The global adoption of cellular phones has prompted development of small, high-resolution color displays to handle the increasingly stringent audiovisual needs of these devices.
Despite shrinking to less than the size of a deck of playing cards, mobile phones have become fully functioned devices that not only make and receive calls but also consider e-mail, a calculator, schedule manager and alarm clock to be basic functions. These phones increasingly can do digital still photography and provide photo transfer, gaming, storage, music playback and Web browsing.
High-resolution color displays are helping phone makers incorporate more visual functions into the mobile handset, since the quality of the image plays a major role in determining the perceived quality of the overall device.
Liquid-crystal displays for mobile phones continue to improve. New red, green, blue composite LED backlights produce an increased color gamut and more-vivid images than the traditional yellow-blue composite LEDs. Color-correction software for handheld displays is under development, and promises to deliver more-accurate and realistic images. New LCD cell structures can increase image contrast, which can make dramatic improvements in the quality of color images. And to reproduce crisp moving images, LCD panels with faster response times are being developed using various material improvements and new controlling software.
As a result, displays will continue to improve and meet the increasing consumer demands for bright, detailed, colorful images and high-content information from their mobile phones.
According to DisplaySearch (Austin, Texas), sales of displays for mobile phones nearly tripled from 2001 to 2005, increasing from 366 million to 975 million units. By 2009, the total will swell to nearly 1.4 billion displays.
Simpler phones can get by with a transflective LCD, which is adequate for simple functions indoors and which performs well in bright outdoor conditions. But audiovisual content is driving the movement toward transmissive displays with superior image quality for still and moving images. These displays deliver higher brightness and higher indoor contrast, along with more-vivid colors and wider viewing angles.
Currently, 2.2-inch-diagonal displays are standard for Japanese mobile phones, either in QCIF+ (176 x 220-pixel) or QVGA (240 x 320-pixel) resolutions.
The trend is toward larger screens with 2.4-inch-diagonal size and VGA (480 x 640-pixel) resolution, to create a more-detailed image. It is unlikely that displays will get any larger than this, at least for the near future, since a larger display will make the phone too big to be convenient.
It is also unlikely that resolutions higher than VGA will be required; a 2.4-inch VGA screen has 333 pixels per inch, which is more than three times as many as a typical SXGA desktop PC monitor. Even when you consider that users tend to view a phone display at about half the distance used to view a computer monitor (50 cm, or 19.6 inches, compared with 30 cm, or 11.8 inches), it still works out that the phone display's apparent pixel density is about twice that of the PC monitor.
One problem with the increased resolution, however, is that the LCD cell aperture ratio is decreased, so a brighter backlight is required to achieve the same image brightness as with a lower-resolution panel. For example, a 2.2-inch QCIF+ panel has a 60 percent aperture ratio, compared with only 10 percent for a 2.4-inch VGA panel. Today's more-efficient LED backlights may alleviate that problem.
While consumers want larger, higher-resolution displays on their phones with brighter images, they also want their phones to be thinner and lighter. This means not only that displays must be thin-a typical 2.2-inch QVGA panel is only 2.6 mm thick, including the backlight-but also that their supporting circuitry must be small.
The typical large-scale-integration driver chips are 2 mm thick, but new designs have cut that dimension in half. And by mounting these along the bottom of the screen instead of along the side, the display can be centered in the phone housing for a pleasingly symmetrical appearance.
Power consumption is another important display design consideration. Brighter backlights and more functions require more power, yet batteries take up more space and add weight as the storage capacity increases. As a result, power savings must be designed into every aspect of the device. The LCD panel of a typical 2.2-inch QVGA display consumes only 12 milliwatts, but the LED backlight draws an additional 216 mW, so the current challenge is to reduce the power consumption of the LED backlight.
Higher levels of integration reduce the parts count for the phone, which can result in lower material costs, reduced assembly costs and increased reliability. A single large-scale integration combining source driver and gate driver functions can reduce costs. A main display combined with a subdisplay, backlight and digital camera in a single module also exemplifies how to cut the parts count.
Shinichi Unayama (firstname.lastname@example.org), associate director of product management for Optrex America Inc. (Plymouth, Mich.)
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