LCD Display – How does it works - Working princple of an LCD Panel
Main component of LCD DisplayLCD Shutter
Supplying voltage to the transparent electrodes between the pixel and common sides changes the arrangement of liquid crystal. By assembling two polarized boards, the transfer of light from the backlight
can be controlled by the transparent ratio of the LCD Shutter.
Liquid Crystal
Liquid Crystal is a material whose state is between a solid and a liquid. It has both characteristics of solids and liquids, and generally it is a white turbid liquid. Its molecules are normally arranged comparatively opaque and change to transparent with the application of voltage or heat.
Transparent Electrode (Film)
An LCD shutter is operated by supplying voltage derived from the video signal. Transparent film is used for its electrode.
Alignment Film
This is a film for arranging liquid crystal molecules and is made of Polymid resin.
Polarized Board
The light with a specified direction passes through a polarized board.
Drive Transistor
The thin film transistor (TFT) is used to drive the LCD shutter of each pixel.
Color Filter
It is a filter with three colors (R, G, B) arranged for each pixel.
Backlight:
Liquid crystal does not emit light. A light source is needed for display. The light source placed on the reverse side of the LCD panel is called “Backlight.”
What is Liquid Crystal?
Liquid Crystal is a material whose state is between a solid and liquid. It has characteristics of both solids and liquids, and generally is a white turbid liquid. Its molecules are normally arranged comparatively
opaque and change to transparent with the application of voltage or heat. Almost all the materials consist of an organic compound taking the form of a slender stick or a flat plate. There are three types of liquid crystal and they depend on the construction and arrangement of molecules. Generally Nematic liquid crystal is used for the display apparatus.
(a) Smectic
Molecules are in layers and arranged parallel to each other. The center of gravity is arranged at random in the layer.
(b) Nematic
Molecules are not in layers. They are arranged parallel. The center of gravity is able to move freely to the major axis.
(c) Cholesteric
Molecules are in layers and arranged parallel. The arranging direction of the major axis for the neighboring layers is shifted gradually. In order to use liquid crystal for display, it is necessary to regularly arrange the molecules of Nematic (Rubbing-process)
Rubbing-process
After chemicals for arranging are put on the glass plate, they are hardened, and then the surface on the plate is rubbed with a cloth to fix the direction of the gaps that are made. The arranging direction of molecules is settled in the gaps. This process is used to change the characteristics so the molecules that touch the rubbed surface are arranged to the major axis of the rubbed direction. This thin film on the glass plate is called “Alignment film.”
Operation of Liquid Crystal
The chemistry substance required for liquid crystal material is one that reacts so that the arrangement direction is changed according to an applied electric field.
In the LCD display, a liquid crystal is placed between two electrodes. When the voltage is supplied between them, an electric field is generated in the liquid crystal, and liquid crystal molecules are moved and arranged. The Backlight applied to the liquid crystal is either passed or blocked according to the arrangement of the molecules. If an electric field from an external source is applied to liquid crystal, electric dipoles will be generated that will react to the intensity and direction of the electric field. Through the operation of these electric dipoles and the electric field, the power changing direction of liquid crystal molecules is generated. Therefore, according to an external electric field, liquid crystal molecules move and change direction from horizontal to vertical.
Operation
of Polarized Board for LCD Panel (Shutter)
Light is an electromagnetic wave that is oscillating at right angles to the direction of advance. In fact, the oscillating directions of all light is mixed. A polarized board can let only the light in the specific direction
pass from the light with which these various oscillating directions were mixed. Therefore, only the light of the same direction as the polarization direction of a polarized board can be taken out by letting the light
pass through this polarized board. That is, if the oscillating direction of light and the direction of a polarized board are in agreement, the light will pass through a polarized board. Moreover, if the direction of a
polarized board differs from the oscillating direction of light, the light cannot pass through a polarized board. When the oscillating direction of a polarized board and light are shifted 90º(right-angled), the light
is blocked completely. The light passes and looks bright if the two boards are in the same direction when looking at two polarized boards in piles, however, if shifted at right-angles, the light is blocked and looks
dark.
Light is an electromagnetic wave that is oscillating at right angles to the direction of advance. In fact, the oscillating directions of all light is mixed. A polarized board can let only the light in the specific direction
pass from the light with which these various oscillating directions were mixed. Therefore, only the light of the same direction as the polarization direction of a polarized board can be taken out by letting the light
pass through this polarized board. That is, if the oscillating direction of light and the direction of a polarized board are in agreement, the light will pass through a polarized board. Moreover, if the direction of a
polarized board differs from the oscillating direction of light, the light cannot pass through a polarized board. When the oscillating direction of a polarized board and light are shifted 90º(right-angled), the light
is blocked completely. The light passes and looks bright if the two boards are in the same direction when looking at two polarized boards in piles, however, if shifted at right-angles, the light is blocked and looks
dark.
Operation of Alignment Film
Liquid
crystal is inserted into alignment films of an upper and lower plate
that have the direction of grooves shifted by 90º on the LCD display.
The liquid crystal molecules of upper alignment plate are arranged along
with the upper alignment film. The liquid crystal molecules of lower
alignment plate are arranged along with the lower alignment film. The
liquid crystal layer between these alignment films is twisted little by
little and is arranged so that a spiral is formed. Light entering
through the first alignment plate will have its oscillating direction
twisted 90º by the liquid crystal layer between the alignment films.Now
the direction of oscillation is aligned with the second alignment plate
and the light will pass through.
Operation of LCD Panel
In
the LCD panel, a liquid crystal is inserted and enclosed between two
glass plates. The polarized board, transparent electrode, and the
alignment film are formed on these glass plates. The light can be
passed
or blocked by supplying voltage or not to this LCD panel. In the
condition (Switch-Off) that the voltage is not supplied, the liquid
crystal molecules are twisted 90º sideways and arranged spirally. The
oscillating direction of the light that passed the upper polarized board
is changed by the twisted liquid crystal molecule arrangement.
Therefore, the direction of a polarized board and the oscillating
direction of the light which is shifted 90º and arranged become the
same, and this light can now pass through a polarized board. This is the
liquid crystal shutter-on condition and an LCD panel (LCD shutter)
passes the light.
On
the contrary, in the condition (Switch-On) that voltage is supplied,
the liquid crystal molecules are arranged in a line at right angles to a
glass plate. Since vertical liquid crystal molecules do not affect the
oscillating
direction of light, the light that passed the upper polarized board
passes as it is without changing the oscillating direction. Since the
oscillating direction of this light differs from direction of the lower
polarized
board which is shifted 90º and arranged, the light collides with this
polarized board and cannot pass. This is the liquid crystal shutter-off
condition and the LCD panel (LCD shutter) blocks the light.
This
is the basic structure (On—Off of the light by the LCD shutter) of an
LCD panel. It is a sandwich structure of the upper and lower sides of
transparent electrodes, alignment films, and polarized boards,
with an enclosed liquid crystal material between them.
The
LCD panel shown in Fig. 10 is a type of panel that changes the light
into a passage condition when voltage is not supplied between the
upper-and-lower polarized boards that are arranged at 90º. This type
of
panel has the advantage that black contrast is improved, and it usually
works well. This mode is called “Normally White Mode.”
An
LCD panel that passes light when voltage is not supplied is referred to
as “Normally Black Mode.” In practice, with this type (when the
upper-and-lower polarized boards are arranged in the same direction),
displaying
perfect black becomes difficult due to the leakage of light caused by
variations in the arrangement of the liquid crystal molecules.
Transparent Electrode
In
order to generate an electric field in liquid crystal, voltage is
supplied to the upper-and-lower electrodes. If metal is used for these
electrodes, the light is interrupted by this metal and cannot pass into
the
liquid crystal.
Twisted Nematic (TN) Type
A
Nematic type of LCD Display where the liquid crystal molecules are
twisted 90º between upper and lower boards is called a Twisted Nematic
type (TN type) liquid crystal. Most LCD displays are of this type and
feature high contrast (ratio) under low voltage and power.id crystal.
Therefore, a transparent electrode that passes light is used for the
electrode of the LCD shutter.
Super TN (STN) Type
Super
TN type (STN type) LCD Displays are used for LCD televisions, personal
computer monitors, cellular phones, etc. A liquid crystal material
developed to improve visual characteristics, such as contrast
ratio
is used. In this STN type liquid crystal molecules are twisted 180º to
270º and arranged between upper and lower electrodes. By supplying
voltage to this liquid crystal, the transparent ratio of light changes
more steeply. Therefore, with the STN type as compared to the TN type,
contrast and rise characteristic of the voltage (response of switch On
and Off) are improved, and a clearer picture on larger screens becomes
possible.
Triple STN (TSTN) Type / Film STN (FSTN) Type
A
fault of the STN type is that the display colors during On and Off of
the LCD shutter become yellowish green and navy blue. (In TN type, they
are white and black.) This is because light of a specific wavelength is
reflected and scattered by the thickness of the LCD panel. Therefore,
even if a color filter of RGB is attached to an STN type liquid crystal,
bluish green is mixed with the colors from black, gray to white, and a
natural color picture cannot be displayed. The triple STN type (TSTN
type) and the film STN type (FSTN type) have been developed as an
advanced type of STN. In the TSTN type, optically compensated films
(high polymer films) which sandwich the upper and lower LCD panels are
used. They compensate for the twist of the light crystal cell, and the
display colors of yellowish green and navy blue are changed to the
correct white and black. The “FSTN” type uses a single optically
compensated film.
Dot-Matrix
System
LCD displays have two drive systems, Segment and Dot-Matrix. The Dot-Matrix system is used for LCD television displays.
The picture elements (pixels) of the display unit are arranged horizontally (X line) and vertically (Y row) by this Dot-Matrix system, and various characteristics and figures can be displayed. Fig. 12 shows a matrix of “X x Y = 10 (pixels)” with the character “Y” displayed. In this Dot-Matrix system, by making the size of a pixel smaller and increasing the whole number of pixels, the big screen with fine character or picture becomes possible. With the present liquid crystal manufacture technology, the number of pixels per inch has reached 200ppi*, and very high definition screen display is possible. Moreover, the number of pixels of an LCD display panel corresponding to bigger screen sizes can be specified and manufactured. For example, the number of pixels of the SXGA* panel is about 1,300,000 (1,280 x 1,024 = 1,310,720 pixels).
ppi: pixel per inch
SXGA: Super eXtended Graphics Array
LCD displays have two drive systems, Segment and Dot-Matrix. The Dot-Matrix system is used for LCD television displays.
The picture elements (pixels) of the display unit are arranged horizontally (X line) and vertically (Y row) by this Dot-Matrix system, and various characteristics and figures can be displayed. Fig. 12 shows a matrix of “X x Y = 10 (pixels)” with the character “Y” displayed. In this Dot-Matrix system, by making the size of a pixel smaller and increasing the whole number of pixels, the big screen with fine character or picture becomes possible. With the present liquid crystal manufacture technology, the number of pixels per inch has reached 200ppi*, and very high definition screen display is possible. Moreover, the number of pixels of an LCD display panel corresponding to bigger screen sizes can be specified and manufactured. For example, the number of pixels of the SXGA* panel is about 1,300,000 (1,280 x 1,024 = 1,310,720 pixels).
ppi: pixel per inch
SXGA: Super eXtended Graphics Array
Colorization
Since an LCD shutter only passes or blocks light, in itself it cannot display a color picture. The color picture is made by mixing the three colors of RGB (three primary colors of light) respectively, like the CRT
color television. The color LCD panel has a color filter of RGB attached to the monochrome panel. See Fig below. In this color LCD panel, by controlling the voltages and the waveforms that are supplied at each
RGB pixel, the transparent ratio is controlled and hue and brightness are adjusted. Therefore, smaller pixels and more numbers of pixels are required for the color LCD Display. For example, although the
SXGA panel described before has about 1,300,000 pixels, in colorization, there are about 4 million dots (sub-pixels)
Since an LCD shutter only passes or blocks light, in itself it cannot display a color picture. The color picture is made by mixing the three colors of RGB (three primary colors of light) respectively, like the CRT
color television. The color LCD panel has a color filter of RGB attached to the monochrome panel. See Fig below. In this color LCD panel, by controlling the voltages and the waveforms that are supplied at each
RGB pixel, the transparent ratio is controlled and hue and brightness are adjusted. Therefore, smaller pixels and more numbers of pixels are required for the color LCD Display. For example, although the
SXGA panel described before has about 1,300,000 pixels, in colorization, there are about 4 million dots (sub-pixels)
Drive
System
The
drive systems for LCD display are divided into the following classifications:
The
Static Drive System, which is seldom used; The Passive Matrix System, which is
used for still pictures, such as calculators and notebook PCs; The Active
Matrix System, which is suitable for high definition and the high-speed
response needed for big screen LCD television.
Passive
Matrix System
In the structure of a passive matrix system, Y electrodes of the vertical direction (Y-direction) are formed in upper glass plate, and X electrodes of the horizontal direction (X direction) are formed in lower glass
plate as a matrix. The liquid crystal molecules are sandwiched between these electrodes. By supplying voltage between the Y electrode and the X electrode in sequence, at a certain time, an electric field is
generated in the liquid crystal where the selected Y electrode and X electrode cross. Therefore, the liquid crystal molecules of this pixel address (X, Y electrode intersection) change arrangement and an LCD
shutter is turned On or Off.
In the structure of a passive matrix system, Y electrodes of the vertical direction (Y-direction) are formed in upper glass plate, and X electrodes of the horizontal direction (X direction) are formed in lower glass
plate as a matrix. The liquid crystal molecules are sandwiched between these electrodes. By supplying voltage between the Y electrode and the X electrode in sequence, at a certain time, an electric field is
generated in the liquid crystal where the selected Y electrode and X electrode cross. Therefore, the liquid crystal molecules of this pixel address (X, Y electrode intersection) change arrangement and an LCD
shutter is turned On or Off.
In
the dynamic drive system, since the electric signal (voltage) is supplied to
the Y electrode and the X electrode in sequence, the number of pixels which
makes all pixels (the total number of pixels are “X x
Y”) turn on or off becomes “X+Y”. Therefore, compared with the static drive system that has an independent electrode for each pixel, the number of electrodes of the dynamic drive system is very few.
However, with this dynamic drive system, since the electrode itself is the wiring, it has resistance that cannot be disregarded in the big screens. This resistance causes the speed of the shutter to become
slower. Therefore, when displaying moving pictures etc., an afterimage is generated. This passive matrix system is not suitable for LCD televisions with big screens that require moving pictures and high resolution. The active matrix system was developed in order to overcome these faults.
Y”) turn on or off becomes “X+Y”. Therefore, compared with the static drive system that has an independent electrode for each pixel, the number of electrodes of the dynamic drive system is very few.
However, with this dynamic drive system, since the electrode itself is the wiring, it has resistance that cannot be disregarded in the big screens. This resistance causes the speed of the shutter to become
slower. Therefore, when displaying moving pictures etc., an afterimage is generated. This passive matrix system is not suitable for LCD televisions with big screens that require moving pictures and high resolution. The active matrix system was developed in order to overcome these faults.
Active Matrix System
In
the active matrix system, a switch element is attached for every pixel
at the intersection of the X and Yelectrodes of a passive matrix system.
Each pixel is now controlled by the switch element (active element).
Since the switch for each pixel is turned On and Off independently, the
response speed is increased. Thin Film Transistor (TFT) is used for the
switch element and is attached on the glass board. The LCD display
using this TFT is called “TFT LCD display”. The upper electrode for the
whole pattern is formed on the upper glass plate and is called the
“Common Electrode”. A pixel electrode (pixel pattern), TFT (switch
element) which drives a pixel electrode, and X electrode for gate input
and Y electrode for source input of TFT are formed on the lower glass
plate. In this structure, the electric field is generated in the area
between the pixel electrode and the common electrode, and the LCD
shutter for 1 pixel is operated. When an electric signal (voltage) is
supplied to the Y and X electrode of TFT, TFT is turned On, and the
liquid crystal molecules are operated as a light switch. Refer to Fig.
The
amplification operation of a transistor is used for the TFT switch in the
active matrix system. In this system, switching speed is unified over the whole
display, increasing drive response speed as compared
with the passive matrix system. Therefore, TFT LCD display (active matrix system) is adopted for the highly efficient display, which can provide the response speed required for big screens or quickly moving
pictures. However, further response speed is needed for high definition LCD television.
with the passive matrix system. Therefore, TFT LCD display (active matrix system) is adopted for the highly efficient display, which can provide the response speed required for big screens or quickly moving
pictures. However, further response speed is needed for high definition LCD television.
Drive
of Active Matrix System
The TFT LCD display consists of a matrix of n lines of X direction (X0 - Xn-1) and of n rows of Y direction (Y0 - Yn-1). The line of X direction is called the “gate line” and the line (row) of Y direction is called the
“data line.”
First, the scan is started from the pixel address (X0, Y0), and when the address (X0, Yn-1) is selected the scan of X0 line is completed. Next, all the pixels from X1 line to Xn-1 line are scanned in sequence, and the final address is (Xn-1, Yn-1). The operation of selected pixel address (X1, Y2) is explained below. First, (signal) voltage is supplied to X1 line (gate of TFT), next voltage is supplied to Y2 row (source
of TFT), and the address of the intersection of X1 line and Y2 row is selected and its TFT is turned On or Off. However, just switching the TFT on and off will not change the brightness of the screen. The brightness of a screen is changed by controlling the voltage of a data line (Y row). The voltage of the data line (Y2) is supplied in the positive direction to a common electrode (DC drive). In practice a uniform AC voltage is supplied to the common electrode (AC drive) to prolong the life of the liquid crystal.
The TFT LCD display consists of a matrix of n lines of X direction (X0 - Xn-1) and of n rows of Y direction (Y0 - Yn-1). The line of X direction is called the “gate line” and the line (row) of Y direction is called the
“data line.”
First, the scan is started from the pixel address (X0, Y0), and when the address (X0, Yn-1) is selected the scan of X0 line is completed. Next, all the pixels from X1 line to Xn-1 line are scanned in sequence, and the final address is (Xn-1, Yn-1). The operation of selected pixel address (X1, Y2) is explained below. First, (signal) voltage is supplied to X1 line (gate of TFT), next voltage is supplied to Y2 row (source
of TFT), and the address of the intersection of X1 line and Y2 row is selected and its TFT is turned On or Off. However, just switching the TFT on and off will not change the brightness of the screen. The brightness of a screen is changed by controlling the voltage of a data line (Y row). The voltage of the data line (Y2) is supplied in the positive direction to a common electrode (DC drive). In practice a uniform AC voltage is supplied to the common electrode (AC drive) to prolong the life of the liquid crystal.
Angle of View
Angle
of view means the normal visible range (angle) of a screen. In an LCD
display, the angle of view is narrow compared with a CRT or PDP (Plasma
Display Panel). The viewing angle of the typical TN type LCD display is
about 100º. However with the new improved technology that has been
developed the angle of view for LCD display has increased to 160º or
170º. This improved system will be described later. (The angle of view
for a CRT or PDP is 180º.)
Response Characteristic
The
response characteristic of the LCD display is the speed at which the
display is refreshed by the input signal (video data signal). If this
response characteristic is slow, an afterimage will appear on the
screen. Therefore, in large screen LCD television, improving this
response characteristic becomes very important.
Angle of View (TN Type)
The
principle of optical penetration and the interception of the LCD
shutter by the arranged direction of cylindrical liquid crystal
molecules controls the direction of light. Therefore, brightness, hue,
and contrast
depend
on the direction of view of the LCD display. The range (angle) where
these look normal is called the “angle of view.” The fault of the TN LCD
display is that this angle of view is narrow. Fig. 22 shows that
brightness changes depending on the angle the screen with a gray picture
is viewed. In this figure, the liquid crystal molecule leans
diagonally. Therefore, the amount of optical penetration will change
depending on the angle when watching the screen from the front or the
side.
Multi-Domain System
The
arrangement of the TN LCD display is one directional. In this
Multi-Domain System, one pixel is divided into two or more different
arranged domains. Fig. 23 shows the example of Multi-Domain System with
two domains. The quantity of the light per pixel from various angles is
equalized by this system. Moreover, the angle of view becomes even
wider by increasing the number of divisions. However, manufacturing is
difficult in the rubbing process
