Tutorial 6 Derek Wright Wednesday, March 2 nd, 2005.

Tutorial 6

Derek Wright

Wednesday, March 2nd, 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

VDDVData

VAddress

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 (LCoS) current leader in projection televisions

• CMOS process means cheap and on-chip integration

LCoS

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 E-field

• Sharp tips release more electrons

• Different than tunneling current

• Coming to a sharp point deforms the E-field 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

Field Emission Tips

Potential Barrier

• 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

metal

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Distance from cathode surface z=0

vacuum

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 = ~2x 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 E-field

• 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.