DISPLAY SYSTEMS AND PHOTOSENSORS
(PART 1)
LCD
TFT
LED-OLED
CCD
CMOS
LCD Display
M.A.MOEENI
SIMPLE TYPES OF LC
PROPERTY OF LIQUID CRYSTALS (LC)
-Three major characteristics of Liquid Crystal
- The Thermal Nature
Solid State
(Crystal)
Liquid Crystaline State
Liquid
State
Low Temp
High Temp
Melting Point
Clearing Point
- The Optical Nature of a LC molecule
NO change in polarization state
Light
Phase retardation will exist
- The Electrical Nature of the LC molecules
Electrodes
No potential field
AC potential
THE HISTORY OF LIQUID CRYSTAL DISPLAY LCD
Before the appearance of LCD, it is the Cathode Ray Tube (CRT) kingdom. The CRT
monitors dominate almost all the display applications. The CRT, invented by Karl
Ferdinand Braun, is also called Braun tube.
Cathode rays exist in the form of streams of high speed electrons emitted from the
heating of cathode inside a vacuum tube at its rear end. The released electrons form a
beam within the tube due to the voltage difference applied across the two electrodes,
and the direction of this beam is then altered either by a magnetic or electric field to
trace over the inside surface of the phosphorescent screen (anode), covered by
phosphorescent material.
Light is emitted by that material at the instant that electrons hit it.
- good image quality
- no problem with response time
- no problem with viewing angle
But it is bulky and high power consumption, So Display engineers tried looking for
alternative technologies like flat panel display.
In 1990s, technology breakthrough brought the birth of active matrix LCD, along with the
plasma display, both of which become the main stream of the flat display markets,
replacing the CRT
LCD DISPLAY
(A) It has a mirror which makes it reflective
(B) & (F) piece of glass with a polarizing film on the bottom side
(C) & (E) a common electrode plane made of indium-tin oxide on top
(D) layer of liquid crystal substance
THREE COMMON TYPES OF LCD
- TRANSMISSIVE TYPE
LCD
Light
(Back Light)
Eyes
POLARIZER ON BOTH SIDES
- REFLECTIVE TYPE
LCD
Incident Light
POLARIZER ON THE FRONT SIDE
- TRANSFLECTIVE TYPE
Day Light
POLARIZER ON THE FRONT SIDE
REFLECTOR ON THE BACK SIDE
LCD
Night Light
(Back Light)
TRANSFLECTOR ON THE BACK SIDE
An alternative way to achieve high-resolution LCD is to use the Liquid Crystal on Silicon
(LCOS) devices. LCOS devices use only one glass substrate, and employ a silicon
wafer for the back substrate. The pixels are then generally coated with a reflective
aluminum layer, and then a polyimide alignment layer.
This technology can also be used in personal viewer such as viewfinder in digital
cameras and camcorders.
LCD MODES
Along with the development of LCD's driving infrastructure, different LCD modes were introduced
to improve the image quality.
-ASV
-BINEM
-Cholesteric
-ECB
-SSFLC
-Guest-Host
-IPS
-LCOS
-MVA
-PDLC
-Pi-Cell
-PVA
-STN
-TN
George Heilmeier in 1968
TWISTED NEMATIC (TN) MODE
The TN mode is the "workhorse" for the LC display. It was first introduced by Schadt and
Helfrich, and also by Fergason in 1971
TWISTED NEMATIC (TN) MODE
The gray scale is achieved by applying intermediate voltages between 0 and the value at
which light is completely blocked.
A TYPICAL TN TYPE LCD CELL
Polarizer
(Axis 0 degree)
Glass with electrodes
NO power
supply
With AC Volts
connected
Polarizer
(Axis 90 degrees)
Cell Gap
The separation between two glasses
The smaller the cell gap, the faster response.
Light
Depending on how the LCD fluid is formulated.
TWISTED NEMATIC (TN) MODE
TWISTED NEMATIC (TN) MODE
For a LCD, each pixel is divided into three subpixels, which have red, green and blue color
filters.
The exact color coordinates of the white point depend on the relative transmission and color
purity of the red green and blue subpixels.
ADVANCED SUPER VIEW (ASV) MODE
The ASV mode was developed by Sharp.
Because of the full circle rotation of the director, the viewing cone is very symmetric and
viewing angle performance is excellent.
POLYMER DISPERSED LIQUID CRYSTAL (PDLC) MODE
-The PDLC display consists of droplets of liquid crystals inside a polymer network
In the off state, the droplets are randomly aligned hence the light is scattered in a large angle
towards the viewer.
In the on state, light can be transmitted with a very high transmission.
POLYMER DISPERSED LIQUID CRYSTAL (PDLC) MODE
The working voltage and response time of the PDLC can be affected by:
- the resistive and dielectric properties of LC inside the droplet and the polymer properties
- size of the droplets
- shape of the droplets
- the viscosity of the of the droplets
There are a few factors influencing the contrast ratio of the PDLC display:
- the cell gap
- the density of the droplets
GUEST - HOST (GH) MODE
In a Guest - Host system, the mixture is prepared by mixing LC and dichroic dyes.
The dichroic dyes absorb the light whose E-field is along the long axis of the dye.
When the LC molecules change their orientation, the dye will also change along with
LC molecules, consequently, the absorption axis is changing, a light transmission
can be modulated.
There are three simple GH displays:
- The Heilmeier type GH
- The Double Layer type GH
- The PDLC type GH.