display devices group(71)
TRANSCRIPT
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Display Devices
Ahmed Mohsen Abdel Hafez, Rami Nabil Faker, Kareem Mohamed Abdel Aziz, Kareem
Mahmoud Mustafa, and Nabil Magdy Hassan
Group (71)
Abstract
This article will focus on demonstrating the
technology of operation for some of the common, new
and experimental electronic display devices such as
Cathode ray tube (CRT), 3D glasses, LCD TV, Laser
TV and surface-conducting electron-emitter display
(SED).
1. Introduction
Nowadays the various display devices are great
tools in many fields of life; Science, Entertainment,
Education & also Finance. So scientists are searching
& developing a better ways to make the use of the
display devices is more comfortable & easier. The
new display devices became now more preferable &
affordable than ever before. We can say that we are
living the adventures of using these devices daily, &
this is because of the huge steps from the scientists tounderstand the way that these devices operate & how
to get the best performance from them.
2. Cathode Ray Tube (CRT)
The cathode ray tube (CRT) is a vacuum
tube containing an electron gun (a source of
electrons) and a fluorescent screen, with internal or
external means to accelerate and deflect the electron
beam, used to create images in the form of light
emitted from the fluorescent screen. The image may
represent electrical waveforms (oscilloscope),
pictures (television, computer monitor), radar targets
and others. The CRT uses an evacuated glass envelope which
is large, deep, heavy, and relatively fragile
2.1. CRT structure
A cathode ray tube is a vacuum tube which
consists of one or more electron guns, possibly
Internal electrostatic deflection plates and a
phosphor target.
Figure (1): CRT
In television sets and computer monitors, the entire
front area of the tube is scanned repetitively andsystematically in a fixed pattern called a raster. An
image is produced by controlling the intensity of each
of the three electron beams, one for each additive
primary color (red, green, and blue) with a video
signal as a reference. [1]
In all modern CRT monitors and televisions, the
beams are bent by magnetic deflection, a varying
magnetic field generated by coils and driven by
electronic circuits around the neck of the tube,
although electrostatic deflection is commonly used
in oscilloscopes, a type of diagnostic instrument.
2.2. Color CRT
Color tubes use three different phosphors which
emit red, green, and blue light respectively. They are
packed together in stripes (as in aperture grille
designs) or clusters called "triads" (as in shadow
mask CRTs). Color CRTs have three electron guns,
one for each primary color, arranged either in a
straight line or in a triangular configuration (the guns
are usually constructed as a single unit). A grille or
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mask absorbs the electrons that would otherwise hit
the wrong phosphor. A shadow mask tube uses a
metal plate with tiny holes, placed so that the electron
beam only illuminates the correct phosphors on the
face of the tube.
Figure (2): Colored CRT
The three beams in color CRTs would not strike
the screen at the same point without convergence
calibration. Instead, the set would need to be
manually adjusted to converge the three color beams
together to maintain color accuracy.
Most CRT television sets and computer monitors
have a built-in degaussing (demagnetizing) coil,
which upon power-up creates a brief, alternating
magnetic field which decays in strength over the
course of a few seconds. This degaussing field is
strong enough to remove most cases of shadow mask
magnetization. [2]
3. Liquid Crystal Display (LCD)
A liquid crystal display (LCD) is a thin,
flat electronic visual display that uses the light
modulating properties of liquid crystals (LCs). LCs
do not emit light directly. LCDs therefore need a
light source and are classified as "passive" displays.
Some types can use ambient light such as sunlight or
room lighting. There are many types of LCDs that aredesigned for both special and general uses. They can
be optimized for static text, detailed still images, or
dynamic, fast-changing, video content.
They are used in a wide range of applications
including: computer monitors, television, instrument
panels, aircraft cockpit displays, signage, etc. They
are common in consumer devices such as video
players, gaming devices, clocks, watches, calculators,
and telephones. LCDs have displaced cathode ray
tube (CRT) displays in most applications.
3.1. LCD vs. CRT
They are usually more compact, lightweight,
portable, and lower cost. They are available in a
wider range of screen sizes than CRT and other flat
panel displays.
LCDs are more energy efficient, and offer safer
disposal, than CRTs. Its low electrical power
consumption enables it to be used in battery-powered
electronic equipment. It is an electronically-
modulated optical device made up of any number
of pixels filled with liquid crystals and arrayed in
front of a light source (backlight) or reflector to
produce images in color or monochrome. [3]
3.2. LCD panel structure
Each pixel of an LCD typically consists of a layerof molecules aligned between
two transparent electrodes, and two polarizing filters,
the axes of transmission of which are (in most of the
cases) perpendicular to each other. With no
actual liquid crystal between the polarizing
filters, light passing through the first filter would be
blocked by the second (crossed) polarizer.
The surface of the electrodes that are in contact
with the liquid crystal material are treated so as to
align the liquid crystal molecules in a particular
direction. This treatment typically consists of a thin
polymer layer that is unidirectional rubbed using, for
example, a cloth. The direction of the liquid crystalalignment is then defined by the direction of rubbing.
Electrodes are made of a transparent conductor
called Indium Tin Oxide (ITO).
Before applying an electric field, the orientation of
the liquid crystal molecules is determined by the
alignment at the surfaces of electrodes. In a twisted
nematic device (the most common liquid crystal
device), the surface alignment directions at the two
electrodes are perpendicular to each other, and so the
molecules arrange themselves in a helical structure,
or twist.
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Figure (3): LCD panel when light is applied.
This reduces the rotation of the polarization of the
incident light, and the device appears grey. If the
applied voltage is large enough, the liquid crystal
molecules in the center of the layer are almost
completely untwisted and the polarization of the
incident light is not rotated as it passes through theliquid crystal layer.
This light will then be mainly polarized
perpendicular to the second filter, and thus be
blocked and the pixel will appear black. By
controlling the voltage applied across the liquid
crystal layer in each pixel, light can be allowed to
pass through in varying amounts thus constituting
different levels of gray.
The optical effect of a twisted nematic device in
the voltage-on state is far less dependent on
variations in the device thickness than that in the
voltage-off state. Because of this, these devices are
usually operated between crossed polarizers such that
they appear bright with no voltage (the eye is much
more sensitive to variations in the dark state than the
bright state). These devices can also be operated
between parallel polarizers, in which case the bright
and dark states are reversed. The voltage-off dark
state in this configuration appears blotchy, however,
because of small variations of thickness across the
device.
Both the liquid crystal material and the alignment
layer material contain ionic compounds. If an electric
field of one particular polarity is applied for a long
period of time, this ionic material is attracted to the
surfaces and degrades the device performance. This
is avoided either by applying an alternating current orby reversing the polarity of the electric field as the
device is addressed (the response of the liquid crystal
layer is identical, regardless of the polarity of the
applied field). [4]
3.3 Color LCD
In color LCDs each individual pixel is divided into
three cells, or sub pixels, which are colored red,
green, and blue, respectively, by additional filters
(pigment filters, dye filters and metal oxide filters).
Each subpixel can be controlled independently to
yield thousands or millions of possible colors for
each pixel. CRT monitors employ a similar 'subpixel'
structures via phosphors, although the electron beam
employed in CRTs do not hit exact 'subpixels'.
Because they utilize red, green and blue elements,
both LCD and CRT monitors are direct applications
of the RGB color model and give the illusion of
representing a continuous spectrum of hues as a
result of the tri-chromatic nature of human vision.
Figure (4): Colored CRT demonstration
Color components may be arrayed in various pixel
geometries, depending on the monitor's usage. If the
software knows which type of geometry is being used
in a given LCD, this can be used to increase the
apparent resolution of the monitor through subpixel
rendering. This technique is especially useful for
text anti-aliasing.
High-resolution color displays such as modern
LCD computer monitors and televisions use an active
matrix structure. A matrix of thin-film
transistors (TFTs) is added to the polarizing and color
filters. Each pixel has its own dedicated transistor,allowing each column line to access one pixel. When
a row line is activated, all of the column lines are
connected to a row of pixels and the correct voltage
is driven onto all of the column lines. The row line is
then deactivated and the next row line is activated.
All of the row lines are activated in sequence during
a refresh operation. Active-matrix addressed displays
look "brighter" and "sharper" than passive-matrix
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addressed displays of the same size, and generally
have quicker response times, producing much better
images. [5]
3.4. LCD Efficiency
LCDs are relatively inefficient in terms of power
use per display size, because the vast majority of
light that is being produced at the back of the screen
is blocked before it reaches the viewer. To start with,
the rear polarizer filters out over half of the original
un-polarized light.
A good portion of the screen area is covered by the
cell structure around the shutters, which removes
another portion. After that, each sub-pixel's color
filter removes the majority of what is left to leave
only the desired color. Finally, to control the color
and luminance of a pixel as a whole, the light has to
be further absorbed in the shutters.
Old LCD sets use several hundred watts of power,
more than would be required to light an entire housewith the same technology. As a result, LCD
televisions end up with overall power usage similar
to a CRT of the same size.
Modern LCD sets have attempted to address the
power use through a process known as "dynamic
lighting". This system examines the image to find
areas that are darker, and reduces the backlighting in
those areas.
CCFLs are long cylinders that run the length of the
screen, so this change can only be used to control the
brightness of the screen as a whole, or at least wide
horizontal bands of it. This makes the technique
suitable only for particular types of images, like thecredits at the end of a movie. Sets using LEDs are
more distributed, with each LED lighting only a
small number of pixels, typically a 17 by 17 patch.
This allows them to dynamically adjust brightness of
much smaller areas, which is suitable for a much
wider set of images.
Another ongoing area of research is to use
materials that optically route light in order to re-use
as much of the signal as possible. One potential
improvement is to use micro prisms or dichromic
mirrors to split the light into R, G and B, instead of
absorbing the unwanted colors in a filter. A
successful system would improve efficiency by threetimes. Another would be to direct the light that would
normally fall on opaque elements back into the
transparent portion of the shutters.
Several newer technologies, OLED, FED and
SED (we will discuss the way of operation for some
of them later), have lower power use as one of their
primary advantages. All of these technologies
directly produce light on a sub-pixel basis, and use
only as much power as that light level requires. The
dramatically lower power requirements make these
technologies particularly interesting in low-power
uses like laptop computers and mobile phones. These
sorts of devices were the market that originally
bootstrapped LCD technology, due to its light weight
and thinness. [7]
3.5. LCD image quality
Early LCD sets were widely derided for their poor
overall image quality, most notably the ghosting on
fast-moving images, poor contrast ratio, and muddy
colors. In spite of many predictions that other
technologies would always beat LCDs, massive
investment in LCD production, manufacturing, and
electronic image processing has addressed many of
these concerns.
Since the total amount of light reaching the viewer
is a combination of the backlighting and shuttering,
modern sets can use "dynamic backlighting" to
improve the contrast ratio and shadow detail. If aparticular area of the screen is dark, a conventional
set will have to set its shutters close to opaque to cut
down the light. However, if the backlighting is
reduced by half in that area, the shuttering can be
reduced by half, and the number of available
shuttering levels in the sub-pixels doubles. This is the
main reason high-end sets offer dynamic lighting (as
opposed to power savings, mentioned earlier),
allowing the contrast ratio across the screen to be
dramatically improved.
Color on an LCD television is produced by
filtering down a white source and then selectively
shuttering the three primary colors relative to eachother. The accuracy and quality of the resulting
colors are thus dependent on the backlighting source
and its ability to evenly produce white light. [7]
3.7. Recent research
Some manufacturers are also experimenting with
extending color reproduction of LCD televisions.
Although current LCD panels are able to deliver
all RGB colors using an appropriate combination of
backlight's spectrum and optical filters,
manufacturers want to display even more colors.
One of the approaches is to use a fourth or evenfifth and sixth color in the optical color filter array.
Another approach is to use two sets of suitably
narrowband backlights (e.g. LEDs), with slightly
differing colors, in combination with broadband
optical filters in the panel, and alternating backlights
each consecutive frame. Fully using the extended
color gamut will naturally require an appropriately
captured material and some modifications to the
distribution channel. Otherwise, the only use of the
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extra colors would be to let the looker boost the color
saturation of the TV picture beyond what was
intended by the producer, but avoiding the otherwise
unavoidable loss of detail ("burnout") in saturated
areas.
4. Field emission display (FED) A field emission display (FED) is a flat panel
display technology that uses large-area field electron
sources to provide electrons that strike colored
phosphor to produce a color image. In a general
sense, a FED consists of a matrix of cathode ray
tubes, each tube producing a single sub-pixel,
grouped in threes to form red-green-blue (RGB)
pixels.
FEDs combine the advantages of CRTs, namely
their high contrast levels and very fast response
times, with the packaging advantages of LCD and
other flat panel technologies. They also offer the
possibility of requiring less power, about half that of an LCD system. To date, however, manufacturing
problems have prevented any FED system from
entering commercial production.
4.1. FED structure
The key lies in giving each pixel separate electron
guns situated very close behind the phosphor coated
screen. Conventionally these guns have been
fabricated using the Spindt process, in which arrays
of small sharp silicon or molybdenum cones are
deposited onto a substrate within an etched hole.
The result is a triode structure of between a few
and less than one micron in diameter, of which there
are thousands per individual pixel. Electrons can
leave the sharp tips with relatively low extraction
voltages at the gate.
The advantages are CRT-like viewing
characteristics using mature phosphor technology for
the anode, and energy efficient low voltage control at
the gate. The main problem is, not surprisingly,
fabricating an array over a large area. To date, full
color 35cm flat panels have been demonstrated, and
12cm versions are in production, but this approach is
highly unlikely to achieve broad area status.
Figure (5): Demonstration for step one.
The traditional tip-based structure relies on a sharp
cone in a small hole, requiring some fine lithography
and difficult processing. This three electrode (triode)
structure enables the emission to be controlled by
lower voltages on the intermediate electrode (gate).
Figure (6): Demonstration for step two.
A predominantly flat cathode like the PFE (Printable
Field Emitters, will be discussed later) printed
composite allows the gated structure to be wider and
produced with greater ease and less expense.
Figure (7): Demonstration for step three.
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Nanotubes (will be discussed later) give hairy
structures that emit at low fields, but would require
much deeper gated structures to accommodate the
irregular morphology and avoid short circuits
between the electrodes.
The solution would be to achieve field emission
from a flat cathode plane. This would reduce the need
for fine lithographic features and relax the tolerances
demanded of the triode structure. Materials that have
been investigated include polycrystalline diamond
thin films, amorphous carbon thin films, and various
other amorphous thin films such as silicon and boron
nitride.
All of these thin film materials are believed to
have the electronic band structure necessary for
electrons to leave the material under the application
of a modest electric field and travel into the vacuum.
However, they all require plasma-based deposition in
costly vacuum equipment. [8]
4.2. Carbon Nanotubes
Carbon nanotubes and related structures (placed in
figure 7) have also been found to field emit electrons
at very low fields. However, their aspect ratio has
made it difficult to fit them into triodes without
losing this advantage.
They are also relatively difficult to produce in
large uniform batches, are very sensitive to adsorbed
gases, and are rather difficult to bind securely within
the device. However, Samsung and Motorola are both
actively researching these materials for displayapplications, and have looked at related carbon
materials that appear to emit electrons well without
some of these drawbacks. [9]
4.3. Printable Field Emitters (PFE)
It’s an emitting structure consists of a tailored
composite of semiconducting or conducting particles
in an insulating matrix.
This approach was derived from investigations of
high voltage hold off between polished but
contaminated electrodes at the University of Aston.
The PFE materials are produced as an ink and can
be deposited using, for example, screen-printing
technology, instantly making them appealing for use
on broad area substrates.
The electronic band structure of the combined
matrix and particle means that each particle in the
composite acts as an individual field emitting site.
Unlike microtips, the PFE cold cathode materials are
extremely robust and relatively insensitive to poor
vacuum.
Figure (8): The concept of printing a field emitter cathode plane
and the components required to assemble a working display.
A further advantage of PFE's materials is that they
are predominantly flat. They can therefore operate
within relatively large triode structure, which means
that the feature size is also compatible with screen
printing technology. Because, as in the PDP case, the
electrodes and dielectric are also screen printable,
fabricating the entire device on slightly modified
PDP production lines is feasible. This will drastically
reduce the time taken for a display to reach themarketplace.
It is worth highlighting that packaging a complete
FED presents other materials related challenges. The
cathode plane needs to be fabricated on a glass
substrate, and, in the case of the PFE device, will
undergo a series of printing and air baking cycles.
The resulting low cost sandwich must be expansion
matched, adhere well at each interface and consist of
material that will be vacuum compatible. The cathode
plane must then be brought up to a suitable anode
plane comprising the phosphors and the black matrix
material that separates the pixels.
These two sheets must be mated with a closely
controlled spacing between the two halves. This is
achieved by using small, high aspect ratio ceramic or
glass spacers, designed to prevent the glass bowing
under the forces of the external atmospheric pressure.
These spacers must be located invisibly at regular
intervals throughout the display, and be conductive
enough to dissipate stray charge that may build up,
but without conducting an excessive leakage between
the anode and cathode.
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from the usually inorganic contacts. To solve this
problem, often the structure includes an electron
transport layer (ETL) and/or a hole transport layer
(HTL), which facilitate the injection of charge
carriers. All of these layers must be grown on top of
each other, with the first grown on a Substrate.
Figure (10): OLED structure
When voltage is applied, one layer becomes
negatively charged relative to another transparent
layer. As energy passes from the negatively charged
(cathode) layer to the other (anode) layer, it
stimulates organic material between the two, which
emits light visible through an outermost layer of
glass.
6.2 Principle behind the emittance of
photons:
A hole is injected into one layer of the device and
an electron is injected into the other side of the
device. The two charge carriers move along the
polymer chains in the emitting layer, and when these
two charge carriers combine, they emit a photon
(they produce light). [13]
6.3 OLED advantages
OLEDs have the following advantages over today's
flat-panel tech (LCD, CRT):
Low power consumption - OLEDs are a far
better choice for portable devices.
Faster refresh rate and better contrast.
Greater brightness - The screens are
brighter, and have a fuller viewing angle.
(Because an OLED becomes self-emissive
through organic material, its viewing angle
is far wider than an LCD. This wide viewing
angle can provide great improvements in a
wide variety of product applications. It also
offers new design concepts to industrial
designers who were before limited by the
constraints of traditional LCD -140 degrees
wide angle visibility-).
Exciting displays - new types of displays,
which we do not have today, like ultra-thin,flexible or transparent displays.
Better durability - OLEDs are very durable
and can operate in a broader temperature
range
Lighter weight - the screen can be made
very thin, and can even be 'printed' on
flexible surfaces
6.4. OLED Disadvantages:
OLEDs have limited lifetime (compared
with the current display panels).
OLEDs can also be problematic in directsunlight, because of their emissive nature.
7. LASER TV
Most projection displays are now using the lamp as
a light source, so the effort for using the laser as a
light source is continued for the merits of laser.
7.1. Laser TV advantages
The advantages of using the laser light for
projection displays are come from the original
characteristics of laser. The main advantages of scanning laser projection displays are high contrast
ratio, excellent expression of natural color and
infinite depth of focus. Laser light is polarized, so it
can yield a higher contrast ratio by using the proper
polarized optics.
The monochromatic property and color saturation
of the laser light can increase the color space about
three times larger than that of the conventional
phosphor system (the ordinary TV). The wavelengths
of lasers cover more than 90% of all colors which can
be perceived by the human eye. Laser has a long
coherence length and a low beam divergence, so one
can achieve infinite depth of focus with the properdisplay technology, such as a raster scanning.
Projection image is no longer limited to a flat screen
and it can be projected at any other surface.
We can conclude the previous talk into the
following points:
be half the weight and cost of plasma
displays
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require around 25% of the power required
by plasma displays
be very slim like plasma and LCD displays
are today
Have a very wide color gamut, twice that of
current HDTVs.
have a very long life maintain full power output for the lifespan
of the laser, resulting in a picture that doesn't
progressively degrade over time (which
happens with technologies such as plasma,
LCD, and CRT)
Never suffer from screen burn-in; Burn-in is
caused by uneven use of color reproduction
elements across a screen's surface, but laser
TVs bypass this completely.
7.2. Laser TV realization obstacles
In spite of these excellent characteristics; laser TVfor home theater could not be realized yet, for the
lack of industrial laser-related technologies, also wecan’t neglect the cost issue.
7.3. Laser TV structure & way of operation
The figure shows a schematic drawing of the basic
layout of the laser TV. It is mainly composed of blue,
green and red laser light sources, three acousto-optic
modulators, a laser beam combining part (a high-
reflection mirror and two dichroic mirrors), a
polygon scan mirror, a galvanometer and optical
lenses.
Figure (11): Schematic drawing of the basic layout of the laser TV.
Blue, green diode-pumped solid state (DPSS)
lasers and a red diode laser are used as a light source.
The wavelengths of the blue green and red are 457
nm, 532 nm and 748 nm, and the output powers are
350 mW, 700 mW and 500 mW, respectively. The
power levels of lasers are adjusted for white color
balance. Diode-pumped solid state (DPSS) lasers are
an exciting tool that combines the beam quality of a
gas laser, small size and efficiency of a diode laser
with single line output.
Blue, green and red laser beams are modulated at
acousto-optic modulators (AOMs) according to the
video signals.
Laser beam modulation in the acousto-optic device
is implemented by varying the amplitude of the
acoustic drive signal, which in turn varies the
amplitude of the light passed to the first order.
Separated RGB color signals are amplified by a high
frequency signal amplifier and are used in
modulating each laser beam by the acousto-optic
modulator.Modulated red, green and blue light beams are
combined by dichroic mirrors and a high-reflection
mirror. Then the combined beam is projected to the
screen by the scanning part. The dichroic mirror
(DM2) for combining the green light with the red
light has a transmittance over 95% in the red light,
and a reflectance of 99% in the green light.
The dichroic mirror (DM1) for combining the blue
light with the green and red light has a transmittance
over 95% in the blue light, and a reflectance of 99%
in the green and red light. All dichroic mirrors are
designed to obtain the best performance with the 45°
incident angle.Combined laser beam is horizontally scanned by a
polygon scan mirror and vertically scanned by a
galvanometer. The galvanometer is running at a rate
of 70 Hz. The polygon scan mirror has 25 facets and
is rotating at the speed of 75,700 rpm for VGA
resolution (740 × 480 Progressive scanning).
Therefore the scan rate is 31.5 kHz in coinciding with
VGA video signal format which has 525 scanning
lines (including blanking signal lines) and video
images of 70 frames per second.
When the RGB video signals are inserted to the
AOM rf drivers, acoustic wave is generated at the
transducer and it is traveled to the AO crystal, and thelaser beam is diffracted and modulated. So some time
delay is exist between the video signal and the
modulation.
This time delay must be controlled to have the
same value at three AOMs. If not, color mismatch at
the image could be happen. It can be done by
adjusting the distance between the laser beam path
and the transducer of the AOM, but some loss of
diffraction efficiency is happened due to the
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manufacturing variations of AOMs. So the control
circuit is applied to solve this problem, and the time
delay is controlled at the RGB video signals.
The controllable time delay is 257 pixels. The
sync signals are used to control the scanning part.
The horizontal sync signal is used to clock for driving
the polygon scan mirror. The vertical sync signal is
integrated to ramp signal for driving the
galvanometer. [14]
8. 3D glasses and other 3D display devices
8.1. How 3D effect works
Our ability to see stereo-vision comes from each
of our eyes seeing a slightly different view of the
world. Our brain integrates these two images into one
three-dimensional picture. The key element in
producing the stereoscopic depth effect is parallax.
Parallax is the horizontal distance between
corresponding left and right image points. Thestereoscopic image is composed of two images
generated from two related perspective viewpoints,
and the viewpoints are responsible for the parallax
content of a view. [15]
8.2. How 3D displays work
Electro-stereoscopic displays provide parallax
information to the eye by using a method related to
that employed in the stereoscope. The 3D display
systems normally in use one of the following
methods:
Separate display for each eye (used in
HMDs)
Shutter glasses (most common method)
Color filter glasses (used in some old 3D
movies)
Polarizing glasses (used in some modern 3D
movies)
8.3. Color filter glasses
Color filter glasses were one of the oldest 3D
glasses. The system works so that both eyes have adifferent color filter in front of them. This causes
that left eye can only see few colors and right eye
some other colors. When the led eye's colors are
used to draw the image which it should see and same
is used for right eye, the combined image can be
viewed with suitable glasses in 3D. The most
common color combinations are red+green and
blue+green. The color filtering limits that there are
only few possible colors in use in the picture so the
images made using this method are not very nice to
look.
Color filter glasses have been used in 3D movies
and some early computer games. The advantage of
this method is that the 3D material can be stored to
any standard color video media and viewed withnormal display devices as long as you wear the right
color filter glasses. The glasses are very inexpensive
because you only need very cheap plastic filters for
them. You can even make your own glasses from
piece of cardboard and suitable filters (standard
lighting GEL numbers R27 and R83 should be quite
suitable for red+green glasses).
Figure (12): red+green glasses.
This technique causes colors in the image to be
compromised because you have too many different
colors in different eyes. Practically you lose almost
all your color, so you can see objects coming out of
the screen but they are gray. The colors also create
some eyestrain and distortion.
8.4. Polarizing glasses
This method is usually used with projection
displays when 3D material needs to be displayed.
Every viewer has to wear special glasses which have
two polarizing lenses which have their polarization
directions adjusted to be 90 degrees different. This
makes is possible that left eye sees its picture without
problems but everything meant to right eye (sent out
at different polarization) seems to be black. Same
applies also to right eye.
Figure (13): Polarizing glasses.
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The material which has to be shown is typically
projected using two projectors (film projector, slide
projector or video projector) which each have
polarizing lenses in front of them (adjusted to meet
the polarization directions of the glasses). The
projection surface must be specially made so that it
does not do any harm to the polarization (many
traditional projection surface materials are not
suitable, silver stripe screen is recommended). The
advantage of this method is that the pictures can be in
full color and the viewing glasses are still quite
inexpensive.
8.5. LCD shutter glass method
In the LCD shutter glass 3D display, the left and
right images are alternated rapidly on the monitor
screen. When the viewer looks at the screen through
shuttering eyewear, each shutter is synchronized to
occlude the unwanted image and transmit the wantedimage. Thus each eye sees only its appropriate
perspective view. The left eye sees only the left view,
and the right eye only the right view.
A field-sequential 3D (stereoscopic) video signal is
a normal video signal (PAL, NTSC or SECAM)
which has been specially recorded with left and right
images stored on the even and odd fields of the video
signal. The 3D video signal is usually viewed while
wearing a pair of LCD shutter glasses which only
allow the left eye to see left images and the right eye
to see right images.
Figure (14): LCD shutter glass
If the images (the term "fields" is often used for
video and computer graphics) are refreshed (changed
or written) fast enough (often at twice the rate of the
planar display), the result is a flickerless stereoscopicimage. This kind of a display is called a field-
sequential stereoscopic display.
The biggest drawback of LC-Shutter glasses
besides the compatibility issue is Crosstalk. Due to
the persistence of the monitor tube, the inability of
the LC-panels to block the light entirely, sync errors
and other factors one see "Ghost images" sometimes.
The right eye sees some residue of the image
dedicated to the left eye and vice versa. [16]
8.6. Head Mounted Displays (HMD)
Head Mounted Displays, or HMD for short, is one
of the oldest stereoscopic 3D technologies on themarket. Unlike their predecessors, modern HMDs are
lightweight helmets or headbands that place
miniature screens directly in front of the viewer’s
eyes.
Figure (15): modern commercial HMD
By having the screens directly in front of their eyes
in an enclosed environment, the viewer is left with
the illusion that they are seeing their favorite games
or movies with a very large screen. While this
illusion is subjective, head mounted displays are
considered one of the most immersive stereoscopic
3D experiences possible.
HMD consists of:
One or two small display units (CRT, LCD)
with lenses.
Semi-transparent mirrors.
Eye-glasses (called data-glasses).
Types of HMD:
Display a computer generated image (CGI).
Display a combination between the real
world image & the CGI (this is called
augmented reality).
Their biggest benefits include full color immersion
and absolutely no ghosting. There is no ghosting
because each eye is getting its own personal screen.
Modern HMDs often include additional featureslike earphones and head tracking that adjust the
game’s perspective as the viewer’s head moves.
The nature of an HMD’s immersion also causes
nausea for the inexperienced gamer. When we move
our heads, our brain expects our vision to correlate
with our movement. When we are wearing an HMD,
and the image doesn’t change according to where our
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brain thinks our eyes should be looking, this
incongruity creates nausea. [17]
Acknowledgment
We thank the anonymous references for their
comments that greatly improve the presentation of this paper.
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