Describe Flat Panel Displays.

FLAT-PANEL DISPLAYS


The term flat–panel displays refers to a class of video devices that have reduced volume, weight, and power requirements compared to a CRT. A significant feature of flat-panel displayed is that they are thinner than CRTs, and we can hang themon walls or wear them on our wrists.

We can separate flat-panel displays into two categories: emissive displays and non-emissive displays. The emissive displays (or emitters) are devices that displays, and light-emitting diodes are examples of emissive displays. Non-emissive displays( or non-emitters) use optical effects to convert sunlight or light from some other source into graphics patterns. The most important example of a non-emissive flat-panel display is a liquid- crystal device.

Major Types of Flat Panel Displays:

Liquid Crystal Displays (LCDs)

LCDs are, by far, the most commercially important flat panel technology because they are used  in notebook computers.  In 1998, around 11 million notebooks were sold.  Almost all (over 99 percent) of those notebooks had an LCD in it.

As their name suggests LCDs exhibit some properties of a liquid and some associated with a crystal. They flow like liquids and share much of a liquid’s dependence upon temperature, stress, electric fields, and chemical environment.
LCDs can be subdivided into the following types:

 

  • twisted nematic LCDs (TN-LCDs), one form of passive-matrix LCD
  • supertwisted nematic LCDs (STN-LCDs), another type of passive-matrix LCD
  • thin-film-transistor LCDs (TFT-LCDs), also referred to as active-matrix LCDs
  • active-addressed LCDs (AA-LCDs), a hybrid of passive- and active-matrix technologies

Advantages:

  • very low power consumption
  • low driving voltage (5-20 volts)
  • very thin display (approaching .5cm without the backlight)
  • lowest cost (TNs)
  • high contrast ratio (STNs)
  • full-color display capability
  • fast writing speed
  • readable in direct sunlight
  • available from many commercial sources

Disadvantages:

  • narrow viewing angle or cone (TNs and STNs)
  • limited contrast (TNs)
  • slow writing speed (STNs)
  • nearly defect-free panels difficult to manufacture (especially TFTs)
  • high capital equipment investment (especially TFTs)
  • low transmissivity of color filters requires strong backlight
  • costly polarizer set required
  • integrated circuits for controllers and drivers are expensive

 

 

Field Emission Displays(FEDs)

This technology is essentially a flat CRT with a thousand cathodes (emitters) per pixel eliminating thereby the long throw distance of the single scanning electron beam of the CRT. Most of the advantages of CRT image quality accrue, without the bulk or power inefficiency of the deflection system, and shadow mask.

Advantages:

  • Full (160 degree) viewing angles horizontally and vertically.
  • Full motion video without artifacts or contrast loss (20 microsecond response time).
  • -45C to +85C operating temperature.
  • Instant-on at any operating temperature.
  • High power efficiency.
  • Costs at or below AMLCD
  • Very high brightness possible.

Disadvantages:

  • New, not yet proven technology.
  • 20 to 85 volt drivers are larger, impacting miniaturization.
  • Life-times (10,000 hrs) below some competitive technologies
     
 

Light Emitting Diodes (LEDs)

Display applications are classified either as low information content (where a limited amount of information is being displayed, refresh rates are slow, and the nature of information being displayed is predictable), or as high information content (where the opposite conditions apply). Broadly speaking LEDs, ELs, and VFDs are best suited to the former, while plasma and all types of LCDs are appropriate for the latter.

 
 

Plasma Display Panels (PDPs)

This Flat Panel Display (FPD) technology is based on the glow discharge that occurs when ionized gas undergoes recombination. Electrons are removed from atoms to produce ions, later recombining with the ions to release energy in the form of light.

Advantages:

  • established technology
  • proven to be rugged and reliable
  • multiple commercial sources
  • simplified driving circuit
  • simple construction lends itself to low-cost, high-volume production
  • color is feasible
  • long lifetime

Disadvantages:

  • high voltage driver requirements (150-200 volts)
  • washout in bright sunlight
  • limited gray-scale ability

 

Electroluminescent Displays (ELs)

This technology is very similar to that of LEDs. However, ELs are doped (as a semiconductor) with specific impurities to provide energy states that lie slightly below those of the mobile electrons and those slightly above states inhabited by electrons bound to atoms. Impurity states are used to provide initial and final states in light emitting transitions.

Advantages:

  • very thin and compact
  • high writing speed (video rates)
  • good readability and brightness
  • some gray-scale ability
  • low-voltage operation
  • no catastrophic failure

Disadvantages:

  • high-voltage drivers (170-200 volts)
  • high cost due to difficulty of manufacture in volume
  • high power consumption (low efficiency)
  • washout in bright ambient light
  • color progressing at a slow rate

 

Vacuum Fluorescent Displays (VFDs)

A form of cathodoluminescent display that can operate at low voltages, since very thin layers of highly efficient phosphors are coated directly onto each transparent anode. These displays are limited to a matrix of widely-spaced phosphor dots, and are designed for use in low-information-content applications.

Advantages:

  • lower voltage than other emissive displays
  • low cost for low-information-content displays
  • multiplex able to large displays
  • viewable in sunlight

Disadvantages:

  • high cost for large screen sizes
  • high cost for high-information-content displays
  • full color not yet available in sizes suitable for consumer home TV or computer monitors

 

Organic Light Emitting Diodes (OLEDs)

Relatively more recent in origin that the other flat panel display technologies, OLEDs use organic compounds as sources of light emissions.  They are quite similar in construction to inorganic EL displays in that they are thick films and are less expensive to construct than LCDs.  It is currently thought that OLEDs will not only be cheaper to make than TFT-LCDs, but that they will make possible the depositing of high-quality displays on plastic and/or flexible substrates.


The following are types of displays that are not flat panel displays that compete with FPDs in the overall display marketplace:

 

Cathode Ray Tubes (CRTs)

Cathode Ray Tubes are well established and form the basis for most displays commonly in use today. Their broad market acceptance is based upon their long-standing in the marketplace and low cost. However, CRTs are being replaced in a wide variety of markets because they require a large physical space in which to operate and comparatively large amounts of power.  People who use CRTs as computer monitors often switch to flat panel displays that have less “flicker” and give off less harmful electromagnetic radiation despite their higher cost.

 

Projection Displays

There are many kinds of projection displays.  One way of classifying them is in terms of the imaging source.  Many projection displays on the market use very bright, small CRTs as their imaging source.  Others use LCDs or digital light processors (DLPs) as their imaging sources.  Projection displays may project the image forward onto a flat wall or screen that is several feet or more distant from the projector (called “forward projection) or they may project the image onto a screen that is relatively close to the projector (called “rear projection”).   Forward projection is used primarily in business and education for the presentation of computer and video materials.  Rear projection is used primarily in homes in large-screen televisions.

 

 

know more at http://www.indiana.edu/~hightech/fpd/


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