2 Outline Image Quality Issues Pixels Color Video Formats Liquid Crystal DisplaysCRT DisplaysProjection Displays
3 Image Quality Issues Screen resolution Size Color Blank space between the pixelsBrightnessContrastRefresh rateSensitivity of display to viewing angleFor each, let’s draw up:Range of commonly available componentsImportanceCostWhich would you want most?
4 Pixels Pixel - The most basic addressable image element in a screen CRT - Color triad (RGB phosphor dots)LCD - Single color elementScreen Resolution - measure of number of pixels on a screen (m by n)m - Horizontal screen resolutionn - Vertical screen resolution
5 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” -> 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?
6 ColorThere 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.
7 Video Formats TV Standards VGA - 640x480, 60f/s, noninterlaced NTSC - 720x480, 29.97f/s (60 fields per second), interlacedPAL - 720x576, 25f/s (50 fields/sec) interlacedVGA - 640x480, 60f/s, noninterlacedSVGA – 800x600, 60f/s noninterlacedXGA – 1024x768+, 60+f/s noninterlacedRGB - 3 independent video signals and synchronization signal, vary in resolution and refresh rateTime-multiplexed color - R,G,B one after another on a single signal, vary in resolution and refresh rate
9 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
10 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
11 LCDs (cont.)Color is obtained by placing filters in front of each LCD elementUsually 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
12 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 elementsEquivalent 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?
13 LCDs (cont.) Passive LCD screens Active 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 timeActive LCD screensEach element contains a small transistor that maintains the voltage until the next refresh cycle.Higher contrast and much faster response than passive LCD
14 Advantages of LCDsFlatLightweightLow power consumption
15 Cathode Ray Tubes (CRTs) Heating element on the yolk.Phosphor coated screenElectrons 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.
16 CRT Phosphor ScreenThe 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.
17 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.
19 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.
20 CRTs (cont.) Strong electrical fields and high voltage Very good resolutionHeavy, not flat
21 Projection DisplaysUse bright CRT or LCD screens to generate an image which is sent through an optical system to focus on a (usually) large screen.
22 Photo courtesy Texas Instruments MEMS projector using three DMD chips Projector Technology seeTwo Basic DesignsTransmittive projectors - Shine light through the image-forming element (CRT tube, LCD panel)Reflective projectors - Bounce light off the image-forming elementIn both types of projectors, a lens collects the image from the image-forming element, magnifies the image and focuses it onto a screenThe projector is the heart of the projection TV system -- this is where the technological advancements have centered. The projectors used in these systems rely on two general approaches:Transmittive projectors - Shine light through the image-forming element (CRT tube, LCD panel)Reflective projectors - Bounce light off the image-forming elementIn both types of projectors, a lens collects the image from the image-forming element, magnifies the image and focuses it onto a screen. Also, it is important not to confuse reflective projectors with rear projection. The terms "transmittive" and "reflective" refer to the optoelectronics inside the projector, not to how the projector is arranged within the projection TV system. Some of the most progressive technologies use the reflective approach, but the transmittive approach has been around longer and appears in many of the small portable projectors on the market today. We'll look at transmittive technologies here and then look in depth at several different reflective technologies.Transmittive Projectors Transmittive projectors use two basic image forming elements:CRTsLCDsBoth types are discussed below. CRT Like conventional TVs, some projectors have smaller CRT tubes built into them. These tubes are small (perhaps 9-inch diagonal), expensive and extremely bright. In the basic layout, you have one or more CRT tubes that form the images. A lens in front of the CRT magnifies the image and projects it onto the screen. There are three CRT configurations used in CRT projectors: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. The rapid succession of color images projected onto the screen forms an apparently single color image (the images are projected too quickly for your brain to distinguish between them).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.One of the problems with CRT projectors is that, with anywhere from one to three tubes and accompanying lenses and/or a filter wheel built in, the projectors can be quite heavy and large. Also, CRT devices do not have the fine resolution that LCD devices do, especially when projected.LCD To make projectors lighter and increase their resolution, newer LCD technologies have been developed (see How LCDs Work for details on LCD panels). Transmitted LCD projectors use a bright light to illuminate the LCD panel, and a lens projects the image formed by the LCD onto a screen. There is not a huge difference between the LCD panels used in projectors and those found in something like a PDA, except that the LCD is smaller and backlit by a very bright halogen lamp. The LCD acts very much like a color slide in a slide projector. The advantage of this approach is that the projector can be very small.The most exciting advances in projector technology can be found in reflective projectors.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.Photo courtesy Texas Instruments MEMS projector using three DMD chipsRecent 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)We'll discuss the new technology of MEMS next.
23 Basic Projector Designs (Images from Phillips Research) Reflective Projection SystemTransmittive Projection System
24 Transmittive Projectors CRT BasedOne 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 technologiesNew ones are tiny
25 Transmittive Projectors LCD BasedUse 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
26 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
27 Advantages/Disadvantages of 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.
28 Recap Raster DisplaysCathode 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).
29 Displays in Virtual Reality Head-Mounted Displays (HMDs)The display and a position tracker are attached to the user’s headMost 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, Stereo monitor
30 Visually Coupled Systems A system that integrates the natural visual and motor skills of an operator into the system he is controlling.Basic ComponentsAn immersive visual display (HMD, large screen projection (CAVE), dome projection)A means of tracking head and/or eye motionA source of visual information that is dependent on the user's head/eye motion.
31 Differences HMD/HTD HTD HMD Distance to display screen(s) varies Line-of-sight to display screen(s) almost never is perpendicularUsually much wider FoV than HMDCombines virtual and real imageryHMDEyes 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
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