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Lovakiranvarma Myla
In this post, we are going to studying how the LCD monitors or display will work and what type of technologies are used to build LCD displays.
What is an LCD?
LCD stands for Liquid Crystal Display, which is used
to shows status of an application, display values, debugging a program, etc.,
Construction of Liquid crystal display
A Liquid crystal display is a passive device, which
means it doesn’t produce any light to display characters, images, video and
animations. But it simply alters the light travelling through it. The internal
construction of LCD describes how the light altered when it passes through it
in order to produce any characters, images, etc.
Consider a single pixel area in LCD, in which there
are two polarization filters oriented at 90 degree angle to each other as shown
in figure 1.1. These filters are used to polarize the unpolarized light. The
first filter (Vertical polarized filter in figure 1.1) polarizes the light with
one polarization plane (Vertical). When the vertically polarized light passes
through the second filter (Horizontal polarized filter) no light output will
produce.
Figure 1.1 Orientation
of two polarization filters in LCD
The vertically polarized light should rotate 90
degrees in order to pass through the horizontal polarized light. This can be achieving
by embedding liquid crystal layer between two polarization filters. The liquid
crystal layer consists of rod shaped tiny molecules and ordering of these molecules
creates directional orientation property. These molecules in the liquid crystal
are twisted 90 degrees as shown in the figure 1.2. The vertically polarized
light passes through rotation of the molecules and twisted to 90 degrees. When
the orientation of light matches with the outer polarization filter light will
pass it and brightens the screen.
Figure 1.2 Liquid
Crystal molecules orientation.
If the Liquid crystal molecules are twisted 90
degrees more precisely, then more light will pass through it. Two glass
transparent electrodes are aligned front and back of the liquid crystal in
order to change the orientation of the crystal molecules by applying voltage
between them as shown in figure 1.3 and figure 1.4. If there is no voltage
applied between the electrodes, the orientation of molecules will remain twist
at 90 degrees and the light passes through the outer polarization filter thus
pixel appears as complete white. If the voltage is applied large enough the
molecules in the liquid crystal layer changes its orientation (untwist) so that
light orientation also changes and then blocked by the outer polarization
filter thus the pixel appears black. In this way, black and white images or
characters are produced. By arranging small pixels together as a matrix will
produce on which it is possible to show different sizes of images and
characters. By controlling the voltage applied between liquid crystal layers in
each pixel, light can be allowed to pass through outer polarization filter in
various amounts, so that it can possible to produce different gray levels on
the LCD screen.
Generally the electrodes is made up of Indium Tin
Oxide (ITO) which is transparent material, hence it is simply called glass
electrodes plates. LCD display is also “twisted nematic LCD” because of twist
and untwist of molecules in liquid crystal layer.
Figure
1.3 Orientation of Liquid crystal molecules altered by applying voltage between
two ITO glass plates.
Figure
1.4 a gif animation shows the LCD working
In order to produce color images a color filter is
placed in front of the outer polarization plate as shown in figure1.5. The red,
green and blue are the three standard colors filters are placed for every three
pixels to produce different color images by varying the intensity of each color.
Figure
1.5 Placement of color filter in front of the polarized filter to produced
color images.
LCD
Technologies:
Based on the technology
implementation LCD is classified into two types.
1.
Passive
LCD’s -- These LCD’s doesn’t produce any light to turn on or
off desired pixels to produce the characters or images. It uses a grid of
conducting material to activate each pixel.
Advantage:
·
Less expensive than Active matrix
displays
·
Ease of implementation.
Disadvantage:
·
Slow response time due to that it
doesn’t produce colors exactly.
·
Poor contrast so that ghost images will
appear.
Types of Passive matrix display
technologies:
a.
Twisted
Nematic (TN).
b.
Super
Twist Nematic (STN).
c.
Film
Compensated Super Twist Nematic (FSTN).
d.
Color
Super Twist Nematic (CSTN).
2.
Active
LCD’s --
These displays are similar to passive, but an active device (transistor) and a
capacitor is connected to each pixel to control the voltage precisely. The
capacitor for each pixel holds the charge for complete refresh cycle.
Advantage:
·
Faster pixel response rate than Passive
matrix displays thus stunning images will be produced.
·
Higher contrast, higher resolution so
that shaper display output.
·
This display’s comes with better viewing
angle than passive matrix display.
Disadvantage:
·
These displays are highly expensive than
passive matrix displays, because each pixel require a transistor and capacitor
pair.
·
More complex hardware is required.
Types of Active matrix display
technologies:
a.
Thin
Film Transistor (TFT).
b.
Thin
Film diode (TFD).
Twisted Nematic
(TN) display – In these types of
LCD displays the molecules in the liquid crystal layer is twisted by 90 degrees
to alter the light passing through the pixel.
Super Twisted
Nematic (STN) display – 180 to 270
degrees twist of molecules, so that more contrast will be achieved in displays
than Twisted Nematic technology. These types of LCDs are used in old mobile
phones. These types of LCD suffer with lower image quality and slower response
time than TFT, because of that blurring of characters or images are occurred. Double
layer STN (DSTN), Monochrome STN (MSTN) and Fast Response STN (FRSTN) is also
use the same STN technology with minor variations.
Film Compensated
Super Twisted Nematic (FSTN) – To
achieve more sharpness and better contrast, it uses a film compensation layer between
STN display and outer polarized filter. Double Film Super Twist Nematic (FFSTN)
display is advanced than FSTN to achieve more contrast.
Color Super
Twist Nematic (CSTN) – These displays
uses red, green and blue color filters to produce color filters.
Thin film
transistor (TFT-LCD) – This type of
display uses a thin film transistor for every pixel in the screen as shown in
the figure 1.6. These transistors are responsible to produce sharper images by
varying voltage applied to it and the cafspacitors are used to keep constant
voltage to activate pixel over the entire refresh cycle. TFT LCD’s are used in
Television sets, Laptops, Mobile phones, etc.
Figure 1.6 a transistor is connected between row and
column lines of every pixel in TFT technology
Types
of LCD Displays
Based
on the type of information displayed, the LCD’s are classified into three
types.
1. Character LCD or Segment LCD displays
2. Graphical LCD displays
3. Color LCD displays
1.
Monochrome Character LCD or Segment LCD Displays:
The LCD displays will display all
alpha numeric values with some special characters including currencies symbols
of different countries. These displays are mainly used in calculators, remote
controllers, and display panels, etc. These displays are not capable to produce
any graphic animations or images.
2.
Graphical LCD displays:
These LCD displays are capable to
produce any monochrome graphical images and animations by energizing set of
pixels in the LCD display. These are also capable to produce characters,
numbers and special symbols.
3.
Color LCD displays:
These LCD displays contains three
sub pixels with primary colors Red, Green and Blue. Each color sub pixel can
produce a range of 256 shades by varying the voltage applied. Therefore Red,
Green and Blue can produce 16.77million (256*256*256) colors. These color
displays require more number of transistors for its construction.
Ex: Generally the laptops contain
these color LCD displays with maximum resolution 1,024x768. general home personal computers got demand due to LCD monitor usage.
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NOTE: One of the Best online website for to buy LCD monitor, Laptops and Hard Drives, DVD players, Computer Parts, Cellphones, Cameras and surveillance, car and GPS, video games play stations, Desktop at discount offers
TigerDirect -- > www.tigerdirect.com
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