Transcript Tutorial 6 (PowerPoint)

Tutorial 6
Derek Wright
Wednesday, March 2nd, 2005
Sensors and Image Systems
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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
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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 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
W0

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
eE1
metal
vacuum
z=0
eE2
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 = ~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
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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.