The perfect mobile display has yet to be invented—but that isn’t stopping eight competing technologies in the quest for bragging rights
Every mobile display technology seems to promise the same benefits: Great readability, even in daylight. Fast pixel-transition speeds for displaying video. A wide color gamut for visual fidelity. And, of course, low power consumption for longer battery life.
Because competition is so fierce, every display manufacturer cloaks its offering with some silly trademarked name so you can’t tell which technology it’s actually based on. To mitigate the confusion, here’s a look at the core technologies behind the trade names. We’ll take a deep-dive into today’s four most common mobile display types, and preview four up-and-comers showing promise.
A-Si TFT LCD
Amorphous Silicon (a-Si) TFT is one of the most common iterations of basic LCD technology. It enjoys widespread adoption, and is found in devices large and small. Transistors fabricated from a thin film of amorphous silicon are arranged in a matrix of rows and columns on a polarized glass substrate. A layer of liquid crystals is placed on top of the transistors, and a second sheet of glass—polarized at a 90-degree angle to the first—is laid on top of that. To address an individual pixel, the relevant row of transistors is switched on, and then an electrical charge is sent down the intersecting column.
The N95 may be packed with Nokia’s special brand of Finnish weirdness, but its a-Si TFT display is as average as they come.
When the transistor is switched on, the liquid crystals above it reorient themselves to allow light from the display’s backlight to pass through. When the transistor is deactivated, the crystals flip back and block the light. Each transistor controls a single pixel, and color is achieved by dividing each pixel into three sub-pixels, colored red, green, and blue. Any color can be achieved by mixing these three sub-pixels, and color intensity can be changed by varying voltage.
A-Si TFT displays deliver good viewing angles and a relatively wide color gamut, but their backlights consume a lot of power and they’re hard to read in direct sunlight—two huge drawbacks for battery-powered mobile devices. According to Paul Semenza, Senior VP for DisplaySearch (www.displaysearch.com), a-Si TFT enjoys the best price/performance ratio of any current mobile display technology and should remain the market leader over the next three years.
LTPS TFT LCD
This technology is very similar to an a-Si TFT LCD, except that its transistors are formed from grains of polysilicon (LTPS stands for low-temperature polysilicon). Because polysilicon grains are larger and more uniform than the random-sized amorphous silicon grains, electrons can flow up to 100 times faster than they can through the grains of a-Si. This enables an LTPS display to respond much faster.
What’s more, because the row/column driver electronics in an LTPS display are integrated into the glass substrate (as opposed to the surrounding screen area), the LTPS pixels can be placed much closer together, resulting in much higher maximum resolutions. All that said, LTPS LCDs aren’t much easier to read in direct sunlight, and their backlights consume at least as much power as a-Si LCDs.
Using LTPS technology, the “Retina Display” in Apple’s iPod touch is well-suited for the high resolutions and frame rate demands of HD movies.
In terms of market share, DisplaySearch predicts LTPS will hold tight to second place over the next three years.
The Retina display in Apple’s iPhone 4 combines LTPS with in-plane switching technology (which we cover next). According to Dr. Raymond M. Soneira, President of DisplayMate Technologies (www.displaymate.com), “The iPhone Retina Display had the brightest and sharpest display” in tests he has conducted. Soneira also said, however, that the Retina Display’s “color gamut is too small, producing under-saturated, somewhat washed-out colors, and its image contrast is too high, which produces punchier images and also partially compensates for its smaller color gamut.”
IPS LCD
When voltage passes through the liquid crystals in an in-plane switching (IPS) LCD, the crystals move parallel to the panel plane rather than perpendicular to it, as they do in twisted-nematic LCDs. This endows the display with much better off-axis viewing angles, and explains why the technology is so popular for displays in phones and tablets (which must look great in both landscape and portrait modes).
Samsung’s Galaxy Tab (reviewed here) uses a Samsung-branded “Super” version of IPS LCD technology. To superfy its display, Samsung integrates the tablet’s touch screen technology directly into the LCD (rather than into a separate layer atop the LCD), providing for a thinner package and potentially brighter screen.
Enhanced IPS panels use smaller transistors, which means the apertures through which light passes are larger. Having a larger aperture enables the manufacturer to use a lower-power backlight, which extends battery life in handheld devices, although not by a wide margin.
Jin Kim, Founder and President of DisplayBlog.com, says, “IPS will rule in tablets and eventually in notebook PCs, monitors, and TVs.” And in the smartphone space, says Kim, “IPS will compete fiercely with OLED technology, especially from Samsung with its Super AMOLED.”
AMOLED
In an AMOLED (Active-Matrix Organic Light-Emitting Diode) display, organic compounds emit light when activated by an electrical current. AMOLED requires a thin-film transistor backplane to turn each pixel on and off, but because the pixels emit light on their own, they don’t require a backlight to produce an image. AMOLED displays also produce very deep blacks—a function of simply shutting off power to the appropriate pixels.
Samsung has become synonymous with AMOLED deployment, as evidenced by the Epic 4G smartphone. Unfortunately, AMOLED is expensive, and the possibility of shortages has influenced configuration decisions among device manufacturers.
For its smartphones, Samsung manufactures a premium AMOLED display dubbed Super AMOLED. While DisplaySearch’s Paul Semenza dismisses Samsung’s “Super” designation as a move in a “marketing arms race,” DisplayMate’s Ray Soneira reports that “Super” OLEDs do indeed perform better than other OLEDs.
“In our lab tests,” Soneira says, “the Galaxy S has a screen reflectance of 4.4 percent, and is 25 percent brighter and uses 21 percent less power than the ‘non-Super’ OLED in the Google Nexus One—meeting or exceeding all of Samsung’s specs. Particularly impressive is the very low screen reflectance, which is among the lowest we have ever measured. Outdoors, it can have a significant impact on screen visibility.”
Soneira, however, does point out that AMOLED isn’t necessarily the be-all and end-all of power efficiency: “In our tests, the IPS iPhone 4 display used roughly half the power of OLEDs for the same brightness.” DisplaySearch says AMOLED’s biggest weakness is its production cost, but predicts AMOLED will grow faster than any other mobile display tech over the next three years.
Multi-Mode LCD
Imagine a display that can perform like reflective electronic paper in one mode, and then a full-color LCD with the flip of a switch. That’s what Pixel Qi (www.pixelqi.com) has achieved with its 3Qi display. Dubbed “multi-mode LCD” by industry watchers, this technology can be manufactured with the same fabrication machinery used in conventional LCD production, and can function like a conventional color LCD. But when its backlight is turned off, the display operates in a low-power, black-and-white reflective mode, rendering content that’s visible in direct sunlight—just like a Kindle and other eReaders.
Pixel Qi (pronounced “chee”) currently offers a 10.1-inch display that delivers a resolution of 1024x768 in RGB transmissive mode, and 3072x600 in B&W reflective mode. Interested? Well, you might need to build your own tablet, notebook, or smartphone as a DIY project (kits are available at MakerShed.com). At press time, 3Qi wasn’t shipping in any retail device, although a company called Notion Ink has been touting a 3Qi-based tablet for quite some time. Dubbed Adam, this multi-mode Android device could be available for sale by the time you read this.
Will the 3Qi-laden Adam ever ship? Notion Ink hopes to have the tablet ready for customers soon.
DisplaySearch rates Pixel Qi’s display as inferior to a-Si LCD, LTPS LCD, and AMOLED in terms of viewing angle, color gamut (in transmissive mode), and maximum resolution. It does, however, give the technology high marks for its low power consumption and readability in sunlight (in reflective mode). Soneira, meanwhile, considers Pixel Qi “a very exciting and useful new technology that out-performs E Ink while also delivering the benefits of color LCD.”
Electrowetting
Electrowetting uses voltage to manipulate the movement of fluids in a confined space, typically a mixture of water and oil sandwiched between two plates of glass. As with Pixel Qi’s technology, electrowetting displays can be fabricated using processes very similar to those used in existing LCD manufacturing.
Dye dissolved in the oil determines the color of the pixel. In the absence of voltage (the pixel’s off state), the oil forms a film between the water and an electrode that’s coated with a water-repellent insulation. When voltage is applied, the electrode changes the water’s surface tension, drawing it down to the bottom plate so that it pushes the oil aside. This results in a semi-transparent pixel—so if the surface beneath the glass is white, you’ll see a white pixel.
Liquavista’s electrowetting displays would be ideal for outside viewing, where traditional mobile screens often let us down.
The transition between white and color is sufficiently fast for an electrowetting display to show full-motion video, giving the technology a significant advantage over other types of reflective displays, such as E Ink (used in Amazon’s Kindle). Liquavista (www.liquavista.com), one of the early players in electrowetting, is targeting the eReader market with its first product. LiquavistaBright will be capable of producing a 64-step grayscale for text, graphics, and video. Liquavista’s next iteration will add filters to produce a full-color version—unimaginatively dubbed LiquavistaColor.
DisplaySearch considers electrowetting to be inferior to LCD and AMOLED in terms of resolution and viewing angle, but far superior in terms of power consumption. “Liquavista could leapfrog over E Ink to bring the first color eReader technology to market,” says Paul Semenza.
MEMS Shutter
MEMS stands for Micro-Electro-Mechanical Systems, a technology consisting of infinitesimally small hardware components ranging in size from 1 to 100 microns. MEMS shutters—one for each pixel—take the place of liquid crystals, opening to allow light to pass through, and closing to block light. The display still relies on a backlight, but because there are no liquid crystals, polarizers, or color filters (which absorb as much as 70 percent of the light before it reaches the eye), MEMS shutter displays can operate with backlights that consume much less power.
Behold: The mechanical assembly for a single MEMS shutter. Think a shutter the size of a pixel couldn’t possibly operate reliably? Texas Instruments’ DLP technology consists of pixel-sized pivoting mirrors, and that technology has been around for years.
As implemented in the PerfectLight display from Pixtronix (www.pixtronix.com), color is produced by red, green, and blue LED backlights. The company maintains that its technology is capable of delivering 24-bit color, 105 percent of the NTSC color gamut, and 170-degree viewing angles. “That’s mighty impressive,” says DisplayBlog’s Jin Kim. “Pixtronix will need to work closely with smartphone brands and OS providers to make sure color is accurate and doesn’t needlessly pop. OLED displays have high color gamut, but the colors pop too much, leading to wholly inaccurate colors.”
DisplaySearch says MEMS Shutter technology will deliver better sunlight readability than AMOLED and any of the LCD technologies, but won’t be superior to Reflective MEMS or electrowetting in that regard.
Reflective MEMS
Reflective MEMS is a micro-electro-mechanical system like MEMS Shutter, but it’s an entirely reflective system that doesn’t rely on a backlight unless you’re using it in the dark. Qualcomm’s Mirasol technology (www.mirasoldisplays.com) is the best current example of a Reflective MEMS display.
Mirasol is based on the concept of interferometric modulation—the creation of color through the interference of reflected light. The display consists of many thousands of sub-pixels formed by extremely tiny (10- to 100-micron) iMOD elements, each with two conducting plates. One plate is a thin-film stack on a glass substrate, and the other is a mirror-like membrane suspended over the substrate with an air gap in between.
By adjusting the air gaps of each red, green and blue sub-pixel, a Reflective MEMS Mirasol display can create about 45,000 different colors, according to Qualcomm.
In the absence of voltage, the two plates are separated and ambient light is reflected off the sub-pixel. When voltage is applied, the two plates are drawn together and ambient light is absorbed by the sub-pixel. The color of each sub-pixel is determined by the size of the gap between its two plates. Triads of sub-pixels—one each for red, green, and blue—form a single viewable pixel, and by precisely manipulating the air gaps between the different sub-pixels, colors are formed. For example, a black pixel is created when the gaps in all three sub-pixels are completely closed.
Mirasol displays are bi-stable, meaning their iMOD elements require very little power to maintain their current state—once a pixel is red, it will remain red. Unfortunately, current iterations of Mirasol have limited color depth—about 45,000 colors total, as opposed to the some 16 million in a desktop LCD.
Qualcomm’s Mirasol Reflective MEMS technology should be rolling out in 5.7-inch eReaders in 2011. Finally: Color electronic paper is almost here!
“I think Mirasol will be best aligned with budget eReaders and mobile handsets—devices that require very low power consumption and are used outside more often than indoors,” says DisplayBlog’s Jin Kim. “Because its color really isn’t as brilliant compared to LCDs or OLEDs, high-end multimedia tablets will probably not sport Mirasol.”
