Download LCD_notes

How does the LCD work?
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The picture that you
see on your laptop
screen is again made
up of many red, green
and blue dots.
Simplified version of the LCD
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On a laptop screen each "dot"
is actually a tiny device rather
than a phosphor.
The “dot” is called a cell.
Light passes thru each cell. At
the end of the cell, light goes
through a red, green, or blue
filter so the cell takes on that
color.
A trio of RGB cells taken
together is a pixel
Color of each pixel is
determined by how bright its 3
cells are.
LCD has Liquid Crystals
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Liquid crystals first discovered in the late 19th century
Transparent substances, exhibiting the properties of
both solid & liquid matter
Light passing thru liquid crystals follows the alignment of
the molecules that make them up
In the 60s, it was discovered that charging liquid crystals
with electricity changed their molecular alignment, and
consequently the way light passed through them
LCD Principles
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The LCD is a non-emissive technology
Liquid Crystals are organic compounds consisting of long rodlike molecules
In their natural state, LC molecules arrange themselves with
their long axes roughly parallel
The alignment of the molecules can be controlled by flowing
the LC along a finely grooved surface.
The alignment follows the grooves, so if the grooves are
parallel, then the alignment is parallel
First Principle of LCD: sandwich LCs between two grooved
surfaces where the grooves of one surface are perpendicular
to the grooves of the other.
Molecules at one surface align vertically & molecules at other
end align horizontally. Those in-between twist.
LCD Principles
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Light follows the alignment of the molecules so it is
twisted through 90o and light is blocked.
When a voltage is applied, the molecules rearrange
themselves vertically
In the LCD screen:
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Liquid crystal is sandwiched between two plates of glass
One plate has horizontal grooves
The other has vertical grooves
Molecules align themselves with the grooves so there is a layer
of horizontal molecules & a layer of vertical ones. Ones in
between do their best to align with their neighbors.
The results is called a “twisted cell” of liquid crystal.
http://www.colorado.edu/physics/2000/applets/dishsoap.html
LCD Principles
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Second Principle:
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Natural light rays are oriented at random angles
A polarizing filter is a set of very fine parallel lines that
act like a net, blocking all light rays except those
parallel to the lines
A second polarizing filter with lines perpendicular to
the first totally blocks this polarized light
http://www.colorado.edu/physics/2000/polarization/blo
cking_light.html
Putting the two principles together
Passive-matrix LCD
 A picture is produced by passing polarized light through a liquidcrystal layer and a electric field that can be aligned to either block
or transmit the light.
 It consists of:
vertical polarizer  vertical grid wires  liquid-crystal layer
 horizontal grid wires  horizontal polarizer
 When the crystals are in an electric field (voltage applied), the
crystalline molecules line up in the same direction, the light is not
twisted (no polarizing effect), thus the light is blocked.
 Characteristics: low weight, low power consumption, small size
How is Color achieved?
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Red, blue, & green filters are used to filter
the light in each cell.
http://www.colorado.edu/physics/2000/lapt
ops/laptop_screen.html
The varying levels of brightness required
to create a full color display is achieved by
changing the voltage strength
Active Matrix
Active-matrix LCD
An extra matrix of transistors is connected to
the LCD panel
There is a transistor at each pixel.
The transistors are used to change the crystal
state quickly
The transistors control the degree (voltage) to
which the state has been changed
The transistors can serve as a memory of the
state of each grid.
LCDs (cont.)
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Passive LCD
screens
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Cycle through each
element of the LCD
matrix applying the
voltage required for
that element.
Once aligned with the
electric field, the
molecules in the LCD
will hold their
alignment for a short
time
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Active LCD screens
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Each element
contains a small
transistor that
maintains the voltage
until the next refresh
cycle.
Higher contrast and
much faster
response than
passive LCD
LCD vs CRT
CRT Advantages
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CRTs use a simple & mature technology and can
therefore be manufactured inexpensively in many
industrialized countries.
While the gap is getting smaller all the time, they remain
significantly cheaper than alternative display
technologies.
The fact that phosphors emit light in all directions means
that viewing angles of close to 180 degrees are possible.
Phosphors have been developed over a long period of
time, to the point where they offer excellent color
saturation at the very small particle size required by
high-resolution displays.
CRT Shortcomings
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However, the CRT has obvious shortcomings
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It sucks up too much electricity
It is simply too big and weighs too much
Its high-voltage electric circuits & strong magnetic
fields create harmful electromagnetic radiation
The scanning technology employed makes flickering
unavoidable, causing eye strain & fatigue
The surface is often either spherical or cylindrical,
with the result that straight lines do not appear
straight at the edges
Contrasting LCDs & CRTs
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LCDs offer advantages in terms of bulk, power consumption, &
flicker
They have a higher price, a smaller viewing angle, & less accurate
color performance
LCDs still don’t support a range of resolutions as well as the CRT
The CRT has three electron beams that must converge faultlessly to
create a sharp image. For an LCD, each cell is switched on and off
individually. Text appears very crisp on the LCD.
LCD cells are either on or off so the flicker problem is not there. An
image displayed at a refresh rate of 40-60 Hz should not produce
any more flicker than one at a 75 Hz rate.
It is possible for one or more cells on the LCD panel to be dead.
Display
Type
Viewing
Angle
PMLCD
Contrast
Ratio
Response
Speed
Brightness
Power
Consumption
Life
49-100 40:1
degrees
300ms
70-90
45 watts
60K
hours
AMLCD
>140
140:1
degrees
25ms
70-90
50 watts
60K
hours
CRT
>180
300:1
degrees
n/a
220-270
180 watts
Years
Input Devices
Logical Devices
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Locator, to indicate a position and/or orientation
Pick, to select a displayed entity or identify a portion of a
picture
Valuator, to input a single value in the space of real numbers
usually between 0 & 1
Keyboard, to input a character string
Choice, to select from a set of possible actions or choices
Locator Devices:
Tablet, Mouse, Trackball, Joystick,
Touch Panel, Light Pen
Pick Devices: mouse, tablet, trackball
Keyboard devices: Alphanumeric keyboard
Valuator Devices:
Rotary dials, Linear sliders, spaceball,
data glove
Choice Devices:
Function keys or bank of buttons
Physical Devices
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Pointing Devices
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Joystick
Mouse
Trackball
Graphics Tablets
3D scanners
Other Physical Devices
Motion Capture