LCD Displays - Improvement Technology of LCD Display

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 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*. [The brightness of a screen is equalized as macro view]

MVA (Multi-domain Vertical Alignment) System
In the MVA system, the (alignment) film is arranged so that the liquid crystal molecules are stood vertically. The MVA system combines vertical alignment with the Multi-domain system. By vertically aligning the liquid crystal molecules, the influence of optical interception is lost, and the angle of view and contrast are improved.
A type of material is used that causes the liquid crystal molecules to become vertical to the glass plate without supplying voltage. (Nega-Nematic liquid crystal*)
In the MVA system, attaching the boss by resin and making the liquid crystal molecules stand diagonally on the transparent electrode make multiple alignment domains. Therefore, since the rubbing process can be skipped at the alignment film production, manufacturing becomes easier compared with the multidomain system. Generally, a Posi-Nematic system is used that aligns the liquid crystal molecules by supplying voltage.

IPS (In-Plain Switching) System

The structure of an IPS system is shown in Fig. 25. The pixel and common electrodes are mounted to the transparent film (drive transistor) side and the electric field is generated horizontally to the glass plate. With this electric field, the alignment direction of liquid crystal molecules is rotated 90º in parallel to the glass plate.

In the IPS system, liquid crystal molecules rotate all at once in the horizontal direction. Since these liquid crystal molecules do not lean like the TN type, there is little change in the picture characteristics (contrast, brightness, hue, etc.) and the angle of view becomes wider. However, there are a few problems.  The quantity of transparent light is reduced, slower response speed, and a white picture becomes a little bluish or yellowish depending on the viewing direction. The S-IPS (Super-IPS) type was developed to improve upon these problems. In the S-IPS type, the structure of the electrode for driving the liquid crystal molecules becomes a zigzag form, which reduces the change of color, increases the viewing angle to about 160º and has high definition equivalent to a CRT.

Optically Compensated Film

By using the optically compensated film, the phase shift of the STN type of LCD display is corrected, and the angle of view and contrast are improved. Three methods for attaching the optically compensated film are shown in Fig.

OCB (Optically Compensated Birefringence) System

The OCB system combines the bend-alignment system where the liquid crystal molecules are bent and aligned between the upper and lower boards and optically compensation film. This system has the features of increased angle of view and quicker response speeds. However, bend-alignment is difficult to make uniform and stable.

Improvement of Response Speed.  Inpulse System
In order to reduce afterimage and dim outline, there is the system that has the backlight blinked for every writing of one picture or an all black picture in inserted in the fixed cycle. It is called the “Inpulse System.”
For example, with the system called “Super Inpulse System,” the black data is written in every 1/60 second, and the afterimage and the ghosts are reduced.

With the usual LCD panel, since the picture is displayed continuously, the front picture becomes dim as the afterimage.  In the impulse system, by inserting black data between the picture data, the afterimage is reduced and the high-speed response is improved.

FFD (Feed Forward Driving) System
The response speed of LCD brightness can be improved by adding over-shoot characteristic to the data line voltage.

 

 LCD Display – How does it works - Working princple of an LCD Panel Main component of LCD Display  

LCD 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.”

Liquid Crystal
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.

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

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)

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 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.

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.

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.

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*.