About LCDs
You probably use items containing an LCD (liquid crystal
display) every day. They are all around us -- in laptop
computers, digital clocks and watches, microwave ovens, CD
players and many other electronic devices. LCDs are common
because they offer some real advantages over other display
technologies. They are thinner and lighter and draw much less
power than cathode ray tubes (CRTs), for example.
But just what are these things called liquid crystals? The
name "liquid crystal" sounds like a contradiction. We think
of a crystal as a solid material like quartz, usually as hard
as rock, and a liquid is obviously different. How could any
material combine the two?
We learned in school that there are three common states of
matter: solid, liquid or gaseous. Solids act the way they do
because their molecules always maintain their orientation and
stay in the same position with respect to one another. The
molecules in liquids are just the opposite: They can change
their orientation and move anywhere in the liquid. But there
are some substances that can exist in an odd state that is
sort of like a liquid and sort of like a solid. When they are
in this state, their molecules tend to maintain their
orientation, like the molecules in a solid, but also move
around to different positions, like the molecules in a liquid.
This means that liquid crystals are neither a solid nor
a liquid. That's how they ended up with their seemingly
contradictory name.
So, do liquid crystals act like solids or liquids or
something else? It turns out that liquid crystals are closer
to a liquid state than a solid. It takes a fair amount of
heat to change a suitable substance from a solid into
a liquid crystal, and it only takes a little more heat to
turn that same liquid crystal into a real liquid. This
explains why liquid crystals are very sensitive to
temperature and why they are used to make thermometers and
mood rings. It also explains why a laptop computer display
may act funny in cold weather or during a hot day at the
beach.
Nematic Phase Liquid Crystals
Just as there are many varieties of solids and liquids, there
is also a variety of liquid crystal substances. Depending on
the temperature and particular nature of a substance, liquid
crystals can be in one of several distinct phases. In this
article, we will discuss liquid crystals in the nematic
phase, the liquid crystals that make LCDs possible.
One feature of liquid crystals is that they're affected by
electric current. A particular sort of nematic liquid
crystal, called twisted nematics (TN), is naturally twisted.
Applying an electric current to these liquid crystals will
untwist them to varying degrees, depending on the current's
voltage. LCDs use these liquid crystals because they react
predictably to electric current in such a way as to control
light passage.
Most liquid crystal molecules are rod-shaped and are broadly
categorized as either thermotropic or lyotropic.
Thermotropic liquid crystals will react to changes in
temperature or, in some cases, pressure. The reaction of
lyotropic liquid crystals, which are used in the manufacture
of soaps and detergents, depends on the type of solvent they
are mixed with. Thermotropic liquid crystals are either
isotropic or nematic. The key difference is that the
molecules in isotropic liquid crystal substances are random
in their arrangement, while nematics have a definite order or
pattern.
The orientation of the molecules in the nematic phase is
based on the director. The director can be anything from
a magnetic field to a surface that has microscopic grooves in
it. In the nematic phase, liquid crystals can be further
classified by the way molecules orient themselves in respect
to one another. Smectic, the most common arrangement,
creates layers of molecules. There are many variations of the
smectic phase, such as smectic C, in which the molecules in
each layer tilt at an angle from the previous layer. Another
common phase is cholesteric, also known as chiral nematic.
In this phase, the molecules twist slightly from one layer to
the next, resulting in a spiral formation.
Ferroelectric liquid crystals (FLCs) use liquid crystal
substances that have chiral molecules in a smectic C type of
arrangement because the spiral nature of these molecules
allows the microsecond switching response time that make FLCs
particularly suited to advanced displays. Surface-stabilized
ferroelectric liquid crystals (SSFLCs) apply controlled
pressure through the use of a glass plate, suppressing the
spiral of the molecules to make the switching even more rapid.
Creating an LCD
There's more to building an LCD than simply creating a sheet
of liquid crystals. The combination of four facts makes LCDs
possible:
* Light can be polarized. (See How Sunglasses Work for
some fascinating information on polarization!)
* Liquid crystals can transmit and change polarized
light.
* The structure of liquid crystals can be changed by
electric current.
* There are transparent substances that can conduct
electricity.
An LCD is a device that uses these four facts in a surprising
way.
To create an LCD, you take two pieces of polarized glass.
A special polymer that creates microscopic grooves in the
surface is rubbed on the side of the glass that does not have
the polarizing film on it. The grooves must be in the same
direction as the polarizing film. You then add a coating of
nematic liquid crystals to one of the filters. The grooves
will cause the first layer of molecules to align with the
filter's orientation. Then add the second piece of glass with
the polarizing film at a right angle to the first piece. Each
successive layer of TN molecules will gradually twist until
the uppermost layer is at a 90-degree angle to the bottom,
matching the polarized glass filters.
As light strikes the first filter, it is polarized. The
molecules in each layer then guide the light they receive to
the next layer. As the light passes through the liquid
crystal layers, the molecules also change the light's plane
of vibration to match their own angle. When the light reaches
the far side of the liquid crystal substance, it vibrates at
the same angle as the final layer of molecules. If the final
layer is matched up with the second polarized glass filter,
then the light will pass through.
If we apply an electric charge to liquid crystal molecules,
they untwist. When they straighten out, they change the angle
of the light passing through them so that it no longer
matches the angle of the top polarizing filter. Consequently,
no light can pass through that area of the LCD, which makes
that area darker than the surrounding areas.
Building a simple LCD is easier than you think. Your start
with the sandwich of glass and liquid crystals described
above and add two transparent electrodes to it. For example,
imagine that you want to create the simplest possible LCD
with just a single rectangular electrode on it. The layers
would look like this:
The LCD needed to do this job is very basic. It has a mirror
(A) in back, which makes it reflective. Then, we add a piece
of glass (B) with a polarizing film on the bottom side, and
a common electrode plane (C) made of indium-tin oxide on top.
A common electrode plane covers the entire area of the LCD.
Above that is the layer of liquid crystal substance (D). Next
comes another piece of glass (E) with an electrode in the
shape of the rectangle on the bottom and, on top, another
polarizing film (F), at a right angle to the first one.
The electrode is hooked up to a power source like a battery.
When there is no current, light entering through the front of
the LCD will simply hit the mirror and bounce right back out.
But when the battery supplies current to the electrodes, the
liquid crystals between the common-plane electrode and the
electrode shaped like a rectangle untwist and block the light
in that region from passing through. That makes the LCD show
the rectangle as a black area.
Backlit vs. Reflective
Note that our simple LCD required an external light source.
Liquid crystal materials emit no light of their own. Small
and inexpensive LCDs are often reflective, which means to
display anything they must reflect light from external light
sources. Look at an LCD watch: The numbers appear where small
electrodes charge the liquid crystals and make the layers
untwist so that light is not transmitting through the
polarized film.
Most computer displays are lit with built-in fluorescent
tubes above, beside and sometimes behind the LCD. A white
diffusion panel behind the LCD redirects and scatters the
light evenly to ensure a uniform display. On its way through
filters, liquid crystal layers and electrode layers, a lot of
this light is lost -- often more than half!
In our example, we had a common electrode plane and a single
electrode bar that controlled which liquid crystals responded
to an electric charge. If you take the layer that contains
the single electrode and add a few more, you can begin to
build more sophisticated displays.
Common-plane-based LCDs are good for simple displays that
need to show the same information over and over again.
Watches and microwave timers fall into this category.
Although the hexagonal bar shape illustrated previously is
the most common form of electrode arrangement in such
devices, almost any shape is possible. Just take a look at
some inexpensive handheld games: Playing cards, aliens, fish
and slot machines are just some of the electrode shapes
you'll see.
Passive and Active Matrix
Passive-matrix LCDs use a simple grid to supply the charge to
a particular pixel on the display. Creating the grid is quite
a process! It starts with two glass layers called substrates.
One substrate is given columns and the other is given rows
made from a transparent conductive material. This is usually
indium-tin oxide. The rows or columns are connected to
integrated circuits that control when a charge is sent down
a particular column or row. The liquid crystal material is
sandwiched between the two glass substrates, and a polarizing
film is added to the outer side of each substrate. To turn on
a pixel, the integrated circuit sends a charge down the
correct column of one substrate and a ground activated on the
correct row of the other. The row and column intersect at
the designated pixel, and that delivers the voltage to
untwist the liquid crystals at that pixel.
The simplicity of the passive-matrix system is beautiful, but
it has significant drawbacks, notably slow response time and
imprecise voltage control. Response time refers to the LCD's
ability to refresh the image displayed. The easiest way to
observe slow response time in a passive-matrix LCD is to move
the mouse pointer quickly from one side of the screen to the
other. You will notice a series of "ghosts" following the
pointer. Imprecise voltage control hinders the passive
matrix's ability to influence only one pixel at a time. When
voltage is applied to untwist one pixel, the pixels around it
also partially untwist, which makes images appear fuzzy and
lacking in contrast.
Active-matrix LCDs depend on thin film transistors (TFT).
Basically, TFTs are tiny switching transistors and capacitors.
They are arranged in a matrix on a glass substrate. To
address a particular pixel, the proper row is switched on,
and then a charge is sent down the correct column. Since all
of the other rows that the column intersects are turned off,
only the capacitor at the designated pixel receives a charge.
The capacitor is able to hold the charge until the next
refresh cycle. And if we carefully control the amount of
voltage supplied to a crystal, we can make it untwist only
enough to allow some light through.
By doing this in very exact, very small increments, LCDs can
create a gray scale. Most displays today offer 256 levels of
brightness per pixel.
Color LCD
An LCD that can show colors must have three subpixels with
red, green and blue color filters to create each color pixel.
Through the careful control and variation of the voltage
applied, the intensity of each subpixel can range over 256
shades. Combining the subpixels produces a possible palette
of 16.8 million colors (256 shades of red x 256 shades of
green x 256 shades of blue), as shown below. These color
displays take an enormous number of transistors. For example,
a typical laptop computer supports resolutions up to
1,024x768. If we multiply 1,024 columns by 768 rows by 3
subpixels, we get 2,359,296 transistors etched onto the
glass! If there is a problem with any of these transistors,
it creates a "bad pixel" on the display. Most active matrix
displays have a few bad pixels scattered across the screen.
LCD technology is constantly evolving. LCDs today employ
several variations of liquid crystal technology, including
super twisted nematics (STN), dual scan twisted nematics
(DSTN), ferroelectric liquid crystal (FLC) and surface
stabilized ferroelectric liquid crystal (SSFLC).
Display size is limited by the quality-control problems
faced by manufacturers. Simply put, to increase display size,
manufacturers must add more pixels and transistors. As they
increase the number of pixels and transistors, they also
increase the chance of including a bad transistor in
a display. Manufacturers of existing large LCDs often reject
about 40 percent of the panels that come off the assembly
line. The level of rejection directly affects LCD price since
the sales of the good LCDs must cover the cost of
manufacturing both the good and bad ones. Only advances in
manufacturing can lead to affordable displays in bigger
sizes.
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