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Visual Displays
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Outline
Image Quality Issues
Pixels
ColorVideo Formats
Liquid Crystal Displays
CRT Displays Projection Displays
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Image Quality Issues
Screen resolution
Size
Color
Blank space betweenthe pixels
Brightness
Contrast
Refresh rate
Sensitivity of displayto viewing angle
For each, lets draw up:Range of commonly available componentsImportanceCost
Which would you want most?
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Pixels
Pixel - The most basic addressable imageelement in a screen
CRT - Color triad (RGB phosphor dots)
LCD - Single color element
Screen Resolution - measure of numberof 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 tocenter of individual pixels.
Cycles per degree How many lines you cansee 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 forhuman vision for:
monitor at 1 m (17-> 10, 22-> 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 smallpixel technologies that can individuallychange color.
Color is encoded by placing different-colored pixels adjacent to each other.
Field sequential color uses red, blue andgreen liquid crystal shutters to changecolor 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, no interlaced SVGA 800x600, 60f/s no interlaced XGA 1024x768+, 60+f/s no interlaced
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 chipswhich change their transparencyproperties when a voltage is applied.
LCD elements are arranged in an nx marray 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 toflow through, or block it.
Electricity applied to a cell cause it tountwist and allow light
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LCDs (cont.)
Color is obtained by placing filters in frontof each LCD element
Usually black space between pixels toseparate the filters.
Because of the physical nature of the LCD
matrix, it is difficult to make the individualLCD pixels very small.
Image quality dependent on viewing angle.
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LCDs (cont.)
LCD resolution is often quoted as number ofcolor elements not number of RGB triads.
R G B R BG R G
B R G B R G B R
R G B R BG R G
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 eachelement of the LCD
matrix applying thevoltage required forthat element.
Once aligned with theelectric field themolecules in the LCDwill hold theiralignment for a shorttime
Active LCD screens
Each element containsa small transistor that
maintains the voltageuntil the next refreshcycle.
Higher contrast andmuch faster responsethan 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 theyolk.
Phosphorcoated 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 withphosphor, 3 colors for acolor monitor, 1 for
monochrome. For a color monitor, three
guns light up red, green, orblue phosphors.
Intensity is controlled by theamount of time at a specific
phosphor location.
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Color CRT
G R B G
B G R B
G R B G
FLUORESCENCE- Light emitted while the phosphor is being struckby electrons.
PHOSPHORESCENCE- Light given off once the electron beam isremoved.
PERSISTENCE- Is the time from the removal of excitation to themoment when phosphorescence has decayed to 10% of the initiallight output.
Red, Green and Blue electron guns.
Screen coated with phosphor triads.
Each triad is composed of a red, blueand green phosphor dot.
Typically 2.3 to 2.5 triads per pixel.
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Beam Movement
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scan line - one row on the screeninterlace vs. non-interlace - Each frame is either
drawn entirely, or as two consecutively drawnfields that alternate horizontal scan lines.
vertical sync (vertical retrace) - the motion ofthe beam moving from the bottom of the image tothe top, after it has drawn a frame.
refresh rate - how many frames are drawn persecond. Eye can see 24 frames per second. TV is30 Hz, monitors are at least 60 Hz.
Beam Movement
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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 LCDscreens to generatean image which issent through anoptical system tofocus on a (usually)
large screen.
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Projector Technology
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 ascreen
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Basic Projector Designs(Images from Phillips Research)
Reflective Projection System Transmittive ProjectionSystem
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Transmittive Projectors
CRT Based One color CRT tube (red, blue,
green phosphors) displays animage with one projection lens.
One black-and-white CRT with a
rapidly rotating color filter wheel(red, green, blue filters) is placedbetween the CRT tube and theprojection lens.
Three CRT tubes (red, green,
blue) with three lenses project theimages. The lenses are aligned sothat a single color image appearson the screen.
Old CRT-based projectors are
usually heavy and largecompared 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 isformed on a small, reflective chip.
When light shines on the chip, theimage is reflected off it and through
a projection lens to the screen. Recent innovations in reflective
technology have been in the thefollowing areas: Micro electromechanical systems
(MEMS) Digital micro mirror device (DMD, DLP)
Grating light valve (GLV)
Liquid crystal on silicon (LCOS)
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Advantages/Disadvantagesof Projection Display
Very large screens can provide large FoV andcan be seen by several people simultaneously.
Image quality can be fuzzy and somewhat
dimmer than conventional displays. (less sothese days).
Light is measured in lumens (1000, 2000common)
Sensitivity to ambient light.
Delicate optical alignment.
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Recap Raster Displays
Cathode Ray Tubes (CRTs), most tubemonitors you see. Very common, but bigand bulky.
Liquid Crystal Displays (LCDs), there aretwo types Transmittive (laptops, thosesnazzy 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 Matrix LCD (ala
laptops)
Head-Tracked Displays (HTDs) Display is stationary, tracker tracks the
users head relative to the display.
Example: CAVE, Workbench, Stereomonitor
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Visually Coupled Systems
A system that integrates the natural visual andmotor skills of an operator into the system he iscontrolling.
Basic Components
An immersive visual display (HMD, large screenprojection (CAVE), dome projection)
A means of tracking head and/or eye motion
A source of visual information that is dependenton the user's head/eye motion.
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Differences HMD/HTD
HMD Eyes are fixed distance
and location from the
display screen(s) Line-of-sight of the
user is perpendicularto the displayscreen(s) or at a fixed,
known angle to thedisplay screen(s).
Only virtual images inworld
HTD
Distance to displayscreen(s) varies
Line-of-sight to displayscreen(s) almost neveris perpendicular
Usually much widerFoV than HMD
Combines virtual andreal imagery
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