Transcript of "Display devices CRT and LCD screen"
The main principle behind liquid crystal
molecules is that when an electric current is
applied to them, they tend to untwist. This
causes a change in the light angle passing
through them. This causes a change in the
angle of the top polarizing filter with respect
to it. So little light is allowed to pass through
that particular area of LCD. Thus that area
becomes darker comparing to others.
WORKING OF LCD
For making an LCD screen, a reflective mirror
has to be setup in the back. An electrode plane
made of indium-tin oxide is kept on top and a
glass with a polarizing film is also added on the
bottom side. The entire area of the LCD has to
be covered by a common electrode and above it
should be the liquid crystal substance. Next
comes another piece of glass with an electrode
in the shape of the rectangle on the bottom and,
on top, another polarizing film. It must be noted
that both of them are kept at right angles.
When there is no current, the light passes
through the front of the LCD it will be
reflected by the mirror and bounced back. As
the electrode is connected to a temporary
battery the current from it will cause the liquid
crystals between the common-plane
electrode and the electrode shaped like a
rectangle to untwist. Thus the light is blocked
from passing through. Thus that particular
rectangular area appears blank.
PASSIVE AND ACTIVE MATRIX
Passive-matrix LCDs use a simple grid to
supply the charge to a particular pixel on the
display. Creating the grid is quite a process!
It starts with two glass layers called
substrates. One substrate is given columns
and the other is given rows made from a
transparent conductive material. This is
usually indium-tin oxide. The rows or
columns are connected to integrated
circuits that control when a charge is sent
down a particular column or row.
The liquid crystal material is sandwiched
between the two glass substrates, and a
polarizing film is added to the outer side of
each substrate. To turn on a pixel, the
integrated circuit sends a charge down the
correct column of one substrate and a
ground activated on the correct row of the
other. The row and column intersect at the
designated pixel, and that delivers the
voltage to untwist the liquid crystals at that
notably slow response time and imprecise
voltage control. Response time refers to the
LCD's ability to refresh the image displayed.
The easiest way to observe slow response time
in a passive-matrix LCD is to move the mouse
pointer quickly from one side of the screen to
the other. You will notice a series of "ghosts"
following the pointer. Imprecise voltage control
hinders the passive matrix's ability to influence
only one pixel at a time. When voltage is applied
to untwist one pixel, the pixels around it also
partially untwist, which makes images appear
fuzzy and lacking in contrast
Active-matrix LCDs depend on Thin Film Transistors
(TFT). Basically, TFTs are tiny switching transistors and
capacitors. They are arranged in a matrix on a glass
substrate. To address a particular pixel, the proper row is
switched on, and then a charge is sent down the correct
column. Since all of the other rows that the column
intersects are turned off, only the capacitor at the
designated pixel receives a charge. The capacitor is able
to hold the charge until the next refresh cycle. And if we
carefully control the amount of voltage supplied to a
crystal, we can make it untwist only enough to allow some
light through. By doing this in very exact, very small
increments, LCDs can create a gray scale. Most displays
today offer 256 levels of brightness per pixel.
Uses Less Power:- LCDs also produce less heat, which means less
load on air conditioning.
Takes up less space:- LCD monitors are small, thin, and weight less.
Highly adjustable:- Many LCD models can be rotated 90 degrees,
allowing you to view websites in portrait mode. LCDs can also be
mounted on the wall or on an arm.
No flicker:- LCDs don't have lines that need to be scanned like in CRTs.
No flicker means a lot less eye strain.
Less glare:- Due to the material of the LCD screen, less light is
reflected at the user. Same with "no flicker", this results in less eye
Brightness:- LCD monitors are brighter than the traditional CRTs.
Less distortion:- Using a direct digital input from the graphics card
produces cleaner 'output'. The monitor's perfect geometry means
images aren't distorted, which is a boon for graphic designers and the
More expensive than CRTs.
Dead pixels:- When the electrical current to one or more pixels
does not operate properly, one or more cells are permanently
aligned, resulting in a dead pixel.
Screen care and fragility:- LCD monitors may be slim, but
they're also more prone to damage and screen breakage.
Native resolution:- LCD monitors can only display information
well at the resolution they were designed for. At any other
resolution, image quality will suffer.
Pixel response time:- In videos and fast-paced video games,
LCDs suffer from the ghosting effect.
Viewing angles:- Looking at an LCD monitor from an angle
causes the image to look dimmer or even disappear completely.
Colours:- The colour accuracy of an LCD monitor does not
match that of a CRT monitor's.
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