VE Display

8/4/2019 VE Display

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Visual DisplaysChapter 2 Burdea

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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 betweenthe pixels

Brightness

Contrast

Refresh rate

Sensitivity of displayto viewing angle

For each, let’s draw up: •Range of commonly available components•Importance•Cost

•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, noninterlaced SVGA – 800x600, 60f/s noninterlaced XGA – 1024x768+, 60+f/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 chipswhich change their transparencyproperties 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 toflow through, or block it.

Electricity applied to a cell cause it tountwist and allow light

http://electronics.howstuffworks.com/lcd2.htm



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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.

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.

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 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 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 struck by 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 of the 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 Technologysee http://electronics.howstuffworks.com/projection-tv.htm

Two Basic Designs

– Transmittive projectors - Shine light through theimage-forming element (CRT tube, LCD panel)

–

Reflective projectors - Bounce light off the image-forming element

In both types of projectors, a lens collects theimage 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 baseddisplays

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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 througha projection lens to the screen.

Recent innovations in reflectivetechnology have been in the thefollowing 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 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 “tube” monitors 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 user’s head

– Most use Active Maxtrix LCD (ala

laptops)

Head-Tracked Displays (HTDs) – Display is stationary, tracker tracks the

user’s 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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