LCD.SEP–II Stanley–pm
Applications drive LCD technology
Despite a plethora of newer LCD technologies, applications demand that
the older ones retain, and even expand, their market share
BY BILL ROTH II Stanley Co., Inc. Irvine, CA
In the competition for market superiority among the various display
technologies, CRTs are slipping as the standard against which all other
display devices are measured. That dominance is eroding because of the
increasing sophistication of flat-panel displays, particularly
liquid-crystal displays (LCDs). Because they need no high-voltage power
supplies and face no danger of implosion, LCDs are safer than CRTs.
Compared to CRTs, flat-panel displays in general are inherently more
rugged and LCDs, in particular, have sharply defined pixel edges that make
them easy to read. Flat-panel displays also do not have the annoying
flicker of CRTs. Although the well-entrenched CRT has a definite
price/performance advantage over any flat-panel technology to date, its
size and fragility precludes it from many applications–most notably
portable and industrial. In these arenas, the CRT has been replaced, or is
battling with, the various flat-panel technologies, especially LCDs. The
undisputed king of LCDs is the active-matrix LCD (AMLCD) (see Fig. 1).
First used on 3-in. miniature TVs, AMLCDs are now finding applications in
multimedia, video, and animation. They are also being offered in some of
the more sophisticated laptop computers, as well as in aircraft instrument
panels.
Because these displays have a thin-film transistor (TFT) and a
capacitor at every pixel, they are, essentially, a huge IC. From the
standpoint of scalability, therefore, AMLCDs are considered to be
extremely expensive to manufacture. They are used only in high-end, rather
exotic applications where performance outweighs cost. They are available
in full color (red, green, and blue subpixels), have a very wide viewing
angle, a contrast ratio of 100:1, and a response time well below that
required for video applications. Because of the high cost of AMLCDs,
passive-matrix LCDs–the low-cost twisted-nematic (TN) and supertwisted
nematic (STN) models–continue to be the flat panel of choice for many
applications.
In mainstream usage, the less-glamorous passive-matrix LCDs began with
dot-matrix applications in such relatively simple products as digital
watches, clocks, and calculators. These applications later expanded to
electronic typewriters and PCs as LCD screen size and capacity increased.
These application-driven display devices, in fact, have been advancing at
a more rapid pace than any of the other competing flat-panel technologies.
Twisted-nematic displays In passive displays, the liquid-crystal pixels,
or picture elements, are arranged in rows and columns (the “matrix”).
Twisted nematic (referring to the way the liquid-crystal rods are
arranged) displays typically have two polarizers, arranged 90 degrees to
each other, to achieve black pixels on a light gray background.
Alternatively, the polarizers can be parallel for a negative mode
display–white characters on a dark background. TN displays can achieve a
contrast ratio of about 3:1, the ANSI-specified minimum. ANSI prefers a
value of 7:1. Relatively inexpensive to manufacture and available in a
temperature range from -20 degrees to 70 degreesC, twisted-nematic
dot-matrix displays are monochromatic and do not have any undesirable
coloration. They are adequate for direct addressing and multiplexed
addressing with a limited number of lines. Their viewing angle and
contrast ratio are, however, the most limited of all LCD technologies.
For higher-information displays, supertwisted-nematic (STN) technology is
preferred (see Fig. 2). A display based on this technology uses nematic
rods twisted in a range of 200 degrees to 240 degrees, compared to the
standard 90 degrees twist of TN displays. STN-type displays have a typical
contrast ratio of between 6:1 and 8:1, and a viewing angle about twice as
wide as TN displays. Orienting the polarizers of an STN LCD so they are
60 degrees and 30 degrees, respectively, to their associated rubbing
directions produces birefringent interference colors. These can be either
blue letters on a yellow-green background (called yellow mode), or, if one
of the polarizers is rotated by 90 degrees, a blue background with yellow
letters (called blue mode). The blue-mode displays have a slightly lower
contrast, and few commercial applications for this type of display have
been found. While STN displays have many application advantages, their
stricter alignment requirement makes them slightly more expensive to
produce than TN type. They also have a narrower temperature range,
typically 0 degrees to 50 degreesC. One color variation–a gray-mode
display–is actually a yellow-mode STN display with a purple filter on the
front. The purple filter cancels the yellow-green background, resulting in
blue on a gray background. This color is often more appealing to users
than the characteristic blue and yellow-green. These displays are also
called supertwisted birefringence effect (SBE) and highly twisted
birefringence (HBE). The next step in display development eliminated the
birefringence-effect coloration to produce the highly desirable
black-on-white display. Called double-layer supertwisted nematic (DST or
D-STN), the display has a second liquid-crystal layer oriented in the
opposite direction to the first layer. This second layer acts to cancel
the color introduced by the first layer. The two layers block so much
light, in fact, that the display must be backlit by a bright fluorescent
tube to have acceptable brightness. This is often undesirable as the
fraction of a watt usually required to power an LCD could jump to as much
as 10 W, depending on the size of the display. The D-STN display has black
pixels on a white background and has the best performance of any
passive-matrix display, with a contrast ratio of 15:1 to 20:1. While D-STN
displays are more expensive, heavier, and require a bright backlight,
their contrast compares to that of a printed page. Some companies,
including Stanley, have replaced the second LCD layer of the D-STN display
with a polymer film retarding layer. In this case, the film cancels the
color of the first layer. These film-supertwist-nematic (FSTN) displays
block less light and so eliminate the need for fluorescent backlighting.
Additionally, FSTN displays are lighter, less expensive to manufacture,
and have a higher contrast ratio than conventional STN displays. They are
offered in reflective mode, or with backlighting. To date, FSTN displays
represent the best the passive LCD has to offer. For higher-end
applications, a designer must resort to active-matrix LCDs.
Newer LCD implementations The high cost of AMLCDs has spurred a search
for alternative LCD technologies offering similar capabilities at a lower
cost. Product research and development is being pursued aggressively by
several companies. Kopin Corp. (Taunton, MA) recently announced a
prototype 640 x 480-pixel AMLCD formed from single-crystal silicon (see
Electronic Products, July, p. 22). The 1.5-in. panel boasts a resolution
of 500 lines/in. or roughly four times the packing density of
current-generation products. The prototype can operate at 50 MHz and
requires only an 11-pin interface with the computing system in which it is
used. The company is also designing a 1,280 x 1,024-pixel display for
head-mounted applications, a project supported by the Advanced Research
Projects Agency. Also recently, Motif, Inc. (Tualatin, OR), a joint
venture of In Focus Systems and Motorola, announced that will domestically
manufacture low cost, video-speed LCDs using Active Addressing. Typical
LCDs are sequentially addressed row by row. Active Addressed LCDs
continually signal the pixels in any sequence to achieve high-contrast
images with a fast response time. The response time is, like AMLCDs,
suited to the display of video images. The technology places the
proprietary pixel-addressing algorithms and circuitry off the screen (see
Fig. 3). This reduces the manufacturing costs, relative to AMLCDs, as
conventional processes can then be used. Prototypes have achieved a 35-ms
response time and a contrast ratio of 60:1. So far, they are capable of a
resolution of 240 x 240 pixels, extendible to 640 x 480 with no change in
technology and no impact on performance. A palette of 262,144 colors was
achieved using In Focus Systems' patented TSTN
(triple-supertwisted-nematic) subtractive color technology. Production of
the LCDs is expected to begin during the first quarter of 1994.
CAPTION:
Fig. 1. The 6.3-million-pixel AMLCD demonstrated by Xerox PARC (Palo
Alto, CA) at the SID show in May this year, represents the
state-of-the-art in LCD design.
Fig. 2. Despite being somewhat fragile, STN LCDs, combined with a touch
screen, find myriad applications in industrial control, and POS terminals.
Fig. 3. Figure and caption to come.
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