Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Display Systems Viewing Images

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science In This Section... We will explore how display systems work. –Cathode Ray Tube Television Computer Monitor –Flat Panel Display Liquid Crystal Display

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube A common device used in televisions, and computer monitors. The tube directs electrons to form an image on a screen.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube A cathode ray tube (CRT) is a special kind of vacuum tube.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube One end of the tube is supplied with electrons by a high voltage power supply. This part of the tube is called the cathode. Power supply + - Cathode

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube The electrons are drawn at high speed toward a plate of metal, called an anode, that is positively charged. The anode accelerate the electrons to very high speed. The electrons hit the anode and return to the power supply Power supply + - Anode Cathode

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube This is how all vacuum tubes work. The thing that makes a CRT tube special is that the anode has a hole in it. Power supply + - Anode Cathode Hole

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube Electron beam, or cathode ray The hole allows most of the high speed electrons miss the anode. These electrons continue to fly toward the other end of the tube as an electron beam (or e-beam). The e-beam is also called a cathode ray. Power supply + - Cathode Anode

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube This part of the CRT is called the electron gun. Electron beam Electron Gun Power supply + - Cathode Anode

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube Deflectors use electric fields to bend the e-beam in a desired direction. (Some CRTs use magnetic coils as deflectors.) Deflectors Electron beam Electron Gun Power supply + -

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube The e-beam collides with a phosphor screen causing it to temporarily glow and become viewable. The electrons then return to the power supply through a wire. Phosphor screen Deflectors Electron beam Electron Gun Power supply + - Light

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube By controlling the deflectors, the e-beam writes on the phosphor screen just like a pencil writing on a piece of paper. Phosphor screen Deflectors Electron beam Electron Gun Power supply + - Light

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cathode Ray Tube This is a general CRT for a black-and-white television system. Phosphor screen Deflectors Electron beam Electron Gun Power supply + - Light

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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)

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science CRT - Color Supplemental Guide The supplemental guide (shadowmask, aperture grill, etc.) for color is placed just before the phosphor screen. Phosphor screen Deflectors Electron beam Electron Gun Power supply + - Light

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Additive Color Mixing The additive primary colors used are RED, GREEN, and BLUE.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Additive Color Mixing Overlapping red, green, and blue light, creates yellow, cyan, and magenta light.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Additive Color Mixing The combination of the three additive primaries gives white light (R + G + B = WHITE).

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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. triad Phosphor Screen Shadowmask E-guns

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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)

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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...

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Liquid Crystal Display Liquid crystals are flattened between two glass plates. –The crystal layer is a few microns thick. Polarizer Liquid Crystal Layer Glass Layer Conductor 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 Liquid Crystal Device Direction of polarization

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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.