working principle of LCD

1. Course: LCD
Liquid Crystal Display
Course Coordinator: Arpan Deyasi
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Arpan Deyasi, RCCIIT, India

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LCD
thin, flat panel display device
displays electronic information (text, audio, video)
uses polarization property of light to display objects
electronically modulated optical device made up of any
number of pixels filled with liquid crystals

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Why LCD is preferred?
smaller size: offers 60% less space than CRT
lower power consumption: consumes 50% less power and
emits less heat than CRT
less weight: 70% less weight to CRT of compatible size
less EM radiation: negligible emission of EM fields, nor
susceptible to it
longer life: longer useful life than CRT
screen shape: completely flat screen

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Material aspect
flows like liquid
molecules oriented like a solid crystal
can precisely be aligned subjected to electric fields
when properly aligned, allows light to pass through
application of electric field causes it to polarize,
and becomes opaque. after removal of field, it
again becomes transparent

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Arpan Deyasi, RCCIIT, India
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unpolarized
light in
polarizer 1
(vertical)
polarizer 2
(horizontal)
LCD
horizontally
polarized
light out

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unpolarized
light in
polarizer 2
(horizontal)
LCD
polarizer 1
(vertical)
no
light
out

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What is the role of polarization property?
It is used in LCD screen to switch
its colored pixel ON or OFF

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Arpan Deyasi, RCCIIT, India
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fluorescent light
glass plate
electrode
liquid crystal
electrode
light filter
glass plate

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Working principle
This light is incident on millions of pixels
(colored Red, green, Blue)
At the back of screen, bright light shines viewer
when switch is ON, light is
trapped between the
polarizers and can’t pass
through
when switch is OFF, light is
passed through the
polarizers

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Phases of liquid crystal

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o transparent or translucent liquid that causes the
polarization of light waves to change as the waves pass
through the liquid
o characterized by molecules that have no positional order
but tend to point in the same direction (along the director)
o used in twisted nematic displays, the most common form
of liquid crystal display
o all the molecules are subsequently arranged in one
dimension structure
Nematic phase

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Smectic phase
 molecules are oriented along the layer normal (Smectic
A), or tilted away (Smectic C) from it
 characterized by a slight degree of translational order in
the crystal molecules which is not found in the nematic
phase
 while these layers as a whole move freely, movement
within the layers is restricted; therefore, it creates a
slightly more solid substance
 has been found to have fast electro-optical response
 all the molecules are subsequently arranged in one
dimension structure

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Cholesteric phase
o all the molecules are arranged same as nematic
crystals
o all the molecules are subsequently layered at angles to
each other
o They organize in layers with no positional ordering
within layers, but a director axis which varies with
layers

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Types of LCD
• Direct Address Display --- display includes limited
variable components (watch, calculator)
• Passive Matrix Display --- intersection of rows and
columns of electrodes forms the pixels, where complex
electrical waveforms controls the voltage
• Active Matrix Display --- allows electrical signal for
each sub-pixel (controlled by isolated TFT) to avoid
influencing adjacent elements

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Disadvantages
cost: more expensive than CRT
response time: slow response time
viewing angle: restricted viewing angle
contrast: lower contrast than CRT due to poor black-level