Display Systems. Viewing Images Rochester Institute of Technology

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1 Display Systems Viewing Images 1999 Rochester Institute of Technology

2 In This Section... We will explore how display systems work. Cathode Ray Tube Television Computer Monitor Flat Panel Display Liquid Crystal Display

3 Cathode Ray Tube A common device used in televisions, and computer monitors. The tube directs electrons to form an image on a screen.

4 Cathode Ray Tube A cathode ray tube (CRT) is a vacuum tube.

5 Cathode Ray Tube - Power supply + The ends of the tube are electrically charged by a power supply.

6 Cathode Ray Tube Electron beam Electron gun - Power supply + An electron gun propels electrons from the negative end of the tube forming an electron ray, or electron beam (e-beam).

7 Cathode Ray Tube Electron beam Electron gun Anode - Power supply + The electrons are accelerated by an anode. The anode has a strong positive charge that forces negative electrons forward.

8 Cathode Ray Tube Deflectors Electron beam Electron gun Anode - Power supply + Deflectors use electric fields to bend the e-beam in a desired direction.

9 Cathode Ray Tube Electron beam Deflectors Phosphor screen Electron gun Anode - Power supply + The e-beam collides with a phosphor screen causing it to temporarily glow and become viewable.

10 Cathode Ray Tube Electron beam Deflectors Phosphor screen Electron gun Anode - Power supply + This is a general CRT for a black-and-white television system.

11 Cathode Ray Tube Color Modifications 1 or 3 Electron Guns Additional E-beam Guide Shadow Mask Aperture Grill 3 Types of Phosphors on 1 Screen Red (R) Green (G) Blue (B)

12 CRT - Color Electron beam Deflectors Supplemental Guide Phosphor screen Electron gun Anode - Power supply + The supplemental guide (shadowmask, aperture grill, etc.) for color is placed just before the phosphor screen.

13 Additive Color The three primary colors of red, green, and blue combine to form other colors. The idea is similar to that of pointillism in certain Impressionistic paintings: Tiny dots or lines are placed closely next to each other When the viewer is far enough away, the dots blur The primaries add together to form other colors

14 Additive Color Mixing The additive primary colors used are RED, GREEN, and BLUE.

15 Additive Color Mixing Overlapping red, green, and blue light, creates yellow, cyan, and magenta light.

16 Additive Color Mixing The combination of the three additive primaries gives white light (R + G + B = WHITE).

17 CRT - Color Shadowmask CRT Three e-beams are shot by 3 electron guns to a phosphor screen. RGB phosphor dots are arranged on the screen in triads making up the corners of equilateral triangles; each dot <.4 mm. The guns are also arranged at the corners of an equilateral triangle. The shadowmask is a metal sheet with a single hole for each triad. Placed just before the screen the shadowmask has the same shape. The holes limit the beam so it hits the correct color phosphor dot. Phosphor Screen E-guns triad Shadowmask

18 Progressive Scanning The method by which electron beam(s) scan over a phosphor screen. The electron beam is systematically moved across the screen Raster lines: The horizontal lines that make up an image. Left to Right (from viewing position) Top to Bottom Refresh Rate: A group of scanned lines forms a picture. Refresh Rate = # pictures per second (pps) Refresh Rate < 30 pps is seen as many individual pictures Refresh Rate > 30 pps is seen as constant motion

19 Interlacing Fields Two fields per picture First, all odd lines are scanned - Odd field Then, all even lines are scanned - Even field Measured as fields per second (fps) Doubles the refresh rate 30 pps yields 60 fps At > 60 fps no flicker is detected between frames US Standards (NTSC) 525 lines per picture 60 fps (30 pps) Great Britain Standards (PAL) 625 lines per picture 50 fps (25 pps)

20 Movies on Television Compare refresh rates: TVs display at 30 pps. Movies display at 24 pps. How are movies adjusted for TV? Every 5th picture is doubled and some dark time is inserted for each frame to increase the refresh rate to 30 pps. The rate is still too fast for the human eye to detect these changes.

21 Flat Panel Display Flat panel displays, or flat screens, are used for systems that have limited space. 2 Widely Used Types Liquid Crystal Displays (LCDs) Light-emitting Diodes (LEDs) Applications Laptop computers Calculators Hand-held organizers Digital clocks VCR/Stereo displays And so on...

22 Liquid Crystal Display What is a liquid crystal? A material that exists between the liquid and solid phases of matter. When an electric field is applied to a liquid crystal the optical properties of the matter change. Causes light to passes through at varying brightness levels.

23 Liquid Crystal Display Liquid crystals are flattened between two glass plates. The crystal layer is a few microns thick. A transparent electrical conductor is placed on the inner sides of the glass. Perpendicularly oriented polarizers are placed over the outer sides of each glass plate Polarizer Direction of polarization Liquid Crystal Layer Conductor Glass Layer Liquid Crystal Device

24 Liquid Crystal Display How does it work? When the voltage is off: The liquid crystals are in a relaxed state and therefore they are aligned (i.e., arranged parallel to one another). Polarized light that has passed through the first polarizer is unaffected by the aligned crystals and is blocked by the second (perpendicular) polarizer. No Light Transmitted

25 Liquid Crystal Display How does it work? When the voltage is on: the conductors transfer an electric field that twists the crystals when the liquid crystals are forced to twist, so does the direction of polarization of the light some or all of the light can pass through the second polarizer. Light Transmitted

26 Liquid Crystal Display How does it work? The amount of voltage controls the orientation of the crystal, or how much they will twist: The maximum amount of light is transmitted when the first and final liquid crystals are perpendicular to each other. The minimum amount of light is transmitted when the first and final liquid crystals are parallel to each other. Any intermediate amount of light can be transmitted when the first and final crystals are oriented at other angles.

27 Liquid Crystal Display Screens An individual liquid crystal device is called a cell. A two-dimensional matrix of cells forms a screen. Wires are connected to the cells to transfer voltages. Certain voltages go to specific cells to control the amount of light through each cell. Color filters are placed over each cell for color screens. The color patterns and shapes change for different displays. Back-lighting The initial light source needs to be bright enough to pass through the LCD cells and be detected by a viewer s eye.