You probably use items containing an LCD (liquid
crystal display) every day. They are all around us -- in
clocks and watches,
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.
A simple LCD display from a
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?
In this edition of HowStuffWorks,
you'll find out how liquid crystals pull off this amazing
trick, and we will look at the underlying technology that
makes LCDs work. You'll also learn how the strange
characteristics of liquid crystals have been used to create a
new kind of shutter and how grids of these tiny shutters open
and close to make patterns that represent numbers, words or
Today, LCDs are
everywhere we look, but they didn't sprout up overnight.
It took a long time to get from the discovery of liquid
crystals to the multitude of LCD applications we now
enjoy. Liquid crystals were first discovered in 1888, by
Austrian botanist Friedrich Reinitzer. Reinitzer
observed that when he melted a curious cholesterol-like
substance (cholesteryl benzoate), it first became
a cloudy liquid and then cleared up as its temperature
rose. Upon cooling, the liquid turned blue before
finally crystallizing. Eighty years passed before
RCA made the first experimental LCD in 1968.
Since then, LCD manufacturers have steadily developed
ingenious variations and improvements on the technology,
taking the LCD to amazing levels of technical
complexity. And there is every indication that we will
continue to enjoy new LCD developments in the future!
Liquid Crystals, Light and Electricity 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 point the same way and
stay in the same position with respect to one another. The
molecules in liquids are just the opposite: They can
point in any direction 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 point the same way,
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
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
rings. It also explains why a laptop
computer's display may act funny in cold weather or during a
hot day at the beach!
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 (see sidebar). 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.
Liquid Crystal Types
Most liquid crystal molecules are rod-shaped and
are broadly categorized as either thermotropic or
Image courtesy Dr. Oleg Lavrentovich,
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 chlorestic, 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.
Building a Simple LCD There's far 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
Liquid crystals can transmit and change polarized light.
The structure of liquid crystals can be changed by
There are transparent substances that can conduct
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
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
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
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, Passive Matrix and Active
Matrix 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
There are two main types of LCDs used in computers,
passive matrix 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.
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
Active-matrix LCDs depend on thin film
transistors (TFT). Basically, TFTs are tiny switching transistors
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 and the Future 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
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.
For more information on LCDs and related topics, check out
the links on the next page!