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Cellular phone displays go hi-res

Posted: 16 Aug 2005 ?? ?Print Version ?Bookmark and Share

Keywords:cellphone? high-resolution? led? oled? backlight?

Global adoption of cellphones has prompted the 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 functional devices that not only make and receive calls, but also consider e-mail, calculator, schedule manager and alarm clock to be basic functions. These phones 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 role in determining the perceived quality of the overall device.

lcds for mobile phones continue to improve. New red, green, blue composite LED backlights produce an increased color gamut and more vivid images than 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 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.

Hence, displays will continue to improve and meet consumer demands for bright, detailed, colorful images and high-content information in their mobile phones.

According to DisplaySearch, 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 performs well in bright outdoor conditions. But audiovisual content is driving the movement toward transmissive displays with improved image quality for still and moving images. These displays deliver higher brightness and 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 sizes and VGA (480 x 640 pixel) resolutions to create a more detailed image. It is unlikely that displays will get any larger than this, at least in 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 333pixels 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 (50cm or 19.6 inches, compared with 30cm or 11.8 inches), the phone display's apparent pixel density is still 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 ease 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 that displays must not only be thina typical 2.2-inch QVGA panel is only 2.6mm thick, including the backlightbut their supporting circuitry must be small.

The typical large-scale-integration driver chips are 2mm 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 symmetrical appearance.

Power consumption is another important design consideration in display. Brighter backlights and more functions require more power, yet batteries take up more space and add weight as storage capacity increases. Thus, power savings must be designed into every aspect of the device. The LCD panel of a typical 2.2-inch QVGA display consumes only 12mW, but the LED backlight draws an additional 216mW, so the current challenge is to reduce 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- and gate-driver functions can reduce costs. A main display combined with a subdisplay, backlight and digital camera in a single module cuts parts count.

Shinichi Unayama

Associate Director of Product Management

Optrex America Inc.

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