In 2007, LCD televisions surpassed sales
of CRT-based televisions worldwide for the first time, and their sales figures
relative to other technologies are accelerating. LCD TVs are quickly displacing
the only major competitors in the large-screen market, the plasma display panel and rear-projection
television. LCDs are, by far, the most widely
produced and sold television display type.
LCDs also have a variety of
disadvantages. Other technologies address these weaknesses, including organic light-emitting diodes (OLED), FED and SED, but as of 2014 none of these have entered
widespread production for TV displays.
Basic LCD concepts
LCD televisions produce a black and colored
image by selectively filtering a white light. The light was provided by a
series ofcold
cathode fluorescent lamps (CCFLs) at the
back of the screen. Today, most LCD-TV displays use white or colored LEDs as backlighting instead. Millions of individual LCD
shutters, arranged in a grid, open and close to allow a metered amount of the
white light through. Each shutter is paired with a colored filter to remove all
but the red, green or blue (RGB) portion of the light from the original white
source. Each shutter–filter pair forms a single sub-pixel. The sub-pixels are so small that when the display is
viewed from even a short distance, the individual colors blend together to
produce a single spot of color, a pixel.
The shade of color is controlled by changing the relative intensity of the
light passing through the sub-pixels.
Liquid crystals encompass a wide range of (typically) rod-shaped polymers
that naturally form into thin, ordered layers, as opposed to the more random
alignment of a normal liquid. Some of these, the nematic liquid
crystals, also show an alignment effect
between the layers. The particular direction of the alignment of a nematic
liquid crystal can be set by placing it in contact with an alignment layer or director, which is essentially a material with microscopic grooves
in it, on the supporting substrates. When placed on a director, the layer in
contact will align itself with the grooves, and the layers above will
subsequently align themselves with the layers below, the bulk material taking
on the director's alignment. In the case of a Twisted Nematic (TN)
LCD, this effect is utilized by using two directors arranged at right angles
and placed close together with the liquid crystal between them. This forces the
layers to align themselves in two directions, creating a twisted structure with
each layer aligned at a slightly different angle to the ones on either side.
LCD shutters consist of a stack of three
primary elements. On the bottom and top of the shutter are polarizer plates set at right angles. Normally light cannot travel
through a pair of polarizers arranged in this fashion, and the display would be
black. The polarizers also carry the directors to create the twisted structure
aligned with the polarizers on either side. As the light flows out of the rear
polarizer, it will naturally follow the liquid crystal's twist, exiting the
front of the liquid crystal having been rotated through the correct angle, that
allows it to pass through the front polarizer. LCDs are normally transparent in
this mode of operation.
To turn a shutter off, a voltage is
applied across it from front to back. The rod-shaped molecules align themselves
with the electric field instead of the directors, distorting the twisted
structure. The light no longer changes polarization as it flows through the
liquid crystal, and can no longer pass through the front polarizer. By
controlling the voltage applied across the liquid crystal, the amount of
remaining twist can be selected. This allows the transparency of the shutter to
be controlled. To improve switching time, the cells are placed under pressure,
which increases the force to re-align themselves with the directors when the
field is turned off.
Several other variations and modifications
have been used in order to improve performance in certain applications. In-Plane Switching displays (IPS and S-IPS) offer wider viewing angles and
better color reproduction, but are more difficult to construct and have
slightly slower response times. Vertical Alignment (VA, S-PVA and MVA) offer
higher contrast
ratios and good response times, but suffer
from color shifting when viewed from the side. In general, all of these
displays work in a similar fashion by controlling the polarization of the light
source.
Addressing
sub-pixels
In order to address a single shutter on
the display, a series of electrodes is deposited on the plates on either side
of the liquid crystal. One side has horizontal stripes that form rows, the
other has vertical stripes that form columns. By supplying voltage to one row
and one column, a field will be generated at the point where they cross. Since
a metal electrode would be opaque, LCDs use electrodes made of a transparent
conductor, typically indium
tin oxide.
Since addressing a single shutter requires
power to be supplied to an entire row and column, some of the field always
leaks out into the surrounding shutters. Liquid crystals are quite sensitive,
and even small amounts of leaked field will cause some level of switching to
occur. This partial switching of the surrounding shutters blurs the resulting
image. Another problem in early LCD systems was the voltages needed to set the
shutters to a particular twist was very low, but that voltage was too low to
make the crystals re-align with reasonable performance. This resulted in slow response times and led to easily visible "ghosting" on these displays on fast-moving images, like a mouse
cursor on a computer screen. Even scrolling text often rendered as an
unreadable blur, and the switching speed was far too slow to use as a useful
television display.
In order to attack these problems, modern
LCDs use an active
matrix design. Instead of powering both
electrodes, one set, typically the front, is attached to a common ground. On
the rear, each shutter is paired with a thin-film transistor that switches on in response to widely separated voltage
levels, say 0 and +5 volts. A new addressing line, the gate line,
is added as a separate switch for the transistors. The rows and columns are
addressed as before, but the transistors ensure that only the single shutter at
the crossing point is addressed; any leaked field is too small to switch the
surrounding transistors. When switched on, a constant and relatively high
amount of charge flows from the source line through the transistor and into an
associated capacitor. The capacitor is charged up until it holds the correct
control voltage, slowly leaking this through the crystal to the common ground.
The current is very fast and not suitable for fine control of the resulting
store charge, so pulse
code modulation is used to accurately control the
overall flow. Not only does this allow for very accurate control over the
shutters, since the capacitor can be filled or drained quickly, but the
response time of the shutter is dramatically improved as well.
Building a
display
A typical shutter assembly consists of a
sandwich of several layers deposited on two thin glass sheets forming the front
and back of the display. For smaller display sizes (under 30 inches),the
glass sheets can be replaced with plastic.
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