Liquid Crystal Display (FAQ) Frequently Asked Question
Fundamental Liquid Crystal Display Technology:
a multi-part introduction for the basic understanding of
Liquid Crystal Displays
Version 1.00 August 18, 1993 By Scott M. Bruck
This FAQ should be printed in 9 POINT MONACO FONT
Since the introduction, rapid decline in price, and increased
availability of notebook computers capable of
operating Graphical User Interfaces (GUIs--MacOS and
Windows), there has been an increased interest in flat
panel display technology. A notebook/palmtop computer
requires a light weight, durable, and reliable display.
Liquid Crystal Display technology has met these requirements
and as a result, virtually all notebook computers are equipped
with some form of LCD. This FAQ is intended to address the
general confusion concerning LCDs that has arisen recently by
explaining the technology, operation, and characteristics of
this important display device.
Part I. Liquid Crystal Display Fundamentals:
1) A general discussion of how liquid crystal displays work.
2) A basic introduction to the chemistry, structure, and
properties of liquid crystals used in displays.
3) An overview of display structure, assembly, and related
technology is summarized.
Part II. Addressing Technology: Passive and Active Matrix
Displays:
1) The differences between Active and Passive matrix display
addressing techniques is examined
2) Color technology applicable to liquid crystal displays is
discussed.
Part III: New Technology and Questions Answered:
1) State of the art displays being prototyped are described.
2) A list of TFT display manufacturers is summarized and a
list of what notebook computers use what TFT display.
3) Solutions to questions proposed on Internet news groups not
covered in the body of the LCD FAQ text.
LCD FAQ Part I: Liquid Crystal Display Fundamentals
1.0 General Characteristics and LCD Modes
Liquid Crystal Displays (LCDs) are categorized as non-
emissive display devices, in that respect, they do not produce
any form of light like a Cathode Ray Tube (CRT). LCDs either
pass or block light that is reflected from an external light
source or provided by a back/side lighting system. There are
two modes of operation for LCDs during the absence of an
electric field (applied Power); a mode describes the
transmittance state of the liquid crystal elements. Normal
White mode: the display is white or clear and allows light to
pass through and Normal Black Mode: the display is dark and
all light is diffused. Virtually all displays in production
for PC/Workstation use are normal white mode to optimize
contrast and speed.
1.1 LCD Cell Switching and Fundamentals
A simplified description of how a dot matrix LCD display
works is as follows: A twisted nematic (TN) LC display
consists of two polarizers, two pieces of glass, some form of
switching element or electrode to define pixels, and driver
Integrated Circuits (ICs) to address the rows and columns of
pixels. To define a pixel (or subpixel element for a color
display), a rectangle is constructed out of Indium Tin Oxide
-- a semi-transparent metal oxide (ITO) and charge is applied
to this area in order to change the orientation of the LC
material ( change from a white pixel to a dark pixel). The
method utilized to form a pixel in passive and active matrix
displays differs and will be described in later sections.
Figure 1 illustrates a cross sectional view of a simple TN LC
display. Figure 2 depicts a dot matrix display as viewed
without its metal module/case exposing the IC drivers.
Looking directly at the display the gate or row drivers are
located either on the left or the right side of the display
while the data or column drivers are located on the top (and
or bottom) of the display. New thin display module technology
mounts the ICs on conductive tape that allows them to be
folded behind the display further reducing the size of the
finished module. An IC will address a number of rows or
columns, not just 1 as pictured in figure 2.
Figure 1: Cross Section of a Simple LC Display
viewer
///////////////////////////////////// Polarizer
_____________________________________ glass
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Liquid Crystal
_____________________________________ glass
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ Polarizer
backlight
Figure 2: LCD panel and IC driver locations
________________________________________
| |
| IC IC | Source/Column ICs
| | |
| | |
|IC---------------------Pixel |
| |
|IC <---- Gate Line/Row IC |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* An IC driver will address a number of row/column lines and
not just the single pixel in the diagram above
Polarizers are an integral part of a LCD display, possessing
the unique property of only passing light if it is oriented in
a specific (oriented) direction. To utilize this
phenomena in TN LC displays, the bottom polarizer orients
incoming light in one direction. The oriented light passes
through the LC material and is either unaltered or "bent" 90
degrees. Depending on the orientation of the top polarizer,
this light will either pass through or be diffused. If the
light is diffused, it will appear as a dark area. Figure 3 is
a simple illustration of the sequence of events that occur
when light passes through a simple twisted nematic LC display.
Figure 3: Polarized Light and its use in a TN LC display
Light (unoriented) will be defined as: !#$%&|-
Polarizer Orientation is defined by: ( $ or # )
($ polarizer will only pass $ light)
(# polarizer will only pass # light) THEREFORE:
Light Polarizer result LC (90 result Polarizer Image
Input type passed degree passed type output
twist)
!##$%%&|-> | # | -> #### -> ~~~~~ -> $$$$ ->| # | ------> Black
!##$%%&|-> | # | -> #### -> ~~~~~ -> $$$$ ->| $ | ------> White
1.2 Liquid Crystal Material
1.21 Fundamentals
Please note, I am not a chemist, so I will keep this
section as simple and concise as possible. Liquid crystals
encompass a broad group of materials that posses the
properties of both a solid and a liquid. More specifically,
they are a liquid with molecules oriented in one common
direction (having a long range and repeating pattern--
definition of a crystal), but have no long range order in the
other two directions. For example, in figure 4 all the lines
are oriented in the Y direction (up and down), but they posses
no common ordering in the x direction (disorder is assumed in
the Z direction). To more easily visualize this, think of
figure 4 as one thin slice (one layer of molecules to be
exact) of a block of material. If you examined another slice,
the molecules would still be oriented in the Y direction, but
they would be in different positions along the X-axis. By
stacking millions of these thin slices, the Z direction is
built up and as a result of the change in relative position on
the x-axis, the Z direction has no long range order.
^ Y
| Figure 4
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* The Z direction is coming out of the page toward the reader
The liquid crystals used for display technology are
thermotropic liquid crystals; they exhibit desired
characteristics over a specific temperature range. This is
the primary reason why LCDs do not operate properly when they
are too cold or too warm. If liquid crystals are too cold,
they will not twist and the display will not form an image.
If the display is too warm, the resistance of the liquid
crystal material changes and this alters the properties of the
display and performance suffers. Liquid crystal material for
display use is normally referred to as TN (STN, DSTN, MSTN,
and etc.) or Twisted Nematic--sometimes known as TNFE or
Twisted Nematic Field Effect. It is called TWISTED since the
crystals are twisted 90 degrees (or more for STN) from the top
piece of glass to the bottom piece of glass. (TN usually
refers only to a 90 degree twist.) Field Effect (a direct
correlation is the semiconductor MOSFET), refers to the LC
material's ability to align parallel or perpendicular to an
applied electric field. As a result, using twisted or
untwisted liquid crystal and two polarizers; an applied
electric field can force the LC material into a particular
alignment effectively diffusing or passing light through the
top polarizer.
As a note of interest, polarizers are also one of the major
reasons that LC displays require bright back lighting. The
polarizers and liquid crystal materials absorb more than 50%
of the incident light. As a result, even though the actual
display is a very low power device, the power hungry back
lighting makes a LCD module one of the primary causes of
short battery life in notebook computers. Due to the fact
that the LC material has optical properties and effectively
bends light, the problem of viewing angle effects occur. When
the user is not directly in front of the display the image can
disappear or seem to invert (dark images become light and
light images become dark). However, LC material and polarizer
technology is rapidly improving and that improvement is
showing up in brighter displays with greater viewing angles.
1.22 Liquid Crystal Alignment
Liquid crystals must be aligned to the top and bottom
pieces of glass in order to obtain the desired twist. In other
words, the 90 degree twist is formed by anchoring the liquid
crystal on one glass plate and forcing it to twist across the
cell gap (the distance between the two glass plates) when
contacting the second plate. Furthermore, The actual image
quality of the display will be dependent on the surface
alignment of the LC material. The method currently used for
aligning liquid crystals was developed by the Dai-Nippon
Screening (English= Big Japan Screening) Company. The process
consists of coating the top and bottom sheets of glass with a
Polyimide based film. The top piece of glass is coated and
rubbed in a particular orientation; the bottom panel/polyimide
is rubbed perpendicular (90 degrees for TN displays) with
respect to the top panel. It was discovered that by rubbing
the polyimide with a cloth, nanometer (1 X 10 - 9 meters) size
grooves are formed and the liquid crystals align with the
direction of the grooves. It is common that when assembling a
TN LC cell, it will be necessary to eliminate patches of non-
uniform areas. The two parameters required to eliminate the
nonuniformities and complete the TN LC display are pretilt
angle and cholesteric impurities. TN LC cells commonly have
two problems that affect uniformity following assembly:
reverse tilt and reverse twist. Reverse tilt is a function of
the applied electrical field and reverse twist is common when
no electrical field is applied. Reverse twist is eliminated
by the introduction of cholesteric additives and reverse tilt
is eliminated by introducing a pre-tilt angle to the LC
material. The pre-tilt angle also determines what direction
the LC molecules will rotate when an electrical field is
applied. Pre-tilt angle can be visualized by considering the
normal position of the LC molecule to be flat against the
glass plate, by anchoring one edge and forcing the other
upward by a specific number of degrees, a pretilt angle is
established.
1.23 Liquid Crystal Display Names and classes
Before discussing the different types of LC displays the topic
of Birefringence must be explained. When a light ray strikes
a crystal ( or crystal-like material), it will be split into
two separate light beams; with one beam perpendicular (offset
by 90 degrees) from the other. Since the beams travel
different paths, they reach the viewer's eyes at slightly
different times. This is an essential point, it may cause the
color or polarity of the display to change when viewed at
angles where the viewer may see both rays.
For active matrix displays, in order to maximize contrast
and gray scale reproducibility, Twisted Nematic (TN) is
utilized. This material is twisted 90 degrees from the top to
bottom glass panels. STN or Super Twisted Nematic is
chemically distinct from TN and the twist angle is usually
greater than 200 degrees. Furthermore, due to the large twist
angle, the actual alignment of the polarizers for STN LCDs are
not perpendicular, but adjusted to find the best direction
(rotation) for optimum display characteristics. The STN
material is rotated in a way so the change from transmission
to dispersion is very abrupt and therefore can respond quickly
to small changes in voltage. Figure 5 illustrates the
response characteristics of a TN curve and Figure 6 shows the
response characteristics of a STN curve which will further
clarify these points.
100% | Figure 5
| Typical Response of a Normal White TN Display
T |
R |************
A | *
N | *
S |<-- Zone I-> *
M | *
I | *
T | *
T | *
A | *
N | <----Zone II--- * --------->
C | *
E | * <--Zone III-->
| *
| *
| *************
0% |________________________________________________________>
Vt (Threshold Voltage)
Applied Voltage
1.24 The TN Liquid Crystal Response Curve
The most prominent feature of the TN response curve is the
central linear region between the two flat areas (Zone II).
Zone I describes the white color of the display when no
electric field is applied. In other words, the display will
transmit virtually all the introduced light. On the other
hand, in Zone III, the display will diffuse light and appear
dark. The middle region can display gray scale or an image
somewhere between White and Black. The key point here is that
you must be able to very carefully control the voltage applied
to the LC cell and maintain it for one duty cycle (before that
pixel is addressed again) in order to produce accurate colors.
For this reason, this type of LC material is primarily used
for active matrix LCDs.
COMMONLY ASKED QUESTION NOTE: because the LC material is
partially twisted in the gray scale area, when looking at a
display at an off angle the colors tend to shift and sometimes
invert due to birefringence.
COMPUTER APPLICATION NOTE: The TN response curve does not have
to be utilized for gray scale, in order to make a simpler
display, improve viewing angle, and use cheaper IC drivers;
the Apple Powerbook 170's TFTs (thin film transistors) drive
the TN response curve directly into region 3. This gives all
the speed/contrast advantages of a TFT display and cheaper
manufacturing cost, but provides no gray scale.
1.25 The STN Liquid Crystal Response Curve
The Key to understanding the STN curve is simply that due to
the addressing method applied, only a small amount of voltage
is available to change the LC material from transmittance to a
dispersion state. For this reason, the shape of the curve has
nearly a 90 degree shift between Zone I and Zone II regions;
in other words, it goes ballistic and nearly straight up !
This property allows the LC material to shift from white to
black at its threshold voltage (VT) without being concerned
with partial transmission (gray scale). Furthermore, the 90
Degree curve shape means that gray scale is not available from
the LC material itself and the driving circuits must provide
the necessary fixes for levels of gray.
STN displays inherently have a yellow on blue appearance
(anyone remember the old Zenith Laptops ?). Because many
individuals found the yellow and blue appearance undesirable,
a number of techniques were developed to convert the STN image
to a black on white scheme. DSTN, developed by Sharp
Corporation, was the first commercial black and white
conversion of the STN display and refers to Double Super
Twisted Nematic. DSTN displays are actually two distinct STN
filled glass cells glued together. The first is a LCD display
as described previously, the second is a glass cell without
electrodes or polarizers filled with LC material for use as a
compensator which increases contrast and gives the black on
white appearance. The drawbacks are a heavier module, a more
expensive manufacturing process, and a more powerful
backlighting system.
FCSTN is Film Compensated STN and is now the most commonly
used STN display technology on the market. FSTN, monochrome
STN, and Polymer film STN are all standard STN displays with a
polymer film applied to the glass as a compensation layer
instead of the second cell as in the case of the DSTN. This
simpler and more importantly cost effective method provides
the preferred black on white image for this display
technology. However, once again, this design lowers the
transmittance of light and requires a more powerful back
lighting system.
COMMONLY ASKED QUESTION NOTE: Why are STN displays slow ?
Due to the method used to address passive matrix (STN/DSTN)
displays and the high density of pixels required for standard
VGA displays, the liquid crystal material must respond to an
extremely small change in voltage. In developing these
materials for this voltage characteristic, there was a
reduction in the switching speed. A slow display can best be
illustrated by the tendency of the cursor to "submerge" or
disappear when rapidly moved across the screen. Another
common example is the blurring of images when they quickly
move across the display as in the case of high speed games. A
fast display is less than 40 milliseconds, most STN type
displays are between 200 and 250 milliseconds. However, some
new LC mixtures are reaching 150 millisecond speeds.
COMMONLY ASKED QUESTION NOTE: What is Contrast ?
Contrast is defined as the ratio of black to white,
more simply put, how black is black when next to a white or
clear pixel. In terms of numbers, passive matrix LCDs are
usually able to produce a contrast ratio of approximately 13
- 20:1; in real terms you get a set of different grays and
blues but no true blacks.
100% ^
| Figure 6
T |
R | ********
A | *
N | *
S |<-Zone I----> *<-----------Zone II--------------------->
M | *
I | *
T | *
T | *
A | *
N | *
C | *
E | *
| *
| *
| *
| *
| **************************
0% ------------------------------------------------------->
Vt Threshold Voltage
Applied Voltage
1.3 Liquid Crystal Display Assembly
Once the switching devices or electrodes have been fabricated
on the glass halves and the polyimide film has been applied &
rubbed, spacer balls (usually 4 to 8 micrometers [1 X 10 - 6
meters] in diameter) are sprayed on one half of the display.
Spacer balls are used to insure that the glass plates remain a
certain distance apart over the entire area of the display;
this is also known as cell gap. If the cell gap is not
uniform, an image will appear different from one end of the
display to the other. If the spacer balls are not applied
correctly, they will collect and the user will be able see
them as strange areas of non-uniform dust or distortion.
(Single spacer balls are too small to see and they are not
black dots.) If the Display has a very large cell gap, when
you apply slight pressure to the display by touching it with
your finger, you will see the image change and the LC material
shift under the glass. Doing this does not damage the
display, but take care when bringing any sharp objects, such
as pen or pencils, near the screen; it is very easy to damage
the polymer film and or polarizers on the display.
The two glass panels are then aligned and glued
together with an epoxy. During panel assembly, if dirt is
trapped between the two glass plates, you most likely will
see these as annoying spots on the display. During the
application of the glue, one corner is left open. In a vacuum
chamber, the liquid crystal material
is drawn into the display through the open corner. Upon
completion, the remaining hole is filled with another epoxy.
The LC material will align itself to the grooves in the
polyimide and spread out around the spacer balls.
After final assembly, excess glass is cut and driver ICs
are mounted. The finished display is mounted onto a backlight
assembly (also known as an inverter assembly) and encased in
metal. There are a number of methods for backlighting a LC
display. STN displays usually have a side, top, or bottom
lighting system. In simple terms, this is where the
fluorescent tube is mounted. For example, in a side-lit
display one or two fluorescent tubes will be located at the
left and or right edges of the display. A fluorescent tube
normally 4 mm in diameter is used. This is dispersed by a
plastic plate around the entire area of the display. A
dispersion plate looks like a white sheet with small holes;
each of the holes provides a small point of light. On top of
the dispersion plate, a diffuser is placed. A diffuser takes
the numerous points of light and uniformly spreads it out over
the entire area of the display. The net effect
is providing a backlighting source around 4 or 5 mm thick !
An Active matrix display, especially color modules, transmit
much less of the incident light and require more elaborate
backlighting systems. An active matrix TFT display has a
matrix fabricated on one piece of glass; the metal lines and
transistor elements are not transparent and block a
significant percentage of light. In order to obtain higher
contrast, newer displays incorporate what is called a black
matrix. This is a black film that surrounds the pixel
elements (this can be on the matrix, but is usually around the
color filters); although this yields higher contrast, it also
reduces brightness. Further complicating this, the polarizers
and the color filters reduce the output to less than 5% of the
incident light. As a result, most backlighting systems
designed for active matrix based displays usually consist of 4
or 5 four mm tubes placed directly behind the display with a
diffuser plate to insure uniform irradiation. Therefore, they
are called backlit. This method of lighting makes the display
slightly larger, heavier, and greatly increases power
consumption. The final metal encased display is called a
display module or sub-assembly and this is what the end user
or notebook manufacturer receives.
References:
Solid State Technology, December 1988, page 65
Amorphous Silicon Technology, Chapter 3 page 77
High-resolution panels target laptop computers, EDN, April 23,
1992
A. Miyaji, M. Yamaguchi, A. Toda, H. Mada, and S. Kobayashi,
Control and Elimination of Disclinations in Twisted Nematic
Liquid Crystal Displays, IEEE Trans. E. D. Vol. ED-24, No.7,
1977, pg. 811
Kaneko, E., Liquid Crystal TV Displays: Principles and
Applications of Liquid Crystal Displays, KTK Scientific
Publishers
Flat Panel Display 1993 (Japanese Publication) from Nikkei
Electronics
Proceedings of SID; May 1993
Various articles from SID Information Display and Electronic
Engineering Times Magazine
Acknowledgments:
I would like to thank Mike Schuster (SCHUSTER@PANIX.COM) for
commenting on clearness and general understanding while
compiling this FAQ.
Written By Scott M. Bruck SBRUCK@EM.DRL.MEI.CO.JP
Matsushita Electric Industrial Co., Ltd.
Liquid Crystal Display Development Center
Development Group #1
Moriguchi-Shi, Osaka 570 JAPAN
In no way does this document represent the views or policies
of Matsushita Electric Industrial Co., Ltd.
Copyright 1992-93 by Scott M. Bruck, Osaka 573 Japan
Copyright Notice: This document is copyright by Scott M.
Bruck. It may be distributed freely electronically in its
complete form including references and copyright notices.
This document may not be included in any publication without
written permission from the author.
Every effort was made to insure the validity of this document.