Feature: Mobile Computing FAQ--Notebook Displays

When it comes to notebooks, the phrase "put it where the sun doesn't shine" is apt. Portable computers allow you to work any time, anywhere--except in bright daylight, where the screen loses all contrast and becomes unreadable. This has left many of you with the following question.

Q: Why does my notebook display wash out in bright sunlight?

A: Because your boss doesn't want you goofing off in the park, silly.

Actually, the answer is more involved, though it all has to do with lighting. There are three types of LCDs (liquid-crystal displays) used in today's notebooks, PDAs, and other portable gadgets: transmissive, reflective, and transflective. I could get really techie describing how these technologies differ, but I'll spare us both the pain and just get to the heart of the matter. Read on for the 411 on the three screen types.

Good Images: Transmissive

The overwhelming majority (like, 99.9999 percent) of current notebooks feature transmissive, color, active-matrix LCDs. That's because transmissive screens offer the best-quality images and are the most affordable option for manufacturers.

Transmissive LCDs are illuminated by fluorescent backlighting and are capable of rendering the greatest color depth, sharpest text, and highest resolutions. The backlighting is a major drag on a notebook's battery, however. And it's easily overpowered by bright sunlight.

"If you shine two lights at each other, the stronger will overpower the weaker," explains Bob O'Donnell, research director for device technology (including LCD screens) for IDC in Mountain View, California. With transmissive screens, backlighting automatically takes a back seat to sunlight. The result is that transmissive screens become nearly unreadable, with little discernible contrast, when exposed to sunlight.

Because of their high quality indoors, though, transmissive color screens have become the most widely produced displays for notebooks and desktop flat-panel monitors. Like any technology, the more transmissive color screens produced, the less costly the displays are to make. And because most notebook users work indoors, it only makes sense to give them the best, least expensive display technology for the job.

Good Outdoors: Reflective

A reflective screen forgoes backlighting in favor of a mirror that reflects ambient light to illuminate the display. As a result, reflective screens are ideal for use in bright sunlight and use the least amount of battery juice. The downside? You guessed it: Reflective screens are typically dim when viewed indoors, which severely limits their use.

With its illuminated reflective screen, NEC's Versa E120 DayLite notebook is an exception to the rule. NEC's notebook, designed for viewing outdoors and only "occasional indoor use," as the company's marketing brochure states, features a switch that turns on the screen's backlighting. For more information, read my hands-on evaluation of the DayLite's dual-mode screen in this issue's Notebooks section.

Somewhere in Between: Transflective

Transflective (or transreflective) LCDs are a kind of hybrid between transmissive and reflective screens. Indoors, transflective screens use backlighting to render images. Outdoors, transflective screens are still legible, because they absorb the sun's rays to produce a bright, readable image (which also saves battery power).

Typically, transflective LCDs render good-quality color and images, but they can't measure up to the high resolutions and sharp images that transmissive screens produce. And because they lack the economies of scale that transmissive screens have, the majority of notebook manufacturers don't see transflective LCDs as a practical option.

There are exceptions, however. Some Panasonic ToughBook notebooks--ruggedized portables for markets such as law enforcement--feature transreflective screens. The ToughBook 28, for instance, is available with a 13.3-inch XGA transmissive, touchscreen, active-matrix color LCD capable of 1024 by 768 resolution or with a 12.1-inch transflective, touchscreen, active-matrix color LCD capable of 800 by 600 resolution. But a 13.3-inch transmissive-screen ToughBook costs $200 less than a similarly equipped model with a smaller transflective screen. For more information about ToughBooks, see "Panasonic Unveils Tougher ToughBooks."

Small transflective screens are often found in today's PDAs, in both color and monochrome versions. Hewlett-Packard's IPaqs feature color transflective displays, for instance, while the Palm I705's screen is monochrome transflective.

In the future, the growing popularity of wireless networks (see the Wireless section) may spur notebook makers to offer more models with transflective or reflective screens, says IDC's O'Donnell. For now, though, most mobile workers will need to stay inside to get their work done. (I've got an inexpensive workaround to the problem, though; see my tip in the Notebooks section.)

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