increasing battery capacity. This is cause for concern, as the power consumed when the display is used is not considered in the talk-time calculation. This was not an issue when cell phones were only used for voice calls. But now, as consumers perform much more "display" intensive tasks, the resulting increased power demand may result in products falling short of consumer expectations for product life between charges.
Operators and mobile-device manufacturers alike are aware of this concern. Some operators now require mobile-device manufacturers to estimate usage time based on a given use case profile which includes the use of the display. Because there are minimal battery energy-density improvements on the horizon, mobile-device manufacturers are forced to employ other creative ways to reduce the power demand. With demand increasing for transmissive AMLCDs, not only is the backlight the most power-consuming element of AMLCDs, now it is also a necessity. There are a few options available to reduce the backlight power.
The first option to reduce backlight power is light-based automatic brightness control (LABC). Using this scheme, the display backlight brightness is optimized for the given ambient conditions. In direct sunlight, the backlight is at maximum brightness. In indoor office lighting or lower natural lighting, the backlight brightness is reduced to a lower brightness, thus lowering the power consumption. Currently, the circuitry necessary to perform this automatic adjustment is separate and costly, but as this technology is embedded into AMLCDs and the total system cost is reduced, demand for this type of solution will increase.
Another option to reduce the backlight power consumption is content-based automatic brightness control (CABC). Similar to LABC, the backlight brightness is automatically adjusted, but based on the content displayed on the AMLCD rather than the ambient brightness. If an image contains a lot of black or colors to which the eye is less sensitive, the backlight is at maximum brightness. If an image contains a lot of white, the eye does not require as much light, and the backlight brightness is reduced, resulting in lower power consumption. Additionally, an algorithm adjusts the gray scales of the content for optimal viewing.
The key question in this area is, "How do other competing technologies such as AMOLED technology compare to AMLCD technology in terms of power issues for mobile devices?"
An advantage of AMOLED technology is the strong relationship between the number of pixels on and the power consumed. If the image contains a lot of black, the corresponding pixels are off, and the display demands much less power than an image that contains a lot of white with many pixels turned on. This provides mobile-device manufacturers the opportunity to optimize their user interface (UI) such that most of the content is light on a dark background, rather than dark on a light background (Fig. 2), and position AMOLED displays to consume less power than AMLCDs, depending on the specific UI.
However, there are limits to how much LABC, CABC, and UI inversion will increase operating time. Additionally, as consumers become educated to a usage-based model, the demand for alternative lower-power-consumption displays will increase.
Some of these new technologies – such as MEMS-based displays, e-paper displays, and some bistable displays – have the potential to significantly reduce this load since they do not require an energy-intensive display-lighting system. Additionally, some of these technologies allow for static images, once displayed, to use little to no power to preserve the image on the screen. With no need for display lighting, the display thickness potentially can be reduced compared to current AMLCDs and AMOLED displays – an additional benefit.
MEMS-based displays also provide incredible reflectivity, with some estimates of more than 10 times that of current transflective AMLCDs. This provides a substantial advantage when displaying multimedia content such as videos, where current transflective AMLCDs in transmissive mode, transmissive AMLCDs, and emissive AMOLED displays cannot compete due to their significant power demands. But that is only one part of the story. How do these highly sunlight-readable displays perform in low-light and no-light conditions?
Mobile-device manufacturers must consider many different ambient-lighting environments, from bright sunlight and indoor lighting to the complete absence of ambient light. Some new displays, such as MEMS, e-paper, and some bistable displays, that perform well in bright ambient-light conditions require some type of lighting system for viewing in conditions absent of light. The addition of a lighting system may negate some of the benefit of reduced thickness and lower power consumption (specific to the lighting system) mentioned earlier. For example, MEMS and some e-paper displays require the use of a light source such as LEDs coupled with an optically clear plastic light guide in front of the display to allow light to be distributed uniformly across the display. However, MEMS-based displays contain such high reflectivity that very little additional light is necessary, allowing for very-thin and lower-power-consuming lighting systems. This could result in the display being pushed back from the viewing surface, sometimes farther than is optimal for the end user. Given this, these highly reflective displays without a lighting system may be best suited to specific use cases to start, such as secondary displays on folding cell phones.
Many new display technologies such as AMOLED, MEMS, e-paper, and other bistable-display technology such as electro-wetting offer unique benefits, and some have the potential to capture market share from AMLCD technology in the mobile-device marketplace. Each of these technologies has its own unique strengths and weaknesses (see Table 1 for a side-by-side comparison).
AMOLED technology offers superior contrast, fast response time, and wide color gamut in a potentially thinner package than AMLCD technology, but clearly at a higher cost. However, as yields improve and volumes grow, this cost delta may disappear.
MEMS technology offers incredible reflectivity and adequate multimedia content without the need of a lighting system, but full-color versions are not yet ready for large-scale commercial implementation.
E-paper technology offers high contrast, bright sunlight viewing, and low power consumption in a thin package, but without a front light (which negates much of the power-consumption advantages) it is not viewable in conditions without ambient light.
Fig. 2: The photograph on the left is the typical Nokia UI idle screen, the one on the right is an edited version with less white, illustrating an option Nokia could consider for a device using an AMOLED display, in an effort to minimize power consumption. Because there is a strong relationship between the number of pixels on and the power consumed for AMOLED displays, the image on the right would consume less power.
Bistable displays such as electrowetting displays offer wide color gamut and fast response times, but full-color versions are not yet ready for large-scale commercial implementation.
These and other attributes such as power consumption and display type (transflective, transmissive, or reflective) have a significant impact on mobile-device design in terms of overall product size, end-user experience, and operational time. AMLCDs were successful in displacing CSTN displays as the color display of choice in most mobile devices today. So what will it take for any one of these technologies to displace AMLCDs?
Only time will tell, but in the meantime, there are certainly opportunities for improving display thickness and high power consumption. Device marketers are always looking for ways to enhance and better differentiate their products by addressing consumer needs and building better user experiences. Depending on product requirements and individual uses, the relative importance of individual items in this matrix may vary (Table 1). That said, assuming that costs continue to decline, AMOLED technology may have the best balance of strengths and weaknesses to supplant AMLCD technology as the display technology of choice. One thing is for certain: The technology that successfully addresses these challenges will be best positioned to contest the supremacy of AMLCD technology. •