sub-pixels are the same as those in
use with color CRT displays. Plasma
technology has benefited from the years
of phosphor developments that have
made conventional color TVs so good at
accurate color reproduction. Because
each sub-pixel can be individually controlled over a wide range of output light
intensity, by appropriately mixing light
from the three pixel group, an amazingly
large palette of colors could be
displayed. The segmented design of plasma pixels, combined with a bubble-like
design that shields individual pixels from
neighboring pixels, further helps create
very accurate color reproductions. The
first generation displays had high-voltage
electrodes placed in front of and at the
back of each discharge cell, but these
quickly gave way to a design where both
high-voltage electrodes are placed at the
front plane, and coplanar-electrode PDP
cell design is now almost universal.
In contrast to the opposing-electrode design, this structure minimizes energetic ion bombardment of
phosphors and thus prolongs phosphor
life. A thin layer of magnesium oxide protects the inner cell surface from energetic particles created in the discharge
plasma. As each pixel is itself a source of
light, plasma panels don’t need backlighting (as is the case with LCD panels). This
makes them especially bright no matter
what the panel size. The resulting rich,
bright, and vibrant imagery provides for
an extraordinary viewing experience.
The advent of High
Definition (HD) TV technology
has found a compatible partner
in plasma display screens as their
combination works exceedingly
well for delivering and displaying
images of outstanding clarity. No
wonder most plasma TVs are
now sold with built-in HDTV
capability. Their large sizes, coupled with the ability to view
them from virtually any angle,
has also made plasma screens the
favorite for electronic billboards and
other outdoor display applications.
FIGURE 2. A section of an LCD panel, showing
the arrangement of various layers.
Progress in the further development of plasma panels has been rapid.
The first ever commercial plasma display TV was introduced by Pioneer in
1997. Last year, Japanese manufacturers displayed sets with screen sizes up
to 103 inches across at the Consumer
Electronics Show in Las Vegas, NV, and
this year will bring even larger units.
Fujitsu and Hitachi are the leading
producers of plasma display panels in
Japan and they supply their panels
to a number of other Japanese and
European companies. Due to their low
weight and naturally planar construction, plasma displays are the technology of choice for large and extra-large
screen TVs and computer monitors.
Liquid Crystal Display (LCD) panels
have been aggressively competing with
plasma panels with several leading manufacturers offering both varieties.
Compared to plasma displays, liquid
crystal panels have better contrast and
lower power consumption, which
explains their widespread use in handheld and portable equipment. On a shop
floor, these two might appear very much
alike. However, closer inspection reveals
interesting differences that originate
from their very different technologies.
Unlike plasma panels, LCD panels
are not self-emissive but rely on a set
of discharge lamps or LEDs to provide
screen illumination. The technology is
the same as that used in laptop
computer displays with the screen
comprised of a large array of LCD cells
that rely on switching polarized light
to generate various light intensities.
A liquid crystal screen is made of
a layer of special, highly-oriented
molecular material (called liquid
crystal) sandwiched between two
sheets of thin, highly-polished float
glass. These glass panes have a pattern
of transparent electrodes printed on
them that define individual pixels.
There are also sheets of light-polarizing
material that cover the whole assembly both at the top and on the bottom.
The entire multilayer is illuminated
with a flat backlight from the rear.
This arrangement is best understood by looking at a small cross-section of an LCD display, shown in
Figure 2. Just as with plasma screens,
each LCD pixel is also divided into
three sub-pixels for red, green, and
blue colors. Instead of phosphors,
however, these displays use colored
filters to define their sub-pixels.
Polarized light — unlike ordinary light —
has a well-defined directional character with its electric field oscillations all
confined to only one direction that is
at right angles to the direction in which
the light is travelling. Such light can
only pass through a polarizing material
if the latter’s polarizing orientation
is the same as the polarised light’s
preferred direction of oscillation.
LCD cells can switch light by rotating the direction of the polarization of
light passing through them. An applied
cell voltage can alter the direction of
orientation or twist the liquid crystal
molecules which, in turn, changes the
polarizing direction of the light passing
through it. If this direction is the same
as that defined by the polarizing material, then the light gets through the LCD
assembly. Otherwise, it gets blocked.
With a mosaic of red, green, and
blue filtered sub-pixels, color can also
be displayed. The cell switching action
is controlled by individual Thin Film
Transistors (TFTs) that are integrated at
the back of the lower glass pane. This
close integration brings the benefits
of high-speed, transistor-controlled
switching to these so-called active
matrix TFT LCDs and enables them to
display television images.
Because polarization is orienta-tion-dependent, LCDs do suffer from
limited angular coverage. Recent
advances have led to significant
improvements in this area, however.
Furthermore, reliability issues connected with the integration of large arrays
of TFTs means that the manufacture of
large liquid crystal panels becomes
quite tricky and, for this reason, LCD-
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