Alternative Display Technologies

Anumber of alternatives to the LCD are presently being propositioned and promoted.
Polymer LCD displays are one option. Certain polymers will conduct electricity
and emit light. These are called Light-Emitting Polymers (LEPs). The example shown
in Figure 4.7 was developed initially for use as a backlight in a handset, produced on
a glass substrate. The longer-term evolution includes plastic substrates and red/
green/blue polymers.
Reverse emulsion electrophoretic displays are a second option. These displays consist
of two glass plates. A color reversed emulsion is injected between the two glass
plates, which are held together like a sandwich. The emulsion consists of a polar solvent
(a liquid with a property like water), a non-polar solvent (a liquid with a property
like oil), one or more surfactants (detergents), and a dye, which is soluble in the polar
solvent and insoluble in the non-polar solvent.

The result is a lot of colored droplets floating in a clear liquid. The droplets can be
electrically charged and made to spread out, which produces color on the display, or
compacted, which makes them transparent. The properties of the emulsion change
with frequency; in other words, the emulsion is frequency or electrophoretically
addressed to provide a dynamic color display—a sort of high-tech lava lamp. (A case
study can be found at www.zikon.com.)
Organic electroluminescent (OEL) displays are a third option. OEL displays use thin
layers of carbon-based (organic) elements that emit light with current passing through
them (electroluminescence). The advantage of OELs is that they do not need a backlight,
because they are by nature luminescent. This saves power. They also have a good
(160°) viewing angle and can be mechanically compact, because the display driver can
be integrated into the substrate. Pixel density is also potentially quite good; Kodak, for
instance, claims to be able to get 190,000 pixels individually addressed into a 2.5-inch
diagonal space.

Another option might be to use miniaturized—that is, thin—cathode ray tubes, as
shown in Figure 4.8. In a thin CRT, an array of microscopic cathodes is deposited on a
baseplate using thin film processing. Each cathode array produces electron beamlets
that excite opposing phosphor dots (i.e., a cold cathode process producing electrons at
room temperature). The process does not need a shadow mask, which means it is relatively
power-efficient, and it uses high-voltage phosphors, which give 24-bit color resolution
of 16 million colors and high luminance.
The CRT would provide a wide-angle view, which is a major performance limitation
with LCDs. It would also produce a 5 ms response time, compared with 25 ms for a
typical LCD, and would consume about 3.5 Watts driving a 14-inch display. Whether
the technology could scale down to a micro-display is presently unproven, but it has
possible future potential, provided the mechanical issues, such as a very high vacuum
gap, can be addressed. Additional information is available at www.candescent.com.
Philips has also proposed 3D displays as a future option. These displays use a lenticular
lens over each pixel segment, which means specific pixels are only visible from
specific angles. Given a reasonably complex driver, a 3D effect can be created.
Although electrophorescent, OEL displays and miniaturized CRTs all offer interesting
longer-term options; LCDs (3D or otherwise) are presently preferred, particularly
for smaller screen displays, where it is proving relatively easy to deliver good resolution,
fast refresh rates, good contrast ratios, an acceptable power budget, and tolerable
cost. Displays and display drivers are generally not the limiting factor in delivering
end-to-end quality, provided cost targets can be achieved. 131