Visual Displays

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

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. LCDs (cont.) Hodges and Babu 2011

10. 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

11. 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

12. 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 Hodges and Babu 2011

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

14. Cathode Ray Tubes (CRTs) Hodges and Babu 2011

15. Color CRT 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. •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. Hodges and Babu 2011

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

17. 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

18. Projector Technologysee 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 Hodges and Babu 2011

19. 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

20. 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

21. 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

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

23. 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

24. 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

25. Differences HMD/HTD • 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 • 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 Hodges and Babu 2011