Tutorial 6 Derek Wright Wednesday March 2 nd

Tutorial 6 Derek Wright Wednesday, March 2 nd, 2005

Sensors and Image Systems • • Liquid Crystal Displays Organic Light Emitting Diode Field Emission and Plasma Displays Electronic Paper

Display Types • Light Generating – Each pixel generates photons based on image data • Light Controlling – Each pixel controls whether or not light passes through based on image data

Passive Matrix • Rows (or columns) can only be driven one at a time • Rows are driven sequentially • Columns determine which pixels are on and which are off (based on image data) • Each row is only being driven for (1/rows) • Each pixel mush be driven extra bright to fool the human eye into thinking it’s always on

Passive Matrix

Active Matrix • Each pixel has it’s own circuit that loads and stores that pixel’s data • This allows the pixel to remain on while data is loading in other rows • Enables bigger higher-resolution displays • Pixels do not have to be driven too hard as in passive matrix

Active Matrix VData VAddress VDD

Liquid Crystal Phases

Liquid Crystal Displays • Liquid Crystal Displays (LCDs) exploit liquid crystal’s ability to bend light • Polarized light enters the back of a liquid crystal pixel • The light passes through nematic phase liquid crystal, which bends the light’s polarization plane • The light passes through another polarizer (NW) • When an E-field is applied, the liquid crystal doesn’t bend the light and it can’t pass through the polarizer (NW)

Twisted Nematic Effect

LCD Benchmarks • Current highest resolution: – 368 ppi – 3840 x 2400 QUXGA @ 22. 2” • Biggest size: – Sharp @ 65” with 1920 x 1080 • Cost: – $400 for 15”, $9, 000 for 45”

Reflective LCDs • • No backlight Designed to reflect ambient light Bad in the dark Good under bright conditions, like outdoors • Low power with no backlight

Reflective LCDs

Transreflective LCDs • Combine features of both transmissive and reflective LCDs • Reflective for high ambient lighting • Transmissive for low ambient lighting • Less power than a fully transmissive LCD • Each pixel is divided into a reflective part and a transmissive part

Projection Displays • An image is produced using either transmissive or reflective means • Optical mirrors and lenses magnify the image to occupy a large area • Liquid Crystal on Semiconductor (LCo. S) current leader in projection televisions • CMOS process means cheap and on-chip integration

LCo. S

OLEDs • Organic Light Emitting Diodes • Organic molecules can be tailored to act as an LED • They can emit photons • Brighter than current TVs • Fast switching • Should eventually be cheap

OLED Deposition • OLEDs can be deposited using many different means – Depends on the physical properties of the organic molecule • Vapour-phase deposition • Liquid-phase deposition – Enables really cheap manufacturing methods, like using an ink-jet printer to pattern the layers

OLED Structure

Single vs. Double Layer

Field Emission • Occurs under high voltage • Electrons are stripped off of an electron emitter • Accelerated using externally applied Efield • Sharp tips release more electrons • Different than tunneling current

Field Emission Tips • Coming to a sharp point deforms the Efield at the tip • This makes it easier for electrons to tunnel across the potential barrier • Material should have a low work function and be resistant to sputtering

Potential Barrier W 0 WF Energy • To strip electrons off the surface they need to overcome the potential barrier • It can be lowered with an externally applied E-field • Tip shape affects local E-field e. E 1 metal vacuum z=0 e. E 2 Distance from cathode surface

Phosphor Screens • Made of inorganic powders with particle grain size 3 to 8 m • Electron impact causes photon emission, just like in CRT • Layer can’t be too thick or emitted photons will get re-adsorbed • Layer can’t be too thin or too many electrons will pass through without impact • Optimal thickness = ~2 x grain size

Plasma Displays • Each pixel in a plasma display is like a tiny fluorescent light bulb • A plasma is “fired” • Xenon gas emits UV photons • A phosphor coating converts the UV into visible light

Electronic Paper • Ultimate goal of display technologies: – Emulate printed paper • • Ultra-low power Perfectly bistable (keeps image with no power) Flexible, foldable High contrast ratio Appear paper white Lightweight User-friendly

Electrophoretic Displays • A solution of black dye and suspended white particles • The white particles move in an applied Efield • There’s a transparent electrode (ITO) and a back electrode • Voltage is used to move the white particles to the surface for a white pixel, or to the bottom for a dark pixel

Electrophoretic Displays

Electrophoretic Displays • Improvements can be made “microencapsulating” dye and pigment • Prevents lateral motion between pixels • Pigments tend to want to stick together under high field – Microcapsules prevent agglomeration of sizes bigger than the capsule – Improves display lifetime

Electrophoretic Displays

Rotating Ball Displays • Tiny balls (~100 m) are made with one half white and one half black • There is a macroscopic charge on the balls, so that black is positive and white is negative (or vice versa) • The balls are suspended in oil and sandwiched between transparent and flexible substrates • An external E-field “printer” is used to write the pattern to the display

Rotating Ball displays

Thank You! • This presentation will be available on the web.