Display Technology

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Display Technology
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Display Technologies

• The Technologies
• CRT
• LCD
• Dual Scan
• Active Matrix
• PDP
• ALiS
• PALCD
• ThinCRT
• LEP

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CRT (Cathode Ray Tube)

• 100 year old technology
• A glass bell envelope contains a vacuum and an
  electron gun. By the application of a current,
  and electron stream is created, which is fired
  through the vacuum towards the inside face of the
  glass envelope. Here it strikes a phosphor layer,
  which converts the beam into visible light,
  colour being achieved through mixing varying
  levels of light intensity from red, green and
  blue phosphors.

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CRT (Cathode Ray Tube) cont.

• As theres only one electron gun, and one beam
  for each colour, the screen needs to be refreshed
  constantly. This is achieved by altering the
  angle of the beam with a magnetic deflector coil,
  which deflects the beam across each part of the
  screen from top left to bottom right in a
  movement known as a raster. If refresh rate is
  set at 75Hz and resolution is 1024x768 (XGA),
  this equates to painting 58,982,400 pixels per
  second.

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CRT (Cathode Ray Tube) cont.

• CRT Types
• FST (Flat square tube)
• much less curvature than standard CRT
• uses shadow mask (smaller than screen size) to
  prevent distortion and overspill of electron
  stream
  
• Trinitron (also Diamondtron)
• to avoid light reduction caused by shadow mask,
  uses an aperture grille, a series of narrow alloy
  strips.
  
• grille improves light emission, but can move and
  is visible in background

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CRT (Cathode Ray Tube) cont.

• CRT Types (cont.)
• ChromaClear
• Developed by NEC to combine shadow mask and
  aperture grille technologies. Uses vertically
  aligned slots in shadow mask focusing onto
  rectilinear phosphors.
• EDP (Enhanced Dot Pitch)
• Developed by Hitachi, focuses on the phosphor
  implementation. Rather than the standard
  equilateral triangle model, reduces distance
  between dots on the horizontal to create
  isosceles triangles of oval rather than round
  dots, giving a greater density of dots and hence
  better resolution.

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CRT (Cathode Ray Tube) cont.

• Advantages of CRT
• robust, well-known technology
• high-quality resolution and image control
• Disadvantages of CRT
• size (footprint) on monitors
• short or mini-neck tubes possible, but
  exacerbates distortion problems
  
• analogue technology

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LCD (Liquid Crystal Display)

• Work by polarisation of light
• Liquid crystals dont emit their own light, but
  depend on a cold cathode backlight being passed
  through a sandwich of glass, liquid crystal and
  polarising filters, at right angles to each
  other.
• The liquid crystal molecules need to be aligned
  to allow the light to refract along the chain and
  out the other side. By anchoring the long crystal
  molecules to each side of the screen by grooves
  in the glass, their natural state creates the
  necessary alignment.

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LCD (Liquid Crystal Display) cont.

• When a current is applied to any screen element,
  the molecules lose the necessary alignment, so
  any light is blocked by the opposing polariser.
  
• Colour is produced in similar way to CRTs, with
  individual liquid crystal cells for red, green
  and Blue. Unlike phosphors, which emit light, the
  liquid crystals filter the light, allowing only
  their corresponding colours through.

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LCD (Liquid Crystal Display) cont.

• LCD Types
• Basically, there a re two types of LCD technology
  that have become successful over earlier, less
  efficient techniques, passive and active matrix.
  
• Dual Scan (passive matrix)
• the matrix refers to the underlying layer of
  conductors, used to activate the screen
  elements.
  
• In passive matrix, this is usually made up of a
  lattice of conductive strips running from edge to
  edge of the display

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LCD (Liquid Crystal Display) cont.

• As these strips are relatively long, the time
  taken to activate each element is longer than in
  active matrix models. This means that it takes
  longer to refresh the screen, an effect that
  increases with the size of the screen and leads
  to submarining and the need to use mouse trails
• To improve the performance of passive matrix,
  dual scan LCD splits the conductor matrix into
  two sections, each of which are addressed
  separately by drivers down both sides of the
  screen.
• Dual scan maintains the low power requirement of
  passive matrix but increases refresh rate, with
  the result that many contemporary notebook
  displays use this technology

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LCD (Liquid Crystal Display) cont.

• LCD Types cont.
• Active Matrix - TFT (Thin Film Transistor)
• uses a much more complex conductor array,
  replacing the lattice with a grid of independent
  transistors that lie on a layer beneath the
  screen elements.
• Far more complex to manufacture, but much faster
  because it independently addresses the liquid
  crystal cells.
  
• Viewing angle is wider, as transistor position
  obstructs backlight less than conductor strips.
  
• Much more expensive and has a higher power drain.

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LCD (Liquid Crystal Display) cont.

• Advantages
• Much smaller footprint than CRT
• Lighter, so appropriate for portables
• Digital technology (analogue connectors - new
  DVI)
  
• Disadvantages
• More expensive than CRT (but less so now!)
• Poorer resolution
• both types based on 100 dpi, but IBM Monet now
  gives 150 dpi and IBM Roentgen will give 200dpi

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Plasma Display Panels

• PDP
• Plasma Display Panels
• Based on the principle that certain gases emit
  light when subject to an electric current
  
• John Logie Baird first considered this technology
  as a possibility for producing an image on a
  screen
  
• Original plasma screens were monochrome, low
  resolution and very power hungry. They worked by
  capturing neon gas between two plates that were
  etched with conductive lines.

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PDPs

• The conductive lines are at right angles to one
  another (similar to LCD) and as current is passed
  along selected lines, or channels, the gas in
  between glows and the image is built up.
• Modern PDPs contain a cocktail of gases that emit
  ultraviolet, rather than visible, light, which is
  then used to excite a phosphor layer, in a
  similar way to the electron stream in CRTs. This
  gives far greater accuracy in the resultant image
  while retaining the brightness and speed of the
  original principle.

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PDPs

• Using this current technology, PDPs cant be made
  to rival the resolution of LCD, so they havent
  been used for monitors and notebook displays.
  However, they can be made to a considerable size
  and their natural brightness makes them highly
  suitable for large advertising displays, such as
  noticeboards at airports and train stations or
  product information screens at trade-shows and
  exhibitions.

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PDPs

• ALiS
• Fujitsu is developing a new plasma display type
  that will overcome the low resolution
  restrictions of current PDPs
  
• Alternate Lighting of Surfaces uses interlaced
  rather than progressive scans to increase the
  resolution of screens. This is achieved by
  creating a sandwich of two layers of the plasma
  display and removing the conducting lines that
  separated the channels.

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PDPs

• The result is that, as well as offering a higher
  level of resolution, the displays are even
  brighter and it actually only uses around half
  the drivers of the older technology.
• AliS has been developed to compete in the digital
  television market, where a resolution of around
  960 lines on screen is demanded.

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PDPs

• PALCD
• A hybrid of PDP and LCD, the Plasma Addressed
  Liquid Crystal Display.
  
• Sony and Tektronix are currently working on
  making a viable commercial PALCD.
  
• Rather than use the ionisation effect of the
  contained gas for the production of an image,
  PALCD replaces the active matrix design of TFT
  LCDs with a grid of anodes and cathodes that use
  the plasma discharge to activate LCD screen
  elements

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PDPs

• With the exception of the plasma activation, the
  rest of the display uses standard LCD
  components.
  
• PALCD displays can be built to provide 42
  (105cm) and greater presentation displays and
  televisions, the resolution achievable being good
  enough to support digital television though they
  wont be used for monitors.
• A significant advantage of PALCD is the lack of
  semiconductor controls.This means that the
  displays can be built in low-grade clean rooms,
  reducing costs.
• Claimed to be brighter, and still thin.

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New Technologies

• ThinCRT
• Developed by Candescent, who have replaced the
  electron guns, deflection yoke and shadow mask of
  the standard CRT with a perforated conductive
  sheet through which conical cold cathode
  emitters, known as Spindt Cathodes, protrude.
• Passing a current through the conductive sheet
  causes the cathodes to emit a stream of
  electrons, which cause phosphor elements to glow
  in exactly the same way as a standard CRT.

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ThinCRT

• Spindt Cathodes are very small, only 200
  nanometres each, so it takes several of them to
  activate individual pixels onscreen. This allows
  a relatively high failure rate, Candescent claims
  20, before any degradation of the image is
  visible. As a result this technology is more
  viable for mass production than LCD.
• Advantages of ThinCRT
• Thin, a fraction of the depth of CRT, so more
  like LCD and PDP

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ThinCRT

• Offers the same, or better, quality of resolution
  as CRT, so better than LCD and PDP.
  
• Brighter, wider viewing angles, lower power
  consumption.
  
• Potential Disadvantage of ThinCRT
• In order to function, the cathodes still need to
  reside in a vacuum. In order to stop the glass
  envelope from collapsing, Candescent has
  developed a non-conductive ceramic compound which
  is made into 0.05mm spacers to keep the sides
  apart. They shouldnt interfere with the image
  but how robust they are is currently an unknown
  factor.

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LEP

• LEP
• Light Emitting Polymers
• Conjugated polymers, such as polyprolle which
  has been known for over 100 years and
  polyaniline, are plastic materials with physical
  properties that confer conductive properties.
• In recent times these polymers have been used for
  battery electrodes, transparent conductive
  coatings, capacitor electrolytes and
  circuit-board plating.

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LEP

• When Cambridge University discovered that certain
  conjugated polymers, specifically
  p-phenlylenevinylene (PPV), could be
  made to emit light in addition to carrying
  current, the idea of using LEP for displays was
  born.
• Cambridge Display Technologies have been
  developing this technology, working on the
  display technology itself, and the creation of
  stable red, green and blue polymers based on PPV.

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LEP

• Essentially the creation of the display itself is
  similar to, but simpler than, LCD. On the surface
  of a layer of substrate a transparent electrode
  layer is applied, on top of which a PPV layer is
  applied, and then a second electrode layer. When
  current is passed between the two electrode
  layers the polymer will emit photons, which pass
  through the transparent electrode layer and the
  substrate to the viewer. The display is
  structured into cells, similar to the picture
  elements of an LCD, and each cell can be
  addressed individually using an active matrix of
  electrodes.

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LEP

• LEPs are about to hit the market, probably
  initially as backlight surfaces for handheld PCs
  and information displays, in the market currently
  served by monochrome LCDs, but they could have a
  major impact in the future.
• Adavantages of LEP
• Can be applied to very large surfaces, so far
  greater than the current 42 PDP maximum.
  
• Polymer can flex with the base surface

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LEP

• Far cheaper to make than competing technologies,
  substrate doesnt require to be glass, simple
  electrode structure, easy to produce.
  
• Lightweight
• Potential Disadvantage of LEP
• Longevity and efficiency of LEP still unknown
  factor
  
• Philips has created a dedicated fabrication plant!