Repeatedly predicted and repeatedly delayed on many occasions, the transition from CRTs (cathode-ray tubes) to LCDs (liquid-crystal displays) has finally occurred, even in cost-sensitive emerging markets and across dominant application segments: computer monitors and televisions. Small-format LCDs also find wide use in diverse portable electronics devices, along with some digital projectors. However, LCDs have lingering imperfections, including low refresh rates and lengthy response times, constrained viewing angles, high power consumption and cost, and poor perceptibility in direct sunlight and other high-ambient-light conditions. As a result, manufacturers are always looking for ways to eliminate these imperfections in this all-important, evolving technology.
Some developers focus their efforts on making incremental improvements to a “vanilla” LCD foundation. Other cases warrant a more revolutionary transition—to OLEDs (organic light-emitting diodes) for an ultrasvelte consumer-electronics device, for example, or to an “electronic-paper” display for a digital reader. Historical trends make it clear that there’s no one-size-fits-all display approach for all applications, no matter how much the resultant volume cost and, therefore, price efficiencies might favor such consolidation. They also make it clear that there’s no shortage of creativity fueling ongoing technological innovation—both in the near term to provide credible alternatives to currently dominant approaches and in the long term to capture the dominance prize.
Subpixel variation shows the basic operational concepts of an LCD. Normally, the perpendicular polarization orientations of two parallel polarizer layers block transmission of ambient-environment-generated light that reflects off a mirrored back panel, self-illumination by a backlight, or both, leading to an array of perceived-black pixels. However, ITO (indium-tin oxide), the same increasingly rare material that touchscreens use, delivers a sufficiently strong applied electric field to alter the intermediary liquid crystal’s modulation properties. This alteration translates to light transmission of varying intensity. Initially popular passive-matrix displays individually and, therefore, sequentially accessed each row-and-column rudimentary circuit intersection, thereby requiring each pixel to hold its state between refreshes and translating into slow response, low contrast ratios, and other shortcomings that became worse as resolutions and screen sizes increased.
IPS (in-phase switching) LCDs emerged in response to display users’ requests for improved viewing angles, deeper black levels, and other enhancements. The IPS LCD horizontally aligns the liquid-crystal cells with subsequent application of the per-pixel electrical field through the crystals’ ends, thereby requiring two transistors per pixel—more costly than TN’s approach. Historically, at least until the unveiling of LG Display’s Enhanced IPS approach, the incremental per-pixel circuitry also negatively affected light-transmission efficiency, thereby necessitating brighter and more power-hungry backlights to compensate.
Speaking of LG, IPS historically found use only in high-end professional displays and other application niches that could tolerate the technology’s inherently higher cost. However, Apple adopted Enhanced IPS LCDs in the company’s latest-generation Cinema Display, iMacs, iPads, and iPhone 4. This adoption should spur price-cutting high-production volumes that other potential customers can also beneficially leverage. An intermediary approach, the VA (vertical-alignment) LCD, offers an approximation of the TN-to-IPS quality improvements. However, VA LCDs, which come in multidomain and patterned variants, require only one transistor per pixel. As such, they have lower per-pixel costs than IPS alternatives.
Kodak’s legendary Bayer-pattern image sensor, which takes its name from Bryce E Bayer, PhD, who patented it in 1976, has a Nuovoyance-equivalent RGBG (red/green/blue/green)-display pattern; both arrangements exploit the fact that the human visual system is most sensitive to green-spectrum information. The RGBG PenTile approach encompasses one-third fewer subpixels than that of a traditional RGB pattern, but primary inventor Candice H Brown Elliott claims that it delivers equivalent perceived display resolution. The RGBG PenTile pattern currently finds use in Samsung’s OLED panels for mobile phones, digital cameras, and other consumer-electronics devices.
Sharp is the only remaining notable Japanese LCD manufacturer, and the company is striving to technically differentiate itself from South Korean, Taiwanese, and Chinese competitors to remain relevant in the future. At the 2009 SID (Society for Information Display) show in San Antonio, TX, the company showed prototype LCDs employing a five-subpixel pattern incorporating not only traditional red, green, and blue additive colors but also the cyan and magenta subset of the subtractive-color palette. Six months later, at the January 2010 CES (Consumer Electronics Show), Sharp unveiled a series of Quattron RGBY (red/green/blue/yellow)-subpixel-pattern-based TVs ranging in screen size from 40 to 68 in., some of which are now in production, with others to follow next year.
The company’s promotional materials dish up no shortage of hyperbole, claiming that this subpixel combination delivers more than 1 trillion distinct colors versus conventional RGB’s billions, “faithfully rendering nearly all colors that can be discerned with the unaided human eye” and delivering “more sparkling golds, Caribbean blues, and sunflower yellows”. Sharp conveniently fails to mention, however, that Panasonic in the 1970s unveiled conceptually similar Quatrecolor CRT TVs, which met with an underwhelming market embrace and which the company quickly discontinued. Sharp also doesn’t seemingly have a compelling answer to the question of why an RGBY display enhances content that gear with only conventional RGB-subpixel cognizance originally captured and processed. However, at least one other company seemingly feels that Sharp is onto something: Apple recently filed patents that one-up Quattron by advocating a pure CMYK (cyan/magenta/yellow/black) subtractive-display approach.
Resolutions, orientations
Modern flat-panel televisions, regardless of their screen sizes, tend to comprehend a native resolution no finer-detailed than 1080p—that is, 1920×1080 pixels in a wide-screen orientation. This upper-end resolution cap makes them easier to manufacture and, therefore, higher yielding and less expensive, and the suppliers rationalize the pixel-count ceiling by pointing out that commercially available video content is 1080p in maximum resolution. Granted, consumers might prefer to view higher-quality versions of the still images their high-resolution digital cameras capture, but there’s insufficient demand for the feature to justify its development and deployment by TV manufacturers and their panel partners.
Samsung this year voiced long-term concern about the resolution cap at a meeting in South Korea. As available screen sizes continue to increase and at close-enough viewing distances, observers will increasingly be likely to discern discrete pixels and the boundaries between them—an undesirable capability. Movie theaters currently project digital content at 2 and 4K resolutions—approximately 2048 horizontal pixels and approximately 4096 horizontal pixels, respectively. No consumer-oriented physical-media format currently supports these resolutions, however. Downloadable and streamed media are more flexible. Similarly, NHK, among other companies, has for several years now at CES demonstrated compelling UHDTV (ultra-high-definition digital television), which NHK brands SHV (superhigh vision), but a broad market rollout remains elusive.
You might believe that computer displays would be more amenable to very-high-resolution pixel configurations, and you’d be right, but probably not to the extent that you might think. Granted, computer monitors’ close-proximity viewing arrangements encourage high pixel resolutions and, therefore, fine pixel pitch. Computer monitors have smaller screen formats than do TVs, somewhat counterbalancing these attributes. And the content, including photographs, text, and the like, that users view on these monitors is more amenable to fine-detail capabilities. But the resolution restrictions of legacy analog VGA, digital single-channel DVI (digital-visual-interface), and HDMI (high-definition-multimedia-interface) connections have to some degree limited display-quality evolution—a limitation that DisplayPort has had limited success in alleviating .
The decreased manufacturing complexity, increased yield, and, therefore, lower cost and higher supply of lower-resolution displays at a given screen size also factor into the price-versus-quality trade-off that has placed higher-resolution alternatives into high-profit-margin but low-volume market niches. You also cannot ignore the dots-per-inch constraints of legacy operating systems and programs. Even with a leading-edge, dot-per-inch-flexible operating system, such as Microsoft’s Windows 7, a legacy application that assumes a traditional 72-dpi density produces unacceptably small or, when interpolated, fuzzy fonts and graphical elements when output to a higher-dot-per-inch display.
Although pixel density is one of several key determinants of the quality of the content you view, aspect ratio—that is, the number of horizontally and vertically arranged pixels—defines how much of the content you can see on the screen at once. The growing popularity of wide-screen-formatted TV programs, movies, and other video content has driven the inexorable migration in recent years of TVs and computer displays to the now-dominant 16-to-9 and similar ratio dimensions. Computer-game players, stock traders, and other power users may even stack multiple displays side by side to further increase the configuration’s horizontal real estate. However, plenty of computer users who predominantly view conventional content while Web browsing, writing, creating spreadsheets and performing calculations using them, and doing similar activities bemoan the perceived “lost” vertical resolution of a wide-screen display versus the predecessor’s 4-to-3 aspect ratio.
On the other hand, mobile electronics’ displays have at least to date largely bucked the wide-screen-conversion trend that has marked their larger-format brethren. In part, this continued reliance on the legacy aspect ratio is due to the fact that a system with a portrait-oriented, 4-to-3-aspect-ratio display tends to fit better into a user’s hand. It’s also partly due to the fact that most of the content users access on such systems continues to be best viewed on a screen other than a wide one. Apple, for example, received no shortage of unwarranted criticism from early reviewers of the iPad, who, in focusing on video-playback applications, overlooked the fact that electronic-publication-reader programs mimic the printed page that has an approximate 4-to-3 aspect ratio in both single- portrait and dual-page-landscape configurations.
Backlight options
Historically, nonreflective LCDs have largely leveraged CCFL (cold-cathode-fluorescent-lamp) backlights, whose dominant attributes included low cost and a diverse supplier base. However, they’re less than ideal in numerous other respects, including inconsistent illumination among lamps, despite intermediary diffuser use; from power-up to stable subsequent operation; and as they age. They also have limited operating life before hard failure, notable incremental effects on both display thickness and display power consumption, environmentally damaging or costly disposal characteristics, and low display ruggedness and reliability. In addition, they cannot deliver deep blacks because the CCFL backlight is always on, and some amount of light leaks through the polarizing filters and to the viewer’s eyes.
Some niche situations have also employed incandescent light bulbs, ELPs (electroluminescent panels), and HCFLs (hot-cathode fluorescent lamps) as backlights, and all have unique combinations of strengths and weaknesses, but LEDs appear to be the emergent widespread CCFL-backlight successor. Initially too expensive to use in any but the smallest display-real-estate settings, they’ve rapidly decreased in price in pace with their burgeoning adoption in diverse applications. As such, now that cost is increasingly not prohibitive, they neatly address CCFLs’ shortcomings. Design engineers love LED backlights’ advantages, and marketers leverage them by giving their products misleading monikers. For example, Samsung in mid-2009 began—and continues—to promote “LED TVs,” despite complaints from Britain’s Advertising Standards Authority and other consumer-advocacy groups. Unfortunately, other manufacturers have followed suit.
The first-generation LED-backlight configuration, which remains in widespread use, is reminiscent of its CCFL predecessor: multiple white LEDs spread across the screen with a diffuser, or light guide. A reflective layer behind the LED array boosts the backlight efficiency by redirecting toward the user emitted light that might otherwise go to waste. Elementary LED-backlight designs drive all of the available LED elements to the same intensity. So-called local-dimming approaches take the architecture to the next logical step. By individually controlling the intensity of each LED element, they boost the display’s effective contrast ratio at the trade-off of increased required processing intelligence and therefore cost built into the display.
An increased color gamut is the key focus of the next increment in LED evolution, which harnesses the fact that LEDs come not only in white but also in red, green, and blue variants. By individually controlling the intensity of each sub-LED within a three-color cluster, a display manufacturer can positively affect not only contrast ratio but also the palette and accuracy of visible colors the LCD outputs. Conversely, edge-lit LED arrangements focus on thickness and, to some degree, backlight cost. In this case, as the name suggests, the LED arrays reside on two or all four border spans of the display, with light guides spreading their illumination across the back panel. Localized contrast and color control are impossible with such a configuration, but, in exchange, the display can be both slightly thinner and potentially cheaper than its backlit counterpart.
Any backlight, no matter its components or its configuration, still incrementally and negatively affects display depth and power consumption. Elimination of the backlight is a key selling point of the OLED, a long-touted supposed successor to LCDs that’s finally beginning to deliver on its promise, at least in small-format applications. OLEDs’ inherent emissive electroluminescence requires no supplemental backlight. Unlike backlight-inclusive displays, they are flexible, opening the doors to new applications, such as electronics-augmented clothing and roll-up signage. Their wide viewing angles, vivid colors, and perceived contrast ratio in dim viewing settings are impressive. They also deliver several orders of magnitude faster response than do LCDs.
But OLEDs’ limited operating lifetime, particularly for blue-spectrum organic materials, has to date left them feasible only with highly disposable electronics devices. Before hard failure, all OLED subpixels exhibit degradation over time, creating undesirable color- balance shifts. OLEDs, being nonreflective in nature, tend to be difficult to discern in high-ambient-lighting settings, such as with direct-sunlight illumination. Unlike LCDs, and like CRTs and plasma displays, they are prone to permanent persistence, or burn-in, in response to the lengthy display of a stationary image
High-volume, cost-effective OLED manufacturing remains a challenge, and there are a limited number of suppliers. For example, although HTC over the past year launched a series of OLED-based mobile phones, the company has subsequently retrofitted them with LCDs in response to supply short falls, leaving currently dominant OLED supplier Samsung to use the small-format displays in its own branded cameras, cell phones, and other products. And Samsung’s frequently stated ambition to obsolete LCD TVs with large-format OLED successors, although an understandable response to the looming threat of emerging LCD competition from companies in Taiwan, China, and elsewhere, seems to be little more than a pipe dream, at least for the near term.
As for the front of the display, an increasing percentage of LCDs employ glossy screens instead of matte, anti-glare counterparts. In response, specialty suppliers have sprung up to, for a warranty-busting and often-substantial fee, retrofit glossy-only computers with matte aftermarket displays. Applying a matte film to the front of a glossy LCD produces a comparable and more cost-effective effect. The heated debates between advocates in both camps are reminiscent of the arguments about matte and glossy photographs, which have comparable sets of pros and cons. Matte screens don’t exhibit egregious reflections and consequently may be easier on the eyes for extended viewing periods and outdoors—that is, if the backlight is strong enough. Detractors point out their decreased brightness and contrast, which promoters alternatively describe as more accurate. Conversely, glossy screens’ vibrant—albeit, according to image professionals, inaccurate—colors make them the often-preferred option for playing games or watching movies, thereby explaining their burgeoning popularity. However, high-ambient-lighting conditions result in glare and reflections from the display’s surroundings. In both cases, the incremental attenuation and other alteration effects of a potentially present touchscreen also beg for your attention.
Whether LCD or OLED, displays’ ever-faster responses have enabled them to finally usurp CRTs in performance-demanding usage environments, such as computer and multiplayer-gaming setups. They’ve also, in combination with evolving backlight improvements, led to a numbers race among suppliers continually striving to one-up or, depending on the specification, “one-down” each other, albeit with often-dubious real-life relevance. Overdriving a pixel to encourage it to more quickly switch states is, for example, a meaningful technique only when moving it from fully open to fully closed or vice versa. Subtler transitions take much longer than their more abrupt counterparts.
Similarly, not too long ago, people considered a 60-Hz display-refresh rate as state of the art, whereas 120-Hz panels are now commonplace, and 240-Hz and higher refresh-rate displays are entering the mainstream. Again, some practical benefit exists to such techniques. These benefits include enabling the display-refresh rate to more evenly match up with the 24-Hz cadence of film-captured material, for example; eliminating “judder” in fast-action sequences, such as sports content; and bringing 3-D playback to the living room. Yet, a bungling implementation may ironically produce results that are worse than those of a slower-refresh predecessor. Intermediary frame creation can take the form of either previous-frame repetition or interpolation between successive source frames. Thanks to LED backlights’ rapid illumination and extinction attributes, intermediary black frames—those with the backlight turned off—often also find use.
Peer at the printed page of a monochrome book or newspaper, and you’ll likely realize that a full-color LCD or OLED represents overkill for an electronic-paper successor. This discrepancy explains the impressive industry embrace of E Ink’s bimodal display medium, which the company based on technology developed at the Massachusetts Institute of Technology’s Media Lab and today finds use in most e-book readers and similar devices. As the company literature explains, “The principal components of electronic ink are millions of tiny microcapsules, about the diameter of a human hair. In one incarnation, each microcapsule contains positively charged white particles and negatively charged black particles suspended in a clear fluid. When a negative electric field is applied, the white particles move to the top of the microcapsule to become visible to the reader. This [approach] makes the surface appear white at that location. At the same time, an opposite electric field pulls the black particles to the bottom of the microcapsules where they are hidden. By reversing this process, the black particles appear at the top of the capsule, which now makes the surface appear dark at that location.”
E Ink’s microcapsules retain their orientations even after removal of the electric field—until subsequent field reapplication and reversal. As a result, E Ink-based devices deliver much longer battery life than LCD or OLED alternatives. The displays are easy to read even in bright-sunlight settings, have nearly 180° viewing angles, and deliver 150- to 200-dpi resolution. E Ink this year unveiled its second-generation Pearl technology, with a claimed 50% improvement in contrast ratio. However, although the manufacturer claims that Pearl has a less-than-1-msec response rate, refresh rates are on the order of only a few frames or less per second, leading to slow page-turning, annoying “ghosting” artifacts, and a practical inability to display even low-frame-rate video content.
Color variants of E Ink displays are not yet in production, and the prototypes at last summer’s SID conference and other recent industry forums have been underwhelming, with limited palettes and low contrast ratios—both in an absolute sense and compared with LCD and OLED counterparts. These shortcomings are problematic for color newspapers, such as USA Today, the Sunday comics, and electronic magazine subscriptions. They also render E Ink displays incompatible with “enlightened” electronic-literature versions, which embrace the new medium’s capabilities by including animation sequences, video clips, and the like. For these and other reasons, Barnes & Noble selected a 1024×600-pixel, 7-in. LCD for its $249 Nook color e-reader, which the company introduced in October. The company is treading a tenuous and unclear pricing path between less expensive monochrome e-books and fully featured color-tablet computers
Qualcomm’s Mirasol display represents a curious move for a company best known for its plethora of wireless-communication patents and the semiconductor devices that employ them. Mirasol, a MEMS (microelectromechanical-system)-based technology, uses IMOD (interferometric modulation), which functions similar to the way in which a butterfly wing refracts light into a rainbow of colors. Each display element comprises two conductive plates, forming an optically resonant cavity. One plate is a thin-film stack on a glass substrate, and the other is a reflective membrane suspended overhead; an air gap separates them. The IMOD element has two stable states. With no applied voltage, the plates remain separate. Applying a voltage differential draws the plates together by electrostatic attraction.
When ambient light hits the element, with no applied voltage to the plates, the light reflects off both the top of the thin-film stack and the reflective membrane above it. Depending on the optical cavity’s height, the light of certain wavelengths reflecting off the membrane is slightly out of phase with the light reflecting off the thin-film structure. Some wavelengths constructively interfere, whereas others destructively interfere. The human eye and brain perceive as color the resultant amplification of some wavelengths and not others. Collapsing the plates’ gap by applying voltage results in constructive interference only at ultraviolet wavelengths, invisible to the human eye, translating to a perceived-black absence of color. Sequentially ordered red, green, and blue constructive-wavelength subpixel elements, as with LCDs and OLEDs, construct pixels that output all color combinations, including white.
To date, Qualcomm’s few Mirasol design wins have been in small-format monochrome displays from a modest Taiwanese facility in partnership with Foxlink. However, Qualcomm has recently begun showing limited-gamut-color and somewhat-dim prototypes and is reportedly building a $2 billion dedicated manufacturing facility after securing a major design win
