Visual Displays Bowman, et al., pp. 29-59 Hodges and Babu 2011

Outline Image Quality Issues Pixels Color Video Formats Liquid Crystal Displays CRT Displays Projection Displays Hodges and Babu 2011

Image Quality Issues Screen resolution Color Blank space between the pixels Intentional image degradation Brightness Contrast Refresh rate Sensitivity of display to viewing angle Hodges and Babu 2011

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 Hodges and Babu 2011

Other meanings of resolution Pitch - Size of a pixel, distance from center to center of individual pixels. Cycles per degree - Addressable elements (pixels) divided by twice the FOV measured in degrees. How much an eye can differentiate one object from another in terms of visual angles. The human eye can resolve 30 cycles per degree (20/20 Snellen acuity). Hodges and Babu 2011

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. Hodges and Babu 2011

Video Formats NTSC - 525x480, 30f/s, interlaced PAL - 625x480, 25f/s, interlaced VGA - 640x480, 60f/s, non-interlaced SVGA – 800x600, 60f/s non-interlaced SXGA – 1280x1024, 60f/s non-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 Hi-Def 1080p – 1900 x 1080, non-interlaced, widescreen aspect ratio 16:9, progressive scan Hodges and Babu 2011

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 called the LCD matrix Level of voltage controls gray levels. LCDs elements do not emit light, use backlights behind the LCD matrix Hodges and Babu 2011

LCDs (cont.) Hodges and Babu 2011

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. Hodges and Babu 2011

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) Hodges and Babu 2011

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 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 Hodges and Babu 2011

Advantages of LCDs Flat Lightweight Low power consumption Hodges and Babu 2011

Cathode Ray Tubes (CRTs) Hodges and Babu 2011

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. Hodges and Babu 2011

CRTs (cont.) Strong electrical fields and high voltage Very good resolution Heavy, not flat Hodges and Babu 2011

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. Hodges and Babu 2011

Photo courtesy Texas Instruments MEMS projector using three DMD chips Projector Technology see http://electronics.howstuffworks.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 (DLP) In both types of projectors, a lens collects the image from the image-forming element, magnifies the image and focuses it onto a screen The 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 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. 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: CRTs LCDs Both 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 chips 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) We'll discuss the new technology of MEMS next. Hodges and Babu 2011

Usually better range of color and brightness CRT 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. CRT-based projectors are usually heavy and large compared to other technologies Usually better range of color and brightness Hodges and Babu 2011

LCD Projectors Use a bright light to illuminate an LCD panel, and a lens projects the image formed by the LCD onto a screen Compact since LCD Chip small compared to CRTs Less Heat, Less Power Screen Door effect One pixel can burn out Different colors are polarized but not in the same orientation. Passive stereo polarization will not work. Hodges and Babu 2011

DLP (Digital Light Processing) Projectors The Chip in a DLP Projector is a Digital Micromirror Device. Essentially every pixel on a DMD Chip is a reflective mirror. Higher resolution is possible than with LCD technology No screen door effect. Consistent Polarization More expensive Video Hodges and Babu 2011

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. Sensitivity to ambient light. Delicate optical alignment. Hodges and Babu 2011

Displays in Virtual Reality Head-Mounted Displays (HMDs) The display and a position tracker are attached to the user’s head Head-Tracked Displays (HTDs) Display is stationary, tracker tracks the user’s head relative to the display. Example: CAVE, Workbench, Stereo monitor Hodges and Babu 2011

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. Hodges and Babu 2011

Differences HMD/HTD HTD HMD 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 Hodges and Babu 2011