Download Chapter 1 Liquid Crystal Display Monitor

Chapter 1
1.
2.
EC5103PA_Apr2008
Types of Electronic displays
LCD displays
ITECC/TP/ECE/SimBN
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Types of Electronic displays
1 Emissive
2 Non emissive
(Based on light control)
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Types of Emissive Displays
1. Neon light
- used in advertising and commercial signage.
- long, narrow glass tubes, often bent into all sorts of
shapes. can spell out a word.
- emit light in different colours.
- Inside the glass tube there is a gas like neon, argon or
krypton at low pressure. At both ends of the tube there
are metal electrodes. When you apply a high voltage to
the electrodes, the neon gas ionizes, and electrons flow
through the gas. These electrons excite the neon atoms
and cause them to emit light that we can see.
- Neon emits red light when energized. Other gases emit
other colours.
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Types of Emissive Displays
2. Cathode ray tube (CRT)
A CRT monitor contains millions of tiny red, green, and blue
phosphor dots that glow when struck by an electron beam
that travels across the screen to create a visible image.
A: Cathode
B: Conductive coating
D: Phosphor-coated screen
E: Electron beam
C: Anode
F: Shadow mask
Fig 1 Inside a CRT
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Types of Emissive Displays
3. Light emitting diode (LED)
- a semiconductor diode that emits incoherent narrowspectrum light when electrically biased in the forward
direction of the p-n junction.
- often used as small indicator lights on electronic devices
and increasingly in higher power applications such as
flashlights and area lighting.
- The colour of the emitted light depends on the
composition and condition of the semiconducting material
used, and can be infrared, visible, or ultraviolet.
- LEDs can also be used as a regular household light
source. Besides lighting, interesting applications include
sterilization of water and disinfection of devices.
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Types of Emissive Displays
4. Electro-luminescent display (ELD)
- A technology used to produce a very thin display screen,
called a flat-panel display, used in some portable computers.
- works by sandwiching a thin film of phosphorescent
substance between two plates. One plate is coated with
vertical wires and the other with horizontal wires, forming a
grid. When an electrical current is passed through a
horizontal and vertical wire, the phosphorescent film at the
intersection glows, creating a point of light, or pixel.
- light and small making them ideal for vehicle dashboards and
various military applications.
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Types of Emissive Displays
5. Gas-Plasma display (GPD) (PDP)
- A type of flat-panel display that commonly used for large TV
displays (typically above 940 mm).
- Theatre, theme park, sale display, home entertainment system.
- 21 to 63 inches, 3 to 6 inches deep, hang on wall.
- 16 millions colours & 160 degree-viewing angle.
- VGA, SVGA, XGA, SXGA, UXGA.
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What is Plasma Display
• Plasma display or Gas-Plasma
Display (GPD) works by sandwiching
neon gas between two plates.
• Each plate is coated with a
conductive print.
• The print on one plate contains
vertical conductive lines and the other
plate has horizontal lines. Together, the
two plates form a grid.
• When electric current is passed
through a horizontal and a vertical line,
the gas at the intersection glows,
creating a point of light, or pixel.
Fig 9
• You can think of a gas-plasma display as a collection of very small
neon bulbs. Images on gas-plasma displays generally appear as
orange objects on top of a black background.
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Advantages and disadvantages of
Plasma Display
Advantages
• The main advantage of plasma display technology is that it can
produce a very wide screen using extremely thin materials.
• Each pixel is lit individually, the image is very bright and looks
good from almost every angle.
• The image quality isn't quite up to the standards of the best
cathode ray tube sets, but it certainly meets most people's
expectations.
Disadvantages
• The biggest drawback of this technology has to be the price.
• Consumes high power.
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Types of Emissive Displays
6. Vacuum fluorescent display (VFD)
- used commonly on consumer-electronics equipment such as
video cassette recorders, car radios, and microwave ovens.
- emits a very bright light with clear contrast and can easily
support display elements of various colours. The technology is
related to both the cathode ray tube and the nixie tube.
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Types of Emissive Displays
7. Field emission display (FED)
- a type of flat panel display using field emitting cathodes to
bombard phosphor coatings as the light emissive medium.
- very similar to cathode ray tubes, however they are only a few
millimeters thick.
- uses a large array of fine metal tips or carbon nanotubes (which
are the most efficient electron emitters known), with many
positioned behind each phosphor dot, to emit electrons through a
process known as field emission.
- FEDs are energy efficient and could provide a flat panel
technology that features less power consumption than existing
LCD and plasma display technologies. They can also be cheaper
to make, as they have fewer total components.
- Nano-emissive display is the name given by Motorola for field
emission display.
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Types of Emissive Displays
8. Organic LED (OLED)
• solid-state devices composed of thin films of organic molecules
that create light with the application of electricity.
• no backlight,
• brighter, thinner, faster & lighter than LCDs
• use less power, offer high contrast & cheaper to manufacture.
• used in PDAs, cellular phones, television screens, computer
displays, portable system screens, advertising, information and
indication.
Fig 2 (a) OLED display for Sony Clie
(b) Sony 11-inch OLED
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Types of Emissive Displays
8. Organic LED (OLED)
• biggest technical problem is the limited lifetime of the organic
materials. In particular, blue OLEDs historically have had a
lifetime of around 14,000 hours when used for flat-panel
displays, which is lower than typical lifetime of LCD, LED or
Plasma display technology – each currently rated for about
60,000 hours, depending on manufacturer and model.
• The intrusion of water into displays can damage or destroy the
organic materials. Therefore, improved sealing processes are
important for practical manufacturing and may limit the longevity
of more flexible displays.
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Types of Non Emissive Displays
1.
Liquid crystal display (LCD)
- composed of liquid crystal suspended between two
transparent sheets.
- can be found in digital watches, hand held displays, laptop
computers, computer projectors, calculators, stereo
equipment and a host of other electronic equipment today.
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Types of Non Emissive Displays
2. Digital micromirror device(DMD)
- Digital Light Processing (DLP), a new technology developed
by Texas Instruments used for projecting images from a
monitor onto a large screen for presentations.
- DLP uses tiny mirrors housed on a special kind of microchip
called a Digital Micromirror Device (DMD). The result is sharp
images that can be clearly seen even in a normally lit room.
- The number of mirrors corresponds to the resolution of the
screen. DLP 1080p technology delivers more than 2 million
pixels for true 1920x1080p resolution, the highest available.
Digital Micromirror Device (DMD)
DLP chip
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Types of Non Emissive Displays
3. Electrophoretic ink (e-ink)
- a specialized type of electronic paper (e-paper) that
combines the uses and advantages of a computer
display and paper.
- e-paper displays are extremely thin, use minimal
amounts of power and provide a high-contrast viewing
surface like paper, but can be easily updated like a
monitor.
http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&stor
eId=10151&langId=-1&categoryId=16184
Fig 3 (a) Sony reader
(b) Seiko watch
(c) 2mm thick flexible active-matrix display
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Types of Non Emissive Displays
3. Electrophoretic ink (e-ink)
•
EPDs are a technology enabled by electronic ink (e-ink)
that carries a charge enabling it to be updated through
electronics.
•
is a reflective technology which requires no front or
backlight, is viewable under a wide range of lighting
conditions, including direct sunlight, and requires no
power to maintain an image.
•
•
http://en.wikipedia.org/wiki/E-paper
http://www.electronista.com/articles/07/11/16/seiko.ultra.d
ense.e.paper/
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Types of Non Emissive Displays
3. Electrophoretic ink (e-ink)
•
There are several different technologies to build e-paper,
some of which can use plastic substrate and electronics,
so that the display is flexible. It is considered more
comfortable to read than conventional displays. This is
due to the stable image which does not need to be
constantly refreshed, the large viewing angle, and the
fact that it uses reflected ambient light. It has a similar
contrast ratio to that of a newspaper and is lightweight
and durable, however it still lacks good color
reproduction.
•
Applications include e-book readers capable of
displaying digital versions of books and e-paper
magazines, electronic pricing labels in retail shops, time
tables at bus stations, and electronic billboards.
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Motorola F3/F3c (MOTOFONE)
•is designed to appeal to the low-end
market and developing countries,
making it less functional, but also less
expensive than most phones.
•the first mobile phone to use
electronic paper in its screen.
•Motorola uses the term ClearVision to
describe the new display, which is
manufactured using E Ink's
electrophoretic imaging film.
•The electronic paper main display
allows for the phone's thinness (no
glass), longer battery life, and outdoor
viewability (paper-like reflectivity).
• It has a backlight for the keypad and
a slit that projects the backlight onto
the screen so the display can be seen
in darkness.
Screen
electronic paper
(ClearVision Display)
Ringtone
Polyphonic
Networks
GSM / CDMA
Physical
size
47 x 114 x 9.1 mm
Weight
68 g
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What is LCD Displays
-
most common type of flat panel display.
-
applications: digital watches, notebook PC, handheld
computers, calculators, televisions, digital camera, video
cameras, monitors.
-
use two sheets of polarising material with a liquid crystal
solution between them.
-
composed of pixels or other shapes which can be turned on or
off with electrical stimulation.
-
typically a light is passed through the LCD to illuminate the
pixels.
-
electric current passed through liquid causes liquid crystals
(rod-shaped molecules) to align so that light cannot pass
through them.
-
each crystal is like a shutter, either allowing light to pass
through or blocking the light.
-
http://en.wikipedia.org/wiki/Liquid_crystal_display
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Types of LCD Display
• Passive matrix LCD Display (PMLCD)
– TN (twisted nematic)
– STN (supertwisted nematic)
– DSTN (dual scan twisted nematic)
• Active matrix LCD Display (AMLCD)
– TFT (Thin Film Transistor)
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Passive matrix LCD displays
Consist of a grid of horizontal & vertical wires. At the
intersection of each grid is an LCD element that
constitutes a single pixel.
Advantages
- use less power and cheaper than AMLCDs.
Disadvantages
- slow response time, smearing occurs when the display
cannot keep up with changes of content.
- causes ghosting, an effect whereby an area of "on" pixels
causes a shadow on "off" pixels in the same rows and
columns.
- narrow viewing angle.
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Active matrix LCD displays
AMLCD displays or called TFT (thin film transistor) displays.
Transistor are built into each pixel to switch each one on or off
within the screen.
Advantages
- sharper, broader viewing angle than passive matrix.
- thinner and lighter.
- faster response time.
Disadvantages
- consume more power and more expensive.
http://en.wikipedia.org/wiki/TFT_LCD
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Basic Operating Principles of LCD Display
The first principle of a LCD
• Consists of sandwiching liquid crystals between two finely
grooved surfaces, where the grooves on one surface are
perpendicular (90°) to the grooves on the other, see Fig 1.
• If the molecules at one surface are aligned north to south, and
the molecules on the other are aligned east to west, then those
in-between are forced into a twisted state of 90 °.
• Light follows the alignment of molecules, therefore is also
twisted through 90 ° as it passes through the liquid crystals.
• When a voltage is applied to the liquid crystal, the molecules
rearrange themselves vertically, allowing light to pass through
untwisted.
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Basic Operating Principles of LCD Display
The second principle of an LCD
0°
Light
Polarising
filter
•
It relies on the properties of polarising
filters and light itself.
•
A polarising filter is a set of incredibly fine
parallel lines. These lines act like a net,
blocking all light waves apart from those
(coincidentally) orientated parallel to the
lines.
Liquid crystal
(rod-like molecules)
•
A second polarising filter with lines
arranged perpendicular (at 90 °) to the
first would therefore totally block this
already polarised light.
•
Light would only pass through the second
polariser if its lines were exactly parallel
with the first, or if the light itself had been
twisted to match the second polariser.
Polarising
filter
90°
Fig 1
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Twisted nematic (TN) LCD at normally white mode
Fig 2 (a) 0v transmitting state
(b) >5v non-transmitting state
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Normally White and Normally Black Mode
•
Normally White Mode
- no voltage applied equals light passing through, see Fig 2a.
- while applied voltage equals no light emerging at the other
end, see Fig 2b.
•
Normally Black Mode
- when voltage applied equals light passing through,
- while no voltage equals no light emerging at the other end.
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DSTN (dual scan twisted nematic) display
The orientation of alignment layers varies between 90 degrees and
270 degrees, depending on the total rotation of the liquid crystals
between them.
Fig 3
normally white mode
No power
is applied
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Backlight and Colour pixel
Backlight
- cold-cathode fluorescent tubes mounted along the top and
bottom edges of the panel, the light are distributed across
the panel using a plastic light guide or prism.
- the image which appears on the screen is created by this
light as it passes through the layers of the panel.
Colour pixel
- In a colour monitor, each pixel is made out of 3 subpixels that
have either red, green, or blue colour filters.
- Each subpixel is energized with different intensities, creating a
range of colors perceived as the mixture of these dots.
- Fig 5 shows that additional red, green and blue coloured filters
are used to create a single multi-coloured pixel.
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Structure of a LCD monitor
Fig 4
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TFT (Thin Film Transistor) Displays
- In a TFT screen, also known as active matrix, an extra matrix of
transistors is connected to the LCD panel,
- one transistor for each colour (RGB) of each pixel.
-
- these transistors drive the pixels, eliminating the problems of
ghosting and slow response speed.
Column select (Data source)
Row
select
(Gate)
Fig 5
Simple TFT Active Matrix Array
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Main reason for expensive TFT displays
- VGA screens need
640 x 480 x 3 = 921,000 transistors,
- XGA screens need
1024 x 768 x 3 = 2,359,296 transistors,
- The complete matrix of transistors has to be produced on a
single, expensive silicon wafer and more than a couple of
impurities means that the whole wafer must be discarded.
- This leads to a high wastage rate and is the main reason for
the high price of TFT displays. It's also the reason why in any
TFT display, there are liable to be a couple of defective pixels
where the transistors have failed.
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Two phenomenon to define a defective LCD pixel
1. A "lit" pixel, which appears as one or several
randomly-placed red, blue and/or green pixel
elements on an all-black background.
2. A "missing" or "dead" pixel, which appears as a
black dot on all-white backgrounds.
Note: A “lit” pixel is more common and is the result of a transistor
occasionally shorting on, resulting in a permanently "turnedon" (red, green or blue) pixel.
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No. of defective pixels accepted per LCD panel
•LCD manufacturers set limits based on user feedback and
manufacturing cost data - as to how many defective pixels are
acceptable for a given LCD panel.
•The goal is to maintain reasonable product pricing while
minimising the degree of user distraction from defective pixels.
-For a XGA panel
1024x768x3 = 2,359,296 pixels
= 2,359,296 transistors
-If 20 defective pixels,
pixel defect rate = (20/2,359,296)*100 = 0.0008%
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a-si (amorphous silicon) and p-si (polysilicon)
TFT LCD panel
Fig 6
p-Si (polysilicon) technology
- allows driver circuitry and peripheral electronics to be made
as an integral part of the display.
- thinner, brighter panels with better contrast ratios, and
tougher than a-si panels. It allows larger panels to be fitted into
existing casings.
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Comparison between CRT & LCD Displays
1. Size
Table 1
Flat Panel size
CRT size
Typical
resolution
13.5in
15in
800x600
14.5in to 15in
17in
1024x768
18in
21in
1280x1024 or
1600x1200
Two specifications for CRT screen size:
1) CRT size (the actual size of the picture tube).
2) viewable screen size (the usable screen area).
- the CRT picture tube is enclosed in the plastic casing,
the viewable screen size is smaller than the overall
CRT size.
http://bravotech.us/info/crt-lcd.htm
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Comparison between CRT & LCD Displays
2. + LCD monitor is thinner and lighter than CRT monitor.
3. + Unlike CRT, the LCD monitors have no convergence
problems, because each cell is switched on and off individually.
4. + LCD monitors are also called "soft" screens. The image does
not flicker thus causing less eye strain.
5. - It is possible for one or more pixels on LCD panel to be
flawed. There's only a slim chance that all pixels will be perfect;
some will be stuck on (a "bright" defect) or off (a "dark" defect).
6. - LCD panels are lit through the backlight; sometimes, will
exhibit brighter lines in some parts of the screen. Ghosting may
occur where a particularly light or dark image can affect
adjacent portions of the screen.
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Comparison between CRT & LCD Displays
7. -Viewing angle problems on LCDs occur because the
technology using transmissive method, which works by
modulating the light that passes through the display. CRTs
are emissive displays, the light is emitted at the front of the
display, which is easily viewed from greater angles.
8. CRT require an analogue signal to produce a picture and
the LCD require a digital signal.
• The graphics signal is generated digitally inside the PC,
converted by the graphics card to an analogue signal,
then fed to the LCD panel where it has to be converted
back into a digital signal.
• This process limits the display's performance and
compromise the image quality.
38
Comparison between CRT & LCD Displays
(a)
Fig 7
Interface between LCD
monitor and PC using
(a) digital interface
(b) analogue interface
(b)
DVI : Digital Visual Interface
VGA : Video Graphics Adapter
Fig 8 A graphics adapter with
both analogue (VGA) and digital (DVI) connectors
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Comparison between CRT & LCD Displays
9. Table 2 compares the difference between a 13.5in PMLCD,
AMLCD and a 15 in CRT monitor.
Display
Type
Viewing
Angle
Contrast
Ratio
Response
Speed
Brightness
Power
Consumpt
-ion
PMLCD
49-100
degrees
40:1
300ms
70 - 90
45 watts
60K
hours
AMLCD
> 140
degrees
140:1
25ms
70 - 90
50 watts
60K
hours
CRT
> 190
degrees
300:1
n/a
220 270
180 watts
Years
Life
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Comparison between CRT & LCD Displays
10. Contrast ratio is a measure of how much brighter a pure white output is
compared to a pure black output. The higher the contrast the sharper the
image and the more pure the white will be. When compared with LCDs,
CRTs offer by far the greatest contrast ratio.
11. Response time is measured in milliseconds and refers to the time it takes
each pixel to respond to the command it receives from the panel
controller.
12. An AMLCD has a much better response time than a PMLCD. Response
time doesn't apply to CRTs because of the way they handle the display
of information (an electron beam exciting phosphors).
13. The higher the level of brightness the brighter the white displays.
14. The life span of an LCD, is the mean time between failures for the flat
panel. If it runs continuously it will have an average life of 60,000 hours
(about 6.8 years) before the light burns out. LCDs simply burn out, CRT's
get dimmer as they age.
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Review Questions on LCD
1.
What are LCD monitors and how do they work?
2.
Name four applications of LCD displays.
3.
Give the full name for the following:
i) AMLCD
ii) PMLCD
iii) TFT
iv) DSTN
v) DVI
4. Give two advantages of an AMLCD as compared to PMLCD.
5. State two types of PMLCD technology.
6. What is the function of the backlight of a LCD panel?
7. What are the two phenomenons which define a defective LCD pixel?
8. State four disadvantages of CRT monitor as compared to LCD monitor.
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