Transcript Liquid Crystal Displays

Liquid Crystal Displays for
Laptops and TV
What they are and how they work
Liquid Crystal Display (LCD) for TV or laptop
Introduction
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In 1980, flat panel TV & laptop displays didn’t exist. LCDs made them possible.
An LCD controls light from a backlight, or from around us: it does not emit light
It does this independently for each of many tiny areas (pixels) on the screen
In a colour LCD, each pixel is comprised of a red, green and blue subpixel, each
with its own colour filter
Colours are produced by applying a small voltage at each subpixel to allow more, or
less, red, green, or blue light to pass through it
This causes a colour picture to be displayed on the LCD
Subpixels can change fast enough to allow moving pictures to be displayed
Where do all the colours come from?
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Each row of subpixels has its own narrow colour filter stripe - some red, some
green and some blue
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You can see these with a x30 magnifier on a laptop, or TV, LCD screen
If all the subpixels in an area are bright, then we see white there. If they are dark, we see black
Screw up your eyes slightly and look at this picture
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Subpixels are too small to see individually by eye, so their colours mix
We see colours that depend on the brightnesses of adjacent R, G & B subpixels
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Each LCD subpixel is controlled by a voltage (applied using conductors you can see
through) to create the right amounts of R, G & B for each part of the display
Liquid Crystal Display (LCD) for TV or
laptop
What is a liquid crystal?
• All materials are comprised of vast numbers of tiny
particles (molecules)
• A liquid crystal has rod-like particles that all point in
the same direction (more or less)
• It is a milky liquid that forms when certain solids melt
and becomes a normal clear liquid at some higher
temperature
– In an LCD, the thin layer of material used will remain a liquid
crystal over a wide temperature range (typically from -20oC to
+80oC)
– Note that a liquid crystal is not solid, liquid or gas. It is an
additional phase of matter.
– It has unusual optical properties, but these can normally only be
seen using Polarisers
Liquid Crystal Display (LCD) for TV or laptop
What are polarisers (often called Polaroid)?
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Polarisers are thin plastic sheets used for the lenses of Polaroid sunglasses
The sheet has a preferred direction (created by stretching it when it is made)
If two sheets have their preferred directions parallel, they will allow light to pass
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Demonstrate this using the two polarisers.
If their preferred directions are crossed at a right angle, they will block light and
look black, or dark blue
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Demonstrate this using the two polarisers. What happens if they are at 45 degrees?
What happens when you look at a laptop, or LCD TV, through a polariser and turn the sheet?
Parallel
45 degrees
Crossed
Liquid Crystal Display (LCD) for TV or laptop
How is liquid crystal used in an LCD?
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In an LCD, it is used in a thin layer
(~0.005 mm thick) between glass
plates, when it appears transparent
Surface coatings on the plates make
its rod-like particles twist by 90o
through the layer
The LCD changes its optical
properties when a small voltage is
applied to clear conducting layers
(made of Indium Tin Oxide, ITO), on
the inside surface of each plate. This
voltage is always AC, since DC
voltage (e.g. from a battery) would
damage the LCD.
Applying the voltage realigns the tiny
particles perpendicular to the plates
The resulting optical change can only
be seen using polarisers, as shown
Liquid Crystal Display (LCD) for TV or laptop
So how does an LCD work?
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An LCD between crossed polarisers “uncrosses” them and allows light to
pass
– See Figure 1
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An LCD between parallel polarisers “crosses” them and light is blocked
– See Figure 2
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This property is temporarily destroyed on applying a small voltage to its
conductors
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An LCD placed between crossed polarisers on a light box allows light to pass.
If ~3Vac is applied to its conductors. The LCD becomes black. See Figure 3
When the 3Vac is removed, the LCD becomes clear after a short delay.
If the polarisers are parallel the LCD goes from black to clear. See Figure 4
Between 0V and 3Vac, a graph of the light intensity against voltage shows that greys
are observed above ~ 1V threshold. See Figure 5
If a coloured filter is placed beneath the LCD and the voltage varied, it goes from
coloured to black, or vice versa
If you prod the LCD in the ON state, you can see waves in the LC layer.
If Cellophane is placed between the LCD and one of the polarisers, it changes
between two colours, since the Cellophane rotates different colours by different
amounts. Overlapping shards of Cellophane will give a changing coloured pattern.
If you drive this LCD with an audio voltage (~5Vac) from an amplifier the colours
change in direct response to the music/speech.
Liquid Crystal Display (LCD) for TV or laptop
So how does a simple LCD on a clock, or a watch, work?
• In a simple LCD the electrodes are patterned so that different
areas can be switched ON and OFF independently
• Applying a small voltage to the electrodes for selected areas
causes a numeral to be displayed
• A reflector is often included so that the LCD can use the light
from its surroundings, daylight for example
• Such reflective LCDs only use a backlight in the dark and, in
daylight, or room light, they take only a tiny amount of electrical
power: about one millionth of a Watt
• This is one reason why they are used in clocks, watches and
many other battery operated devices.
Liquid Crystal Display (LCD) for TV or laptop
So how does a laptop, or TV, LCD work?
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In a laptop, or TV, display there may be a million, or
more, subpixels: too many to connect externally
A thin layer containing millions of tiny electronic
switches (called Thin Film Transistors, or TFTs) is used,
in contact with liquid crystal in the LCD, to route
appropriate drive voltages from the edge of the display
to the corresponding subpixels, in order for them to form
the image – click here to see illustration.
An external electronic circuit controls the switching of
the TFTs to apply the correct voltage to each subpixel.
The polarisers are usually mounted at ±45o to improve
the viewing angle of the LCD and let it be viewed wearing
Polaroid sunglasses.
Similar, but much smaller, LCDs are used in data
projectors, mobile phones & camera viewfinders
Since a liquid crystal is an insulator, LCDs consume little
current, or energy, but the backlight takes quite a lot.
This limits the operating time for a laptop, or other
battery operated product.
SUMMING UP
Important Points:
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Liquid crystal materials are made of rod-like tiny particles (molecules)
whose arrangement and directions define their optical properties
Liquid crystals can exist over wide temperature ranges (e.g. -20oC to 80oC)
Liquid crystals are different from solids, liquids and gases
Low AC voltages can change a thin layer of liquid crystal, so that the
amount of light passed through the layer and polarisers is varied
Liquid crystal displays are flat panels that can be used to present
information, colour pictures and movies
Reflective LCDs take almost no power: most of the power in a laptop/TV
LCD is consumed by its backlight
In simple LCDs, each segment can have its own connection
In complex LCDs, millions of Thin Film Transistors (TFTs) are used as onboard switches to control the voltage applied to each subpixel
Each subpixel has its own colour filter and almost every colour can be
displayed using different combinations of voltages on the subpixels
Laptops not possible without LCDs and TVs are no longer large boxes
RED
GREEN
BLUE
RED+GREEN
+BLUE
BLUE+GREEN
CLICK TO RETURN
RED+BLUE
RED+GREEN
Construction of a Liquid Crystal Display (LCD)
for TV or laptop
CLICK TO RETURN
By kind permission of Merck KGaA, Liquid Crystal
Division, D-64271 Darmstadt, Germany
www.licristal.com
LC Shutter between Polarisers
Figure 1
Figure 2
OFF
Crossed
LC Shutter
Parallel
ON
Figure 3
Figure 4
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LC Shutter between Polarisers
Figure 1
Figure 2
OFF
Crossed
LC Shutter
Parallel
ON
Figure 3
Figure 4
CLICK TO RETURN
LC Shutter between Polarisers
Figure 1
Figure 2
OFF
Crossed
LC Shutter
Parallel
ON
Figure 3
Figure 4
CLICK TO RETURN
LC Shutter between Polarisers
Figure 1
Figure 2
OFF
Crossed
LC Shutter
Parallel
ON
Figure 3
Figure 4
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Transmitted Light
Intensity (Lux)
white
Graph of Transmitted Light Intensity
against Voltage applied to an LCD
with Crossed Polarisers
black
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1V
2V
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3V
LCD Voltage