New Color Screen Combines Beauty, Readability
If you’re looking to buy an e-reader, you can choose between a beautiful but battery-draining liquid crystal display (LCD), like the one in Apple’s new iPad, or a slow-switching but easy to read black-and-white one, like the one in Amazon’s Kindle. At the Society for Information Display’s annual conference this week in Seattle, Qualcomm MEMS Technologies is demonstrating prototypes of a screen that meets somewhere between these extremes. It shows video in color, and under full sunlight, but without draining the battery. The display will be in products by the end of the year.
The backlights in conventional LCDs consume the majority of the power in portable electronic devices. That’s because a significant amount of that light is lost in polarizers and filters inside the device. These displays also require continuous power to maintain an image. “Batteries are evolving slowly, and there’s increasing pressure to reduce this power consumption,” says Brian Galley, senior director of product management at Qualcomm MEMS Technologies.
The iPad has raised the bar, says Paul Semenza, senior analyst at Display Search, by showing that LCDs are getting more energy-efficient while the glass they’re built on is getting tougher and lighter. “Once things go full color, and go video, it’s difficult to go back,” he says.
Qualcomm’s Mirasol display, which can play video in color, extends battery life by 51 percent relative to an LCD, according to a report by Pike Research.
Many companies are working to develop better reflective displays, which provide considerable power savings because they don’t require a backlight and, in most cases, can maintain an image without needing additional power. E-Ink has been a leader in this area but has yet to come out with a color, video-capable display, though company representatives at the conference say one will be ready at the end of the year.
E-Ink pixels contain electrically charged black and white particles; when a small voltage is applied, one or the other moves to the surface to make the pixel reflect light or appear black. To make a color screen, filters are added to the top. Early versions of E-Ink’s color screen appeared washed out because of light lost due to the filter, and were relatively slow to refresh, taking about a quarter of a second to refresh a page. Black-and-white E-Ink prototypes at the conference have higher resolution, faster-switching screens that looked crisp; company representatives say these improvements will lead to better color technology, too.
The pixels in Qualcomm’s Mirasol displays can switch fast enough to show video, and don’t use filters to generate color. These displays generate color by harnessing the interference effects that occur when light bounces off certain structures.
Each pixel is made up of rows of two-layered reflective surfaces separated by air, and acts as a tiny interference chamber. When ambient light hits each of these subpixels, some will reflect off the top surface, and some will pass through and be reflected off the bottom surface. The two waves constructively interfere with each other, combining to create light whose color is determined by the distance between the two surfaces–on the order of hundreds of nanometers. Within each pixel are three of these chambers, each a different depth, to create red, green, and blue. To turn the pixel off, a small voltage is applied to collapse the bottom reflector against the top. Once a subpixel is either on or off, it will stay there until power is applied again.
These Mirasol pixels can switch between on and off in 10 millionths of a second. “Because it’s mechanical, it’s very fast,” says Galley. Touch-screen Mirasol prototypes at the conference showed video almost as color-saturated and rich as an LCD, and the response rate felt faster than an E-Ink screen.
The company showed other prototype displays incorporating a front light, which had a crisper picture. Front lights are more energy-efficient than the backlights, but are tricky to engineer. Galley says the first product will likely incorporate a touch screen, and the front light will be added to future generations of products. Monochrome Mirasol displays are already in a few products in Asia with very small screens, including a Bluetooth headset and a cell phone.
“The battle in reflective displays is, who can get reasonable color and motion,” says Semenza. “That’s where Qualcomm has leapfrogged other companies.” The interference-based displays may prove to have some cost advantages as well. Most displays must be built on a thin-film transistor backplane, which is one of the most expensive components; the MEMS system in the Mirasol doesn’t require a transistor array. However, the real cost of the displays won’t be known until they’re manufactured in large quantities.
Qualcomm is currently manufacturing the devices at sizes of about a meter squared at a plant in Taiwan, but the company would not comment on their current manufacturing capacity. Galley says Qualcomm has addressed problems with earlier prototypes, but would not go into detail. In addition to the e-reader prototypes, company representatives are showing small static displays that demonstrate further improvements in brightness. Qualcomm is partnering with another manufacturer to develop e-readers using Mirasol displays.
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