Visual Displays

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Visual Displays
Chapter 2 Burdea

2
Outline

• Image Quality Issues
• Pixels
• Color
• Video Formats
• Liquid Crystal Displays
• CRT Displays
• Projection Displays

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Image Quality Issues

• Screen resolution
• Size
• Color
• Blank space between the pixels

• Brightness
• Contrast
• Refresh rate
• Sensitivity of display to viewing angle

• For each, lets draw up
• Range of commonly available components
• Importance
• Cost
• Which would you want most?

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Pixels

• Pixel - The most basic addressable image element
  in a screen
• CRT - Color triad (RGB phosphor dots)
• LCD - Single color element
• Screen Resolution - measure of number of pixels
  on a screen (m by n)
• m - Horizontal screen resolution
• n - Vertical screen resolution

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Other meanings of resolution

• Pitch - Size of a pixel, distance from center to
  center of individual pixels.
• Cycles per degree How many lines you can see in
  a degree of FOV.
• The human eye can resolve 30 cycles per degree
  (20/20 Snellen acuity).
• So how many lines of resolution are needed for
  human vision for
• monitor at 1 m (17 -gt 10, 22 -gt 13)
• projector screen at 2 m (4), 4 m (8)
• REVE at 4m (18 high)
• How far should you make someone sit in front of a
  42 (34 rotated vert) plasma running at 720p?

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Color

• There are no commercially available small pixel
  technologies that can individually change color.
• Color is encoded by placing different-colored
  pixels adjacent to each other.
• Field sequential color uses red, blue and green
  liquid crystal shutters to change color in front
  of a monochrome screen.

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Video Formats

• TV Standards
• NTSC - 720x480, 29.97f/s (60 fields per second),
  interlaced
• PAL - 720x576, 25f/s (50 fields/sec) interlaced
• VGA - 640x480, 60f/s, noninterlaced
• SVGA 800x600, 60f/s noninterlaced
• XGA 1024x768, 60f/s noninterlaced
• RGB - 3 independent video signals and
  synchronization signal, vary in resolution and
  refresh rate
• Time-multiplexed color - R,G,B one after another
  on a single signal, vary in resolution and
  refresh rate

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Interlacing
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Liquid Crystal Displays

• Liquid crystal displays use small flat chips
  which change their transparency properties when a
  voltage is applied.
• LCD elements are arranged in an n x m array call
  the LCD matrix.
• Level of voltage controls gray levels (amount of
  light allowed through).
• LCDs elements do not emit light, use backlights
  behind the LCD matrix

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Liquid Crystal Displays (LCDs)

• LCDs have cells that either allow light to flow
  through, or block it.
• Electricity applied to a cell cause it to untwist
  and allow light

http//electronics.howstuffworks.com/lcd2.htm
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LCDs (cont.)

• Color is obtained by placing filters in front of
  each LCD element
• Usually black space between pixels to separate
  the filters.
• Because of the physical nature of the LCD matrix,
  it is difficult to make the individual LCD pixels
  very small.
• Image quality dependent on viewing angle.
• Black levels not completely black

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LCDs (cont.)

• LCD resolution is often quoted as number of color
  elements not number of RGB triads.

Example 320 horizontal by 240 vertical elements
76,800 elements Equivalent to 76,800/3 25,500
RGB pixels "Pixel Resolution" is 185 by 139
(320/1.73, 240/1.73) How many pixel transistors
for a 1024x768 display?
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LCDs (cont.)

• Passive LCD screens
• Cycle through each element of the LCD matrix
  applying the voltage required for that element.
• Once aligned with the electric field the
  molecules in the LCD will hold their alignment
  for a short time

• Active LCD screens
• Each element contains a small transistor that
  maintains the voltage until the next refresh
  cycle.
• Higher contrast and much faster response than
  passive LCD

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Advantages of LCDs

• Flat
• Lightweight
• Low power consumption

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Cathode Ray Tubes (CRTs)
Heating element on the yolk. Phosphor coated
screen Electrons are boiled off the filament and
drawn to the focusing system. The electrons are
focused into a beam and shot down the
cylinder. The deflection plates aim the
electrons to a specific position on the screen.
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CRT Phosphor Screen

• The screen is coated with phosphor, 3 colors for
  a color monitor, 1 for monochrome.
• For a color monitor, three guns light up red,
  green, or blue phosphors.
• Intensity is controlled by the amount of time at
  a specific phosphor location.

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Color CRT
Red, Green and Blue electron guns. Screen
coated with phosphor triads. Each triad is
composed of a red, blue and green phosphor
dot. Typically 2.3 to 2.5 triads per pixel.

• FLUORESCENCE - Light emitted while the phosphor
  is being struck by electrons.
• PHOSPHORESCENCE - Light given off once the
  electron beam is removed.
• PERSISTENCE - Is the time from the removal of
  excitation to the moment when phosphorescence has
  decayed to 10 of the initial light output.

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Beam Movement
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Beam Movement

• scan line - one row on the screen
• interlace vs. non-interlace - Each frame is
  either drawn entirely, or as two consecutively
  drawn fields that alternate horizontal scan
  lines.
• vertical sync (vertical retrace) - the motion of
  the beam moving from the bottom of the image to
  the top, after it has drawn a frame.
• refresh rate - how many frames are drawn per
  second. Eye can see 24 frames per second. TV is
  30 Hz, monitors are at least 60 Hz.

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CRTs (cont.)

• Strong electrical fields and high voltage
• Very good resolution
• Heavy, not flat

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Projection Displays

• Use bright CRT or LCD screens to generate an
  image which is sent through an optical system to
  focus on a (usually) large screen.

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Projector Technologysee http//electronics.howstu
ffworks.com/projection-tv.htm

• Two Basic Designs
• Transmittive projectors - Shine light through the
  image-forming element (CRT tube, LCD panel)
• Reflective projectors - Bounce light off the
  image-forming element
• In both types of projectors, a lens collects the
  image from the image-forming element, magnifies
  the image and focuses it onto a screen

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Basic Projector Designs(Images from Phillips
Research)
Reflective Projection System
Transmittive Projection System
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Transmittive Projectors

• CRT Based
• One color CRT tube (red, blue, green phosphors)
  displays an image with one projection lens.
• One black-and-white CRT with a rapidly rotating
  color filter wheel (red, green, blue filters) is
  placed between the CRT tube and the projection
  lens.
• Three CRT tubes (red, green, blue) with three
  lenses project the images. The lenses are aligned
  so that a single color image appears on the
  screen.

Old CRT-based projectors are usually heavy and
large compared to other technologies New ones
are tiny
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Transmittive Projectors

• LCD Based
• Use a bright light to illuminate an LCD panel,
  and a lens projects the image formed by the LCD
  onto a screen.
• Small, lightweight compared to CRT based displays

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Reflective Projectors

• In reflective projectors, the image is formed on
  a small, reflective chip.
• When light shines on the chip, the image is
  reflected off it and through a projection lens to
  the screen.
• Recent innovations in reflective technology have
  been in the the following areas
• Microelectromechanical systems (MEMS)
• Digital micromirror device (DMD, DLP)
• Grating light valve (GLV)
• Liquid crystal on silicon (LCOS)
• Images from howstuffworks.com

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Advantages/Disadvantagesof Projection Display

• Very large screens can provide large FoV and can
  be seen by several people simultaneously.
• Image quality can be fuzzy and somewhat dimmer
  than conventional displays. (less so these days).
• Light is measured in lumens (1000, 2000 common)
• Sensitivity to ambient light.
• Delicate optical alignment.

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Recap Raster Displays

• Cathode Ray Tubes (CRTs), most tube monitors
  you see. Very common, but big and bulky.
• Liquid Crystal Displays (LCDs), there are two
  types transmittive (laptops, those snazzy new
  flat panel monitors) and reflective (wrist
  watches).

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Displays in Virtual Reality

• Head-Mounted Displays (HMDs)
• The display and a position tracker are attached
  to the users head
• Most use Active Maxtrix LCD (ala laptops)
• Head-Tracked Displays (HTDs)
• Display is stationary, tracker tracks the users
  head relative to the display.
• Example CAVE, Workbench, Stereo monitor

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Visually Coupled Systems

• A system that integrates the natural visual and
  motor skills of an operator into the system he is
  controlling.
• Basic Components
• An immersive visual display (HMD, large screen
  projection (CAVE), dome projection)
• A means of tracking head and/or eye motion
• A source of visual information that is dependent
  on the user's head/eye motion.

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Differences HMD/HTD

• HTD
• Distance to display screen(s) varies
• Line-of-sight to display screen(s) almost never
  is perpendicular
• Usually much wider FoV than HMD
• Combines virtual and real imagery

• HMD
• Eyes are fixed distance and location from the
  display screen(s)
• Line-of-sight of the user is perpendicular to the
  display screen(s) or at a fixed, known angle to
  the display screen(s).
• Only virtual images in world