in August 2003
LCD & Plasma Displays:
By Kirt Yanke
Note: The author takes us through a comparison between
LCD and plasma displays here. However, one caveat is that
LCDs are not as available in the larger sizes at this point,
so the 30- to 40-inch size is the only direct comparison.
LCDs and plasmas will co-exist for awhile, until LCD manufacturers
can achieve sizes as large as plasma’s 70 inches (and
growing!)—and unit pricing becomes more equitable.
Improvements are constant on both sides, and have been for
Plasma displays have been
utilized for information display applications for many years
and have provided users with several benefits. However,
with the growing popularity of larger-screen liquid-crystal
displays (LCDs) in this market, users now have a choice
to make for their visual-display needs. Here, we’ll
explain the many differences between the two technologies.
LCD and plasma displays incorporate
different fixed matrix technologies that provide superior
clarity and definition when used at their native resolution.
How the two displays produce an image, however, is quite
different. An active matrix LCD’s light source normally
is generated by small fluorescent bulbs (Figure
1). The light from
these bulbs is diffused to create uniform light across the
back polarizer, which allows light to go in through only
one direction. Individual cells are turned “on”
and “off” by applying a small electric charge
to the thin film transistor (TFT) located in each sub pixel.
This small charge causes the liquid crystal to “twist,”
allowing light to be passed through the color filters and
the front polarizer. If the LCD cell is not turned on, then
the light doesn’t pass through the front polarizer,
which is perpendicular in respect to the back polarizer.
plasma display (Fig. 2)
is composed of two parallel sheets of glass, which enclose
a mixture of discharged gases composed of helium, neon or
xenon. Dike-like barriers, or ribs, keep the glass plates
parallel and separate. Groups of electrodes sit at right
angles between the panes, forming rectangular compartments,
or cells, between the glass sheets. Phosphors embedded within
each cell individually emit red, green or blue light and
collectively create a single color pixel when excited.
Selectively applying voltages to the electrodes causes them
to generate a discharge in the panel’s dielectric
layer and on its protective surface. This generates ultraviolet
light that excites the phosphors, stimulating them to emit
light. This principle of operation is similar to that of
a fluorescent lamp. In this sense, it is possible to think
of a plasma display as a screen incorporating thousands
of miniature fluorescent lights of different color. An LCD
display absorbs much of the ambient light, while a plasma
reflects more of it.
Brightness and Contrast
The specifications for LCD
and plasma displays are measured differently for brightness
and contrast. LCDs measure brightness according to the Video
Electronics Standards Association (VESA) Flat Panel Display
Measurements (FPDM) Standard Version 2.0 (June
1, 2001), by using a full-screen white pattern. Contrast
ratio according to the VESA standard is measured as the
difference between full-screen white and full-screen black
in a dark room.
Plasma display specifications usually are measured in the
same way a cathode ray tube (CRT) would be measured. The
brightness is specified by using a peak value rather than
a typical value. This is done by generating a small white
square on the screen, concentrating all of the display’s
energy in this small area. The contrast ratio is calculated
as the difference between the small white area and the black
area surrounding it.
VESA FPDM standard is more of a “real-world”
test because there aren’t many applications that use
a small portion of the screen while leaving the rest blank.
In a true comparison using the VESA FPDM standard method
(Fig. 3), typical results
in dark room conditions might be as indicated in Table 1.
LCD displays also generally
perform better than plasma displays under higher ambient-lit
conditions (Figure 4). The contrast of both displays is
reduced under these conditions, but it is reduced to a much
higher degree for the plasma.
indicates that plasmas operate better in darker rooms,
while LCDs are better in brighter environments.
improvement of the NTSC color spectrum (Fig 6) has been
the target for all display technologies. Currently, plasma
displays have a larger color spectrum than LCD displays,
but new technologies are being developed that will soon
improve the color spectrum of LCDs. With new color filters
and backlighting technologies, the color spectrum of an
LCD may soon meet or exceed the NTSC color spectrum.
The color spectrum of a display affects its ability to reproduce
colors accurately. The color spectrum of displays varies
between LCD and plasma manufacturers, and is also affected
by other devices (DVD player, computer video card, etc.)
in the graphics system. Based on all of these factors, as
well as the perception of the human eye and the application
being used, usually it is left to the user to determine
preference and calibration.
Brightness, Color Uniformity
Because LCD displays use backlights
that consist of many fluorescent bulbs, many precautions
have to be designed into the diffusion layer to distribute
light evenly. To prevent “shading” or “hot
spots,” many bulbs and more than one diffusion layer
The result is a uniform image in both brightness and color,
which is different than the LCDs of the past, and comparable
to the uniformity of a plasma display.
There are many concerns with
both LCD and plasma displays when using a static image.
LCDs can exhibit a phenomenon known as “image retention.”
This is caused by a pixel being in the “on”
state for an extended period of time, causing a small charge
to remain in the cell in the “off” state, similar
to a memory effect. However, this phenomenon is not permanent.
To reduce the chance of this phenomenon, many advances have
been made in the liquid-crystal material itself, and the
display may include a “screen-saver” pixel-shifting
technology to further reduce the risk.
Plasma displays use phosphor and can suffer from phosphor
burn-in just like CRT products that use phosphor. Some manufacturers
include pixel-shifting technologies to reduce the chance
of this happening. When phosphor burn-in occurs, the phosphor
material is damaged and this cannot be removed.
The amount of time required
for both image retention and phosphor burn-in is almost
impossible to calculate because of all the variables involved.
The color of the image, the pattern, the display’s
brightness setting and many other factors can affect the
time before this occurs.
There are some concerns with
using plasma displays at high altitudes. Most plasma manufacturers
have an operational altitude specification between 6500
and 9500 feet, while an LCD’s operating altitude specification
is more than 13,000 feet. This allows LCD technology to
be used and transported in some areas where plasma displays
cannot be used because of this limitation to the technology.
The energy cost to operate
an LCD or plasma display can be a determining factor in
large installations. The power consumption of desktop displays
has always been an advantage for LCDs when compared to CRT
technology. Large-screen LCDs continue to be the more cost-effective
solution related to power consumption when compared with
plasma as well. See Figure 7 for an example of the operational
Although the viewable image
size of an LCD or plasma may be identical, the native resolution
can be quite different. Many of the 40-inch to 43-inch plasma
displays have a native resolution of 853x480, and others
have a native resolution of 1024x768. LCDs are capable of
higher resolutions within the same size parameters and have
a native resolution of 1280x768. The native resolution affects
how much information can be displayed on the screen at one
Note: This material is based on information current as
of March 2003. All facts, figures and comparisons are general
in nature and are used to illustrate the theoretical differences
between LCD and plasma technologies. Technical information
is subject to change.