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Outdoor Color LCDs

The State-of-the-Art in Display Technology
(by Geoff Walker, 2001)

Ever since the development of the first 9.5-inch color VGA LCD by Toshiba in 1985, the ability to use a portable computer outdoors in sunlight has been on many users' wish lists. While some have been granted their wish (e.g., Compaq iPAQ owners), and some progress has been made in the underlying technology, we're still not there yet.

In this article I will attempt to explain the technological and market issues involved, as well as express a few opinions on what's likely to happen in the near future regarding outdoor color. But first, a few disclaimers: (1) This article covers only color LCD technologies that are currently on the market, not new technologies such as OLED (organic light-emitting displays, the next hot LCD technology) that won't be available until 2003 or later. (2) This article does not cover any mono LCD technologies. (3) This article is strongly biased towards technologies used in products in North America and Europe, since I don't have a good conduit of information on what's happening in the very different Japanese market. (4) This article includes a number of my personal opinions (such as the quality rankings in Table 3) that should not be taken as unalterable fact.

Four LCD characteristics
There are four key characteristics that describe any color LCD: construction, technology, resolution and size. Construction, which refers to the arrangement of layers in the LCD (including the light guide), has the most impact on outdoor viewability. There are three basic constructions used today in color LCDs: transmissive, transflective and reflective. These three constructions are described and illustrated in the "LCD Constructions" sidebar. Note that throughout this article, all LCD constructions are assumed to have a backlight unless they are explicitly identified as having a frontlight or no light source (unlit). (See the sidebar on "Backlights, Sidelights & Frontlights" for some help with this sometimes-confusing terminology.)

There are two basic technologies used today in color LCDs: active-matrix, commonly called TFT, and passive-matrix, commonly called STN (see definition of these acronyms and other specialized terms used in this article). Comparing the two technologies, TFT is higher contrast (it therefore seems brighter and easier to see), has more saturated colors, is faster (able to display moving images without "ghosting"), can be viewed at wider angles, and is generally more expensive than STN. LCD manufacturers are in general moving away from STN and towards TFT. Sanyo, for example, recently ended all production of STN LCDs and now makes only TFTs.

Any of the three constructions can be combined with either of the two technologies, for a total of six types of color LCDs. Of the six, all except transmissive STN can be used outdoors. Table 2 lists the five remaining LCD types (two of which have variations, for a total of seven types) and shows the number of currently shipping products that use each LCD type, along with the manufacturers of the products.

The number of products that use a given LCD type doesn't necessarily correlate with the quality of the LCD type. Many different factors are involved, including the LCD type's age, cost, power consumption, market demand in a specific product category, ease of integration into a product, system manufacturer's risk tolerance for new technologies, LCD vendor's level of innovation, etc. Table 3 lists the same seven LCD types as Table 2, with outdoor and indoor quality ranking and the primary limitations of each type.

Resolution and size, the remaining two key characteristics, are closely related -- in general, the more pixels (dots) there are in an LCD, the larger it is. Actually the number of dots per inch (dpi) in LCDs shipping in products today ranges from a low of 70 (for a 5.7" display) to a high of 150 (for a 6.7" SVGA). Most current LCDs fall in the range of 90 to 125 dpi. Table 4 lists the 11 standard resolutions that are used in today's portable computers, handhelds, PDAs and phones. For each resolution the table lists the size range (based upon products that are on the market today) and the primary application.

A fifth obscure but important key characteristic of an LCD is aperture ratio. This value, which isn't usually on an LCD spec sheet (but can be calculated from other specs), is the ratio of the area of a pixel (sub-pixel, actually) to its total screen area, which includes all the support structures required by the sub-pixel (e.g., the "transistor" in TFT). A larger aperture ratio means that more light gets through each pixel, which makes the LCD brighter. For a given size LCD, such as 10.4", lower resolutions have a higher aperture ratio because fewer pixels occupy more screen area. This is why you'll often see two different LCD resolutions for indoor and outdoor models of a portable computer. For example, the Panasonic Toughbook 28's standard indoor transmissive TFT is XGA, but the optional indoor/outdoor transflective TFT is only SVGA. Because outdoor light has to go through transflective and reflective LCDs twice, the aperture ratio must be larger to achieve the same relative brightness as a transmissive LCD where the light only has to go through the LCD once. For this reason the LCD vendors tend to offer transflective and reflective LCDs in lower resolutions than transmissive LCDs of the same size. This is also why newer TFT technologies (such as polysilicon, versus the older amorphous silicon) work better as transflective and reflective LCDs -- the support structures in a polysilicon LCD are much smaller, which makes the aperture ratio larger, which in turn makes the LCD brighter. Or, to put it another way, a polysilicon TFT can have higher resolution than an amorphous silicon TFT and still achieve the same brightness level.

Getting down to the meat
Now that we've covered the 4+1 key characteristics of LCDs, let's get down to the meat of this article: what combination of characteristics (outdoor color LCD type) is best for each product category, and why. Outdoor color definitely isn't a "one type fits every product" situation. The preferred LCD type for each category of product varies widely -- it depends on a large number of factors, some of which will be discussed in the following paragraphs. The starting point is Table 5, which lists the outdoor color leader in terms of current products, along with the first & second choice of LCD types for each of eight distinct product categories.
Color LCDs of any kind in cellphones in the US and Europe are in their infancy (although a market researcher from DisplaySearch recently said that "up to 85% of new phones in Japan are selling with color," driven by the rich content that NTT DoMoCo delivers). Because of the usage model, any color LCD in a cellphone should be outdoor-readable (can you imagine not being able to read the screen on your cellphone outdoors?). I know of only two currently shipping phones with color LCDs in the US and European markets. The first is the Sprint (US) PCS phone SCP-5000 by Sanyo; this product uses a 2" reflective TFT with frontlight. The second is the Nokia 9210, a "smart phone/communicator" currently shipping in Europe and scheduled to be available in a North American version (Nokia 9290) in mid-2002. This device uses a 1.3" x 4.2" modified transmissive TFT. There are more color phones expected in Q4 this year, such as the Microsoft/Sendo Z100 ("Stinger"). According to the press release, this phone will use "a 65,000-color TFT display, comparable in quality to a high-end laptop display" -- which means that it's probably a transmissive TFT that won't be very useable outdoors. The press releases of two more -- the Ericsson T68 and the Motorola Accompli -- simply say "256-color LCD display." Hopefully they'll use transflective STN.

In PC Magazine's review of the Sanyo phone, the screen is described as being "clear inside and even brighter in daylight" -- this is characteristic of a reflective LCD. I exchanged emails with a Nokia 9210 user in Europe, who told me that "to be honest, direct sunlight kills the [9210] display -- it's still useable outdoors, but far from ideal. The screen is so dark in sunlight that wearing any kind of sunglasses makes it almost unreadable." Reflective TFT with a frontlight is therefore the First Choice LCD for phones in Table 5. A white LED light source makes more sense for a phone than a CCFL light source, since a phone doesn't have to be as bright as a handheld computer. Since transflective TFTs aren't available yet in small sizes, transflective STN currently is the best Second Choice. It would have provided a better balance of indoor/outdoor readability in the 9210 than Nokia's modified transmissive TFT -- although it wouldn't be as bright and contrasty indoors.

The Compaq iPAQ H3600 is the first product with an outdoor color LCD to win truly broad market acceptance in the US (the current sales rate is said to be more than 100,000 per month). The iPAQ uses a 3.8" 1/4-VGA, reflective TFT with frontlight from Sony. Because of the great success of this product, many other LCD vendors (such as Philips, Sanyo, NEC, Casio and Panasonic) have announced or started shipping very similar LCDs (all in 1/4-VGA resolution). You can therefore expect reflective TFTs with frontlights to become widely used in Pocket PC and Palm-OS handhelds (the new Sony Clie PEG-N710c uses a 3.1", 320x320 reflective TFT with frontlight). Unfortunately, this LCD type is unlikely to migrate to larger/higher-resolution products in the near future because frontlights don't work very well above 6" (see "Light Guides" on page 37).

The iPAQ wasn't actually the first PDA to use a reflective TFT. Compaq launched the Aero 2100 PDA in 1999 with a reflective TFT, but the frontlight used white LEDs as the light source (instead of the much brighter CCFL lamp used in the iPAQ). The combination of the very dim light source (10-15 nits) and the first-generation reflective LCD from Sharp caused the product to be rejected by the market. It simply wasn't bright enough either outdoors or indoors.

After reflective TFT, there really isn't a Second Choice LCD. Transmissive STN, such as used in the HP Jornada 520/540 series and the Fujitsu PenCentra 200 CTM, can't be used outdoors at all. Transflective STN just can't compete with reflective TFT on quality terms. Transflective TFT would work in a PDA, but none of the LCD vendors seem to be willing to invest in developing a 3.8" transflective TFT (all transflective TFTs so far are 10.4" and above). Unlit reflective TFT (such as used in Nintendo's very successful GameBoy) won't work in a PDA because PDA users, unlike GameBoy users, aren't willing to go find "task lighting" -- they want to use the PDA wherever they are, whenever they want, without having to go find some light.

Consumer Notebooks
The NEC Versa DayLite is shown as the leader in outdoor color in Table 5 (not linked yet) because it's the first consumer notebook to use transflective TFT. Actually, when the DayLite was launched in April 2000, it used a reflective TFT without a frontlight. (It didn't have a frontlight because its 10.4" LCD is too large to support a frontlight -- see the sidebar on "Light Guides".) The DayLite was originally described as having "a reflective LCD without a backlight" and was advertised as a notebook for outdoor use ("for architects, contractors and others who need to compute outdoors"). A review of the product on CNN.com observed that "If you take the Versa DayLite indoors, it's nearly impossible to see the screen."

At the end of June, after only two and a half months of shipments, NEC quietly revised the DayLite to use a transflective TFT instead of a reflective TFT. According to an NEC representative, the reason was that "feedback from the target market indicated that users wanted to be able to use the product both outdoors and indoors." Now the DayLite is described by NEC as having "an illuminated reflective LCD, viewable outdoors and indoors." The same NEC representative said that NEC didn't want to use the term "transflective" because consumers didn't easily understand it. Actually, "illuminated reflective" can describe both reflective TFT with a frontlight, or transflective TFT with a backlight, so from the point of view of simplifying things for the consumer, it's a pretty good choice!

After transflective TFT, there really isn't a Second Choice LCD for consumer notebooks. Transmissive TFTs are used in 95% of today's notebooks because the market strongly prefers the high quality of transmissive TFTs (the remaining 5% use transmissive STN). Transflective STNs are even worse than transmissive STNs -- they look so bad compared to transmissive TFTs that nobody would even think about buying one in a notebook today.

Rugged Notebooks
Rugged notebooks are often used in vertical applications, which creates more demand for outdoor color in that category. If your job is gathering data from oil and gas wells, and you need a notebook to record the data, it's not practical to hunt for shade whenever you need to use your PC. Since 95% of all notebooks use transmissive TFTs, the logical solution in the past has been to modify the transmissive TFT so that it works better in bright light, without affecting indoor use. A modification that costs a few hundred dollars is generally not a big problem to a vertical market buyer (unlike the dollar-conscious consumer).

However, now that Panasonic has launched a notebook that uses a 12.1" transflective TFT (the Toughbook 28), the days of modified transmissive TFTs are numbered. They may continue to be used in some situations where the highest available resolution is required (i.e., where the smaller aperture ratio of a high-resolution, amorphous silicon, transflective TFT makes it unacceptably dark). Generally speaking, though, as the range of available sizes of transflective TFTs increase over the next year or two, they will replace most applications of modified transmissive TFTs. An alternative Second Choice LCD for a rugged notebook is still a high-brightness transmissive TFT, particularly if plenty of power is available, such as in a truck or at a fixed outdoor test station on an oilrig.

Vertical Handhelds
There aren't any PDA-sized vertical handhelds with outdoor color on the market yet. The dominant products in this category are Symbol's monochrome Palm-OS and Windows CE handhelds (SPC-1500/1700 and PPT-2700). However, given the activity of the LCD vendors in this space, it's only a matter of time until a PDA-sized vertical handheld with outdoor color appears.

In 5"-8.2" vertical handhelds, the situation is very difficult -- the only outdoor color LCD type available in these sizes is transflective STN, which generally has very low market acceptance. (The market acceptance of the reflective STN with frontlight used in one model of Fujitsu's PenCentra is even lower -- a Fujitsu representative told me that it was literally unsaleable until the product was enhanced with a liquid-filled touchpanel, which significantly reduces reflected ambient light. Reflective STN isn't used in any other vertical market handheld in this category, so it really doesn't count.) While none of the existing products in this category have very good outdoor color LCDs, the TouchLite from Two Technologies is probably one of the best. It uses an unusually large 5.7" transflective STN.

LCD choices in this category are limited mostly because LCD vendors can't afford to build LCDs for low-volume markets. Compared to the PDA and consumer notebook markets, the vertical handheld market is miniscule. For an LCD vendor to consider building a new LCD (e.g., a 6" VGA transflective TFT, which many handheld vendors would love to get their hands on), the minimum annual volume commitment is 50K-100K, which is out of the ballpark for vertical handheld manufacturers. In addition, LCD vendors typically want an up-front NRE (non-recurring engineering charge) of one million dollars to cover their development costs, and development takes 9-12 months before the new LCD is in full production. Often LCD vendors won't develop a new LCD, even if the system manufacturer is willing to pay for it, because the business risk or the opportunity cost is too high.

The absence of good outdoor color LCDs in this category may have an interesting side effect on vertical application software. With the proliferation of good 1/4-VGA 3.8" and SVGA 10.4" outdoor color LCDs, software developers may be forced to migrate their applications to one size or the other. Unless Kyocera, the leading developer of transflective STNs, has a real quality breakthrough, transflective STNs may completely die out in this category. The result could be a total lack of any outdoor-color handhelds on which to run vertical software written for half and full-VGA.

Tablet PCs
8.4" tablet PCs suffer from the same limitation as the 5"-8.2" vertical handheld category. 8.4" is not a high-volume LCD size, so there are very few choices available for outdoor color. The Fujitsu Stylistic LT C-500's modified TFT is representative of the category -- and the market acceptance of this LCD is only fair. Via and Strata both offer 8.4" tablet PCs with unlit reflective TFTs, but the market for this configuration is tiny (e.g., Strata sells mostly to surveyors). A Sharp representative told me a few months ago that Sharp will probably obsolete their 8.4" unlit reflective TFT by the end of this year due to low sales. A Via representative told me at PC Expo 2001 that Via plans to add a Kyocera 8.4" transflective STN to their product line later this summer, presumably to replace the Sharp 8.4" unlit reflective TFT. His evaluation of the Kyocera LCD was, "it's not a great outdoor color screen, but there simply isn't anything else available in this size for indoor-outdoor use." If one of the LCD vendors (e.g., Toshiba) would take pity on this market and build an 8.4" VGA transflective TFT, several system manufacturers would probably immediately adopt it.

The 10.4" tablet PC category is in better shape because LCD vendors are more interested in building larger LCDs due to potential applications in industrial, medical and instrumentation markets. Fujitsu recently introduced the Stylistic 3500R, an unlit reflective TFT tablet PC. While this product might seem risky since it can really only be used outdoors, Fujitsu hedges by offering the 3500X (transmissive TFT) and the 3500S (modified transmissive TFT). The 10.4" and 12.1" transflective TFTs just announced by Sharp, Philips and Toshiba should energize this category. I expect to see tablet PC products using these LCDs from several system manufacturers sometime next year.

If you take any one of the product categories and ask the LCD vendors why better outdoor color LCDs aren't available in products in the category, you'll probably get an answer that includes many of the following factors:
  • LCDs are low margin and thus very price-sensitive products, so very high volume is required

  • Many LCD applications outside of consumer notebooks and PDAs are not mobile and therefore can use high-brightness transmissive TFTs -- there's actually not that much demand for outdoor color
  • The risks of new product failures in LCDs are high, particularly where the success of a product depends on the user accepting an inherently subjective quality such as sunlight readability
  • Market acceptance of new LCD technologies such as transflective TFT are unknown until a system manufacturer has a big hit with a new product, and it's very difficult to get to that point (chicken & egg)
  • The cost and time to develop and put into production a new LCD are so enormous, and resources are so limited, that most new products have to be very low risk
  • Choosing the sweet spot from a matrix of three constructions x two technologies (with variations such as polysilicon versus amorphous silicon) x 11 resolutions x an average of four sizes per resolution is very difficult -- that's over 500 combinations!
The bottom line is that outdoor color LCDs are limited by a combination of economic and technology factors. The most promising technology for small devices (reflective TFT with a frontlight) is limited by the LCD vendors' focus on a single size and resolution (1/4-VGA 3.8"). In addition, the maximum size of a reflective TFT with a frontlight is limited by the fact that frontlights don't work well over 6". The most promising technology for larger devices (transflective TFT) is limited by the LCD vendors' understandable reluctance to offer a wide range of sizes and resolutions of a brand-new technology that hasn't achieved market acceptance yet.

Higher-resolution transflective TFTs (especially the older, amorphous silicon TFTs) are also limited in outdoor brightness by their smaller aperture ratio. Some of the outdoor color LCD types in current use, such as transflective STN, are likely to fade away in the next year or two, which may severely affect vertical handhelds in the 5"-8.2" category, as well as have an unanticipated side effect on vertical application software. Finally, the whole LCD industry moves relatively slowly, so all of these issues are likely to be resolved in a matter of years rather than months. - -- Geoff Walker

Based in Silicon Valley, Geoff Walker is a consultant with Walker Mobile. Geoff has worked on the engineering and marketing of pen computers since 1990 at GRiD Systems, Fujitsu Personal Systems (now Fujitsu PC) and Handspring. He can be contacted at geoff.walker@att.net.