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
|           
|
|
|    ||||    |||     ||      |       |||||
|
|        ||||||||     |||||||||||||     ||||    |
|
|    ||||||       |||||        ||||    ||||
|  |||||||
|
|------------------------------------------------------> X

* 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.