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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 Liquid Crystal Display (LCD) for TV or laptop Introduction In 1980, flat panel TV & laptop ... – PowerPoint PPT presentation

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Title: Liquid Crystal Displays for Laptops and TV


1
Liquid Crystal Displays for Laptops and TV
  • What they are and how they work

2
  • 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

3
  • 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

4
  • 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

5
  • 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
6
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

7
  • 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.

8
  • 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.

9
  • 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.

10
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
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