Lcd

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Principle of
LCD Display
By-Supriya

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Contents
1.   What’s Liquid Crystals (LC)
2.   Introduction to Liquid Crystal Displays
3.   Operating Principle
4.   Display Addressing
5.   Applications
A) Thin Film Transistor (TFT)
B) Alpha-numeric Display
C) Back Lighting System
6.   Reference

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1. What’s Liquid Crystals
(LC)
 intermediary substance between a liquid and solid
state of matter.
e.g. soapy water
 light passes through liquid crystal changes when it
is stimulated by an electrical charge.

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2. Introduction to Liquid
Crystal Displays
 Consists of an array of tiny segments
(called pixels) that can be manipulated to
present information.
 Using polarization of lights to display
objects.
 Use only ambient light to illuminate the
display.
 Common wrist watch and pocket calculator
to an advanced VGA computer screen

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Different types of LCDs
Passive Matrix LCDs (AMLCD)
and Active Matrix LCDs (AMLCD)
 Passive Twisted Nematic Displays (TNLCD)
 Super Twisted nematic LCD (STNLCD)
 Thin Film Transistor LCD (TFT LCD)
 Reflective LCD
 Rear Projection LCD

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 The parallel arrangement of liquid crystal
molecules along grooves
 When coming into contact with grooved surface in
a fixed direction, liquid crystal molecules line up
parallel along the grooves.

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Molecules movement
Offline (no voltage is applied)
 Along the upper plate : Point
in direction 'a'
 Along the lower plate : Point
in direction 'b‘
 Forcing the liquid crystals
into a twisted structural
arrangement. (Resultant
force)

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Light movement
Offline (no voltage is applied)
 Light travels through the
spacing of the molecular
arrangement.
 The light also "twists" as it
passes through the twisted
liquid crystals.
 Light bends 90 degrees as it
follows the twist of the
molecules.
 Polarized light pass through
the analyzer (lower
polarizer).

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Molecules movement
Online (voltage is applied)
 Liquid crystal molecules
straighten out of their helix
pattern
 Molecules rearrange
themselves vertically (Along
with the electric field)
 No twisting thoughout the
movement
 Forcing the liquid crystals
into a straight structural
arrangement. (Electric force)

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Light movement
Online (voltage is applied)
 Twisted light passes straight
through.
 Light passes straight through
along the arrangement of
molecules.
 Polarized light cannot pass
through the lower analyzer
(lower polarizer).
 Screen darkens.

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Offline Online
Sequences of offline and
online mode
Offline
1. Surrounding light is
polarized on the upper
plate.
2. Light moves along with
liquid crystals and twisted
at right angle.
3. Molecules and lights are
parallel to the lower
analyzer.
4. Light passes through the
plate.
5. Screen appear
transparent.

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Sequences of offline and
online mode
Online
1. Surrounding light is
polarized on the upper
plate.
2. Light moves along with
liquid crystals which moves
straight along the electric
field.
3. Molecules and lights are
perpendicular to the lower
analyzer.
4. Light cannot pass through
the plate.
5. Screen appear dark.
Offline Online

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Polarization of light
 When unpolarized light passes through a polarizing filter, only
one plane of polarization is transmitted. Two polarizing
filters used together transmit light differently depending on
their relative orientation.
Online Offline

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Construction of
Liquid Crystal Display
 Two bounding plates (usually
glass slides), each with a
transparent conductive coating
(such as indium tin oxide) that
acts as an electrode;
 A polymer alignment layer :
undergoes a rubbing process as
grooves.
 Spacers to control the cell gap
precisely;
 Two crossed polarizers (the
polarizer and the analyzer);
 Polarizers are usually
perpendicular to each other.

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Properties of LCD Display
 Small footprint (approx 1/6 of CRT)
 Light weight (typ. 1/5 of CRT)
 power consumption (typ. 1/4 of CRT)
 Completely flat screen - no geometrical
errors
 Crisp pictures - digital and uniform colors
 No electromagnetic emission
 Fully digital signal processing possible
 Large screens (>20 inch) on desktops
 High price (presently 3x CRT)
 Poor viewing angle (typ. 50 degrees)
 Low contrast and luminance (typ. 1:100)
 Low luminance (typ. 200 cd/m2)
Maximum luminosity : 50%
of CRT as 50% of light is
blocked by the upper
polarizer.

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Advantage of LCD over CRT
 Smaller size—AMLCDs occupy approximately 60
percent less space than CRT displays—an
important feature when office space is limited.
 Lower power consumption—AMLCDs typically
consume about half the power and emit much
less heat than CRT displays.
 Lighter weight—AMLCDs weigh approximately
70 percent less than CRT displays of comparable
size.
 No electromagnetic fields—AMLCDs do not
emit electromagnetic fields and are not
susceptible to them. Thus, they are suitable for
use in areas where CRTs cannot be used.
 Longer life—AMLCDs have a longer useful life
than CRTs; however, they may require
replacement of the backlight.
Maximum luminosity : 50%
as 50% of light is blocked by
the upper polarizer.

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 Addressing is the process by which pixels are turned on
and off in order to create an image.
 There are two main types of addressing, direct and
multiplexing.
 Direct addressing is convenient for displays where there
are only a few elements that have to be activated. With
direct addressing, each pixel in the display has its own
drive circuit. A microprocessor must individually apply a
voltage to each element. A common application of direct
addressing is the traditional seven segment liquid
crystal display, found in wristwatches and similar devices.

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 In multiplex addressing, a larger number of pixels are
involved. When the elements are in a regular order, they
can be addressed by their row and column instead of
each element being driven separately. This reduces the
complexity of the circuitry because each pixel no longer
needs its own driver circuit.
 If you have a 10x10 matrix of pixels, with direct addressing,
you need 100 individual drivers. However, if you use
multiplex addressing, you only need 20 drivers, one for
each row and one for each column.
 This is a tremendous advantage, especially as displays
become larger and larger.

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Optical Response
 twisted nematic displays can switch between light and dark
states, or somewhere in between (grayscale).
 Electro-distortional curve is shown as follows :
 the electro-distortional response determines the transmission
of light through the cell.
 Different light intensity of an image projected on the screen is
determined by different voltage suppy. Thus the level of
blocking of light may vary.

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5. Applications
A) Thin Film Transistor (TFT)
 Constructed on a glass surface using a photolithographic
process.
 The source and gate are the control electrodes. The drain
electrode connects to the liquid crystal pixel. The thin layer
of amorphous silicon is the semiconducting material that
allows the TFT to function. The capacitor is attached to the
pixel electrode, but is not an integral part of the TFT.

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5. Applications
B) Alpha-numeric display
 Digital letters can be displayed by blocking the lights in
different plates we place.
 For applications such as digital watches and calculators,
a mirror is used under the bottom polarizer. With no
voltage applied, ambient light passes through the cell,
reflects off the mirror, reverses its path, and re-emerges
from the top of the cell, giving it a silvery appearance.
 When the electric field is on, the aligned LC molecules do
not affect the polarization of the light. The analyzer
prevents the incident light from reaching the mirror and
no light is reflected, causing the cell to be dark. When the
electrodes are shaped in the form of segments of
numbers and letters they can be turned on and off to form
an alpha-numeric display.

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5. Applications
C) Back lighting systems
 Alpha-numeric displays are not very bright because the
light must pass through multiple polarizers which
severely cut down on the intensity of the light, in
addition to the various layers of the display which are only
semi-transparent. Therefore a more intense source is
employed in the form of a back lighting system.
For brighter displays
 Light bulbs mounted behind
 At the edges of the display replace the reflected ambient
light.
 Disadvantage : very power intensive. Back lighting
systems are used in more complex displays such as laptop
computer screens, monitors, LCD projectors, pda,
digital devices such as digital camera and DV.

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6. Reference
Reference webpages
 LCD Principle
http://infochem.hanyang.ac.kr/researches/researches_lcd_
english.html
 Sharp
http://www.sharp.ca/lcd_principles.html
 Liquid Crystals
http://www.ee.calpoly.edu/~dbraun/courses/lcd.html
 PC Technology Guide
http://www.pctechguide.com/07panels.htm
 Casio official homepage
http://www.casio.co.jp/edu_e/product/2line/

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