Liquid Crystal Displays for Laptops and TV

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Liquid Crystal Displays for Laptops and TV

• What they are and how they work

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• Liquid Crystal Display (LCD) for TV or laptop
• Introduction
• In 1980, flat panel TV laptop displays didnt
  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 (called picture elements, or pixels) on the
  screen
• In a colour LCD, each pixel is comprised of even
  smaller red, green and blue subpixels, 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 alters the pixel colours and allows a colour
  picture to be displayed on the LCD
• Pixels and subpixels can change fast enough to
  permit moving pictures to be displayed

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• Liquid Crystal Display (LCD) for TV or laptop
• Where do all the colours come from?
• Each row of subpixels has its own narrow colour
  filter stripe - some red, some green and some
  blue
• 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
• 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
• 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

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• Liquid Crystal Display (LCD) for TV or laptop
• What is a liquid crystal?
• All materials are comprised of vast numbers
  of tiny particles (called 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

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• Liquid Crystal Display (LCD) for TV or laptop
• What are polarisers (often called Polaroid)?
• 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
• 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
• 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
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Liquid Crystal Display (LCD) for TV or laptop
How is liquid crystal used in an LCD?

• 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

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• Liquid Crystal Display (LCD) for TV or laptop
• So how does an LCD work?
• An LCD between crossed polarisers uncrosses
  them and allows light to pass
• See Figure 1
• An LCD between parallel polarisers crosses them
  and light is blocked
• See Figure 2
• This property is temporarily destroyed on
  applying a small voltage to its conductors
• An LCD placed at 45o 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 affects 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.

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• 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.

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• Liquid Crystal Display (LCD) for TV or laptop
• So how does a laptop, or TV, LCD work?

• 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.

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SUMMING UP

• Key Points
• Liquid crystal materials are made of rod-like
  tiny particles (molecules) whose arrangement and
  directions define their optical properties
• Liquid crystals are different from solids,
  liquids and gases and can exist over wide
  temperature ranges (e.g. -20oC to 80oC)
• Low AC voltages can change a thin layer of liquid
  crystal, so that the amount of light passing
  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 on-board 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