Advanced Display Technologies

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Nov 18, 2013 (3 years and 11 months ago)

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Advanced Display
Technologies


Paul Anderson










This report was peer reviewed by
:



Dr. Wayne Cranton,

Reader in Visual Technology,

Nottingham Trent University
.


Mark Fihn,

Publisher,

Veritas et Visus
.


Chris Williams
,

Logystyx UK Limited
.


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TABLE OF CONTENTS


Page

1. Executive Summary


3

2. Introduction


4

3. Why is this relevant to education?


4

4. Conveying information: the human eye and factors in display
systems


5

4.1 Brightness and contrast


5

4.2 Visual acuity,

information co
ntent and resolution


6

4.3 Field of view and angle of viewing


6

4.4 Perception of colour


7

4.5 C
olour gamut

(Chromatic range)


7

4.6 Perception of motion, refresh rate and CFF


9

4.7

HVS v
ariations

and accessibility


10

5. Display Technologies: th
e state of the art


10

5.1 Discussion of the strengths and
weaknesses of current technologies


11

5.1.1 Discussion of display characteristics relating to HVS


1
2

5.1.2 Discussion of non
-
HVS related display characteristics


1
8

5.2 Projection


an alte
rnative for multi
-
viewer scenerios


19

5.3 CRT


getting thinner and still in the market


19

6. The future of display technologies


the next ten years


2
1

6.1 Specific Trends: FPDs
-

Flattening of the screen


2
1

6.1.1 Field Emission Displays (FED): ca
rbon nanotubes


2
2

6.1.2 Organic light emitting diodes


OLEDs


2
2

6.1.3 Polymer light emitting diodes


PLEDs


2
3

6.1.4 Surface
-
conduction electron
-
emitter display


SEDs


2
4

6.1.5 Some final thoughts on FPDs


2
4

6.2 Specific Trends: Flexible display
s and e
-
paper


2
6

6.3 Specific Trends: High Definition TV (HDTV)

and video


2
7

6.4 Specific Trends: 3D systems


28

6.4.1 How we perceive 3D


2
9

6.4.2 Stereo P
air systems


29

6.4.2.1 Glasses: Liquid C
rystal shutters


29

6.4.2.2 Without glasses: A
utost
ereoscopic 3D


29

6.4.2.2.1 Parallax Barrier


30

6.4.2.2.2 Lenticular systems


30

6.4.2.2.3 Observer or head tracking systems


30

6.4.3 Volumetric 3D systems


31

6.4.4 Holography


3
2

6.5 Specific Trends: Higher resolution

and visualisation technologi
es


3
3

6.5.1 Scalable display walls and Power walls


33

6.5.2 The future of visualisation


34

6.6 Specific Trends: Personal and near
-
to
-
eye displays


3
5

6.6.1 Scanned beam


35

7. The longer term: the future of display technology


36

8. Conclusion

and

recommendations


37

9. References


39

Appendix A: Review of the technology of existing solutions


49

Appendix B: Further notes on projection systems


51

About the author

and reviewers


5
3




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1. Executive Summary


In common with the rest of society, i
ncreasing amounts of time in education are spent
interacting with display screens. Through computer displays, by watching projections or
viewing TV we handle day
-
to
-
day office information, process research, view e
-
learning content
and engage with multimedi
a materials. Display technologies are in a period of flux, driven by
the recent introduction of flat panel displ
ay (FPD) technologies, LCD and
Plasma, which

are
rapidly replacing the traditional cathode ray tube (CRT). This report reviews this flux in the
context of the human visual system and discusses a range of novel forthcoming technologies
that will take the process further.


The human visual system (HVS) is a complex and highly evolved system and, to date, no
display technology has been able to match
its capacity. There is a range of defining
characteristics
of the HVS that the system employs to resolve and make sense of an image
and the report outlines these with particular reference to their impact on the design of display
technology. Having establis
hed this context the report outlines the
current state of the
art,
focus
ing in particular, on the current rapid uptake of LCD and, to a lesser extent, Plasma
screens. The th
ree technologies, CRT, LCD and
Plasma are compared with particular refer
ence
to how

they handle

aspects of the HVS.
In this

respect
,

CRT has tremendous benefits,
particularly when the cost is factored in, but its bulk
, size and weight are

r
apidly consigning it

to history. Despite possessing some drawbacks, such as its relatively limited
viewing angle,
LCD has the biggest commercial momentum and in 2004 replaced CRT as the technology of
choice for computer displays.


Despite thi
s momentum, new technologies

in development
are aimed at replacing

the LCD,
continuing the trend to flatter, mor
e powerful displays.
The report review
s

the
se

forthcoming

developments

and discusses the
ir

implications, again in the context of the factors of the
human visual system. These developments include nanotube
-
based field emission displays and
Organic and Polym
er LEDs.
Large number
s

of researchers and t
echnology companies are
compet
ing to take these developments out of the prototype stage and into commercial
products in the next five years. All of these technologies pro
mise greatly improved capabilities

with reg
ard to HVS factors such as resolution and brightness. Whether they will be able to
compete with the momentum already built up in the manufacturing and supply chain for LCD
is a moot point. Most manufacturers are hedging their bets and awaiting developments

in the
market place.


We then scan the horizon a little further
,

looking at the development and implications of more
novel technologies such as 3D displays, near
-
to
-
eye systems, electronic paper,
rollable/
flexible
screens and
increased immersion through r
oom
-
sized display walls. Although these
developments are at least several years away from general uptake within the education
market
,

some of them may have profound implications for the content and setting in which
education is delivered and
the
context an
d process in which research
is
undertaken. They can
be seen in the context of new developments in classroom and laboratory design and part of a
wider story of a move to mobile and blended learning and new forms of collaborative, remote
and multi
-
site

resea
rch visualization.


All these developments will offer new opportunities for the development of content for display.
In fact it is likely that the generation of new content, for example High Definition TV and
video, will be the key driver to the adoption of

new display screens. This will be a key
challenge for education, as

young people in particular will

become use
d

to being engaged with
high
-
end multimedia, high def
inition and, perhaps

even, 3D content.


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2
. Introduction


Despite periods of huge technologi
cal change in the computing industry we remain wedded to
the electronic display screen as the principal channel for the transfer of information from
machine to person (Wisnieff

and Ritsko
, 2000). In the modern educational setting considerable
amounts of ti
me in learning, research and administration is 'screen' time, devoted to
absorbing and manipulating display
-
mediated information. For this reason the functionality,
design and technology of display systems profoundly affects our everyday working lives. Wit
h
this in mi
n
d
,

this repor
t will review the current state of the
art with regard to displays and look
at some of the forthcoming developments and trends.


Until relatively recently the TV and computer display have b
een largely static technologies

based on
the cathode ray tube, an underlying platform technology almost unchanged since the
1920s. However, in the last decade, we have witnessed a dramatic change as we have moved
to flat screens, based on new Liquid Crystal Display (LCD) and Plasma technologies.
This
report will show that this trend will continue and, in fact, we are only at the beginning of a
period of huge
flux

in display technologies, and that these developments
are likely to
have
a
profound effect on education. As designers and engineers conti
nue to strive to match their
technologies' capabilities to those of the human visual system, they will introduce a range of
new platforms and novel materials. We will see flatter, lighter displays with much higher
resolutions. We will see new, flexible mat
erials being employed to allow for larger screens
which will be incorporated into novel display interfaces and surfaces. We'll see a move into 3D
content and we may

also see displays

disappear altogether with the introduction of near
-
to
-
eye technologies th
at scan light directly into our retinas. It may read like science fiction, but
within a few years we could be seeing the disappearance of the computer display monitor
altogether.



3
. Why is this relevant to education?


In common with the rest of society,

increasing amounts of time in education are spent
interacting with day
-
to
-
day office information, e
-
learning content and multimedia materials
through either computer displays, by watching projections or viewing TV. In learning and
teaching this has been f
ortified by the trend towards 'blended learning' incorporating a rich
mixture of paper
-
based, lecture, online material and interactive multimedia (Newby, 2004).
These developments mean that within education we are becoming much more aware of the
quality of

the screens we are viewing and how that affects the work we undertake.


There are also some special drivers in e
ducation and research

at the moment with respect to
displays:




E
-
science dataset visualisation: t
he ever larger datasets
created

by scientific

discovery

are presenting new problems in visualisation
and
are pushing the requirements for
displays
'

size and resolution (see section
6
.5)

(White Rose Grid, 2003)
.



Collaborative work: increasingly, groups of researchers are
required to work on such
datas
ets simultaneously (Cameron, 2005)



Training: some professions will need to increasingly incorporate training techniques
using advanced displays such as simulators, VR and augmented reality. For example,
medical training for remote surgery (Brodlie, 2004)



M
obile learning and research: developments in handheld devices, smartphones and
electronic books will all make use of advanced display technologies such as electronic
paper and near
-
to
-
eye systems (
National Research Council, 2002;
Wagner, 2005)



Engagement:
expectations over the quality and perceptual experience gained from
viewing educational materials through displays will be partly driven, particularly
amongst younger students, by the increasingly high quality of visual experience
obtained through home TV
and video gaming.



Classroom design: new types of display surfaces will be a significant feature in
restructured classroom and learning spaces that emphasise flexibility, collaboration
and multi
-
site working (Long and Ehrmann, 2005)

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4
. Conveying informati
on: The human eye and factors in display systems


‘The human visual system evolved over millions of years. The disparity between the
natural world 'display' and that 'sampled' by year 2000 technology is more than a
factor of one million. The grand challeng
e for display technology is to close this
fantastic 10
6

gap between devices and the human visual system.’

(Hopper, 1999)


We are a particularly visually
-
oriented species and have evolved a highly complex human
visual system (HVS) based on over 100 million
light
-
detecting photoreceptors (IST, 2004).
What we 'see' when we view a display, or anything else, is the response of the eye (by the
photoreceptors) to the detection of electromagnetic radiation in the form of small packets of
energy (photons) from the v
isible part of the electromagnetic spectrum
1
. In a sense
,

the eye
is a radiant energy detector. This is the medium through which display devices must convey
information
,

and aspects of the HVS and its operation have profound implications for the
methods us
ed by displays and their operational features (such as resolution, contrast,
brightness etc.). The context and environment in which the HVS and display device are
operating is also im
portant, for example, the level of

ambient light available. The human
vis
ual system is highly evolved, complex, able to function across a wide range of
environments, and offers superior capabilities compared to existing forms of information
display technology. The grand goal of displays research and development is therefore to
try to
match the information output of the display to the input and processing capacity of the human
visual system (Wisnieff

and Ritsko,

2003), allowing, for
exam
ple,

the ease of readability
afforded by paper (Dillon, 2004). Therefore, before we embark on
a discussion of new display
technology developments and trends it is important to place the key factors at work in the
context of the human visual system and, in the following sections, we outline these factors
and their interplay with display systems.


4.
1
Brightness and contrast


The
brightness

of a viewed object is defined in a psychological sense as the level of light
in
tensity perceived by the viewer. T
his is directly related to
both
the physical quantity or
intensity of the light involved (either emit
ted or reflected)

and

the section

of the dynamic

range the
absolute
ambient brightness falls

in
to
. There are

various related photometric
2

measurements for such brightness, but the key physical measure of brightness is luminance or
luminosity, the amount of

light emitted, or reflected, from a source in a given direction,
measured in candela per square metre (cd/m
2
). As an indication of the range of HVS
perception, moonlight exhibits around 0.1 cd/m
2
, indoor lighting is around 100 cd/m
2
, and
sunlight is

aroun
d 10,000

cd/m
2
. In terms of displays, brightness is formally defined as the
luminance of white colour in the centre of the screen and is measured in candela per square
metre (cd/m
2
) or 'nits'. For display technologies the other important consideration is t
he
dynamic range
, that is, the ratio between the maximum and minimum intensities that can be
generated.


In order to make 'sense' of an image, the HVS uses patterns presented through the
comparative, differing luminance levels within an image (the contras
t). The information that
can be conveyed to a user through a display technology is fundamentally limited by the
human ability to discern such contrast, and the ability to display contrast is therefore a key
factor in the design of display technologies. The

eye's ability to discern contrast is known as
'contrast sensitivity' and is a measure of how faded or 'greyed
-
out' an image can be before it
becomes indistinguishable from a uniform background. Higher contrast is required to detect or



1

Technically, visible light consists of the spectrum with wavelengths from 380nm (blue light)
and 730nm (red light).

2

Photometry is the measurement and specification of light relating specifically to its effect on
human vi
sion

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'resolve' smaller ob
jects and this resolution limit is related to contrast and to visual acuity (see
below) (Woods and Wood, 1995). The minimum contrast sensitivity the HVS can detect is an
intensity difference between two patches where the ratio of their intensities differs
by at least
about one percent (Poynton, 1998).


4.2
Visual
Acuity,

information

content and resolution


The amount of

visual

'
information
'

that a display is able to convey is a very important factor in
display design. This is related to the
'
information con
tent
'

of a display
(
which is defined as the
total number of pixels,
the size of the pixels (resolution) and
the size of
the display
)

and the
eye
's ability to discern detail
-

our
visual acuity

(Wisnieff and Ritsko, 2000).

The large number
and density of ph
otoreceptors present in the HVS means we have a considerable ability to
discern detail at a distance and this is affected by the degree of luminance and the contrast
present in a scene or image and hence these are particularly important factors in display
design (Kolb, 2005).



Figure
1
: Visual acuity of human eye is 1/60
th

of a degree

of the visual context


The acuity of the average human eye can resolve an individual pixel of
approximately
1/60th
of a
degree (or one arc minute) of the visual context wide (what is known as the ocular
resolution)

(figure 1)
. This is what an optician

is measuring when you read the letters on a
chart at the far end of the

room (the Snellen chart).
Visual acuity determines t
he level of
detail
that the eye can absorb from the
pattern of pixels
present
on a screen
.

The nearer you
are

to the object being viewed
,

the smaller the level of detail that can be determined.
This
means that the distance of the user from a TV or computer

display is an important factor in
determining resolution requirements. As a
simplified

example, t
o match the levels provided by
t
he HVS
, a typical display of 20 inches, when

viewed at 24 inches, would need to offer 221
4 x
1664 pixels or around 3.5 m
ega
pix
els (Brown University, 2002).

Such a measure is somewhat
artificial since users in the real world are not likely to stay fixed at exactly 24

inches and

it is
generally accepted within the displays industry that the key measure for acuity is the pixels
per
inch (ppi) of a display
,

where

200 ppi is a good approximation of the HVS requirement
within a computer display environment.


4.3
Field of view and angle of viewing


Most of the photoreceptors in the eye are distributed around a central point, known as the

fovea

and for this reason the bulk of our visual information is derived from a small portion of
our total field
-
of
-
view (Lantz and Spitz, 1997).
3

The eye compensates for this by constantly



3

Most of the receptors in the fovea (known as cones) are geared to colour vision, whereas the
peripheral receptors are known as rods and are geared towards processing grey
-
scale
information.

Angle α = 1/60
th

degree

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darting around to bring objects of interest into the foveal area.
In fact a human can typically
see 155 degrees vertically and 185 horizontally (Rheingold, 1991).


In d
isplay technologies this is

a consideration
for

the position of the viewer and the angle of
viewing. In general, a viewer should be positioned so that the

display is placed centrally, in
front of the eyes. However, this is not always possible, for example, in a tutorial group with a

number of people viewing a single screen. In these cases
, the display's capacity

to offer a
wider
'
angle of view
'

with similar

levels of resolution, brightness and contrast becomes
important.
The

angle
of view
is formally defined as the angle at which the viewer must be
posit
ioned in relation to the screen

in order to clearly see the image on a display (zero degrees
being directl
y in front, 180 degrees directly to the left or right hand side). Th
e LCD industry
takes this one step further by saying that it is also the point at

which the brightness is halved
for the viewer.
It is worth noting that some display technologies may offer

a theoretical
viewing angle of 180 degrees but in actual fact this would involve viewing the screen
tangentially across the surface of the display
,

which is not
physically
possible.


4.4
Perception of colour


The science of colour and its perception by th
e HVS is complex and draws on many disciplines
such as physics, colorimetry, physiology and psychology (Foley, 1990). In physical terms the
colour of an object is complex, but in the main it is a function of the different wavelengths of
light that an objec
t emits or reflects. A colour can be fully specified in terms of three
perceptual properties: hue, brightness/lightness and saturation, which in turn are related to
underlying physical, colorimetric properties such as the dominant wavelength as below:




Hue
: related to dominant wavelength of the light



Brightness/lightness: related to the brightness (i.e. perceived intensity), which is
related to the luminosity of the colour. If an object is reflecting light then we use the
term lightness,
but
if an object is

emitting light (as is the case with a CRT
display)
then we call it brightness



Saturation (or 'chroma'): related to the relative strength or paleness of a colour. This
is related to the spread of the wavelengths involved or, in simpler terms, the amount
of

mixing of white light with the hue (also known as the
excitation purity
). A highly
saturated colour will consist of a very narrow range of wavelengths centred around its
hue.


The average human can perceive hundreds of thousands of different colours and c
an perceive
128 fully saturated colour hues (Foley, 1990).
Humans perceive d
ifferent wavelengths o
f light
as

different colours,
being able to differentiate wavelengths of around 3nm
,
although there are
complex physiological and psychological processes at p
lay so that one person's experience of a
shade of blue may not actually be the same as another

person
's. The HVS is able to perceive
this through the use of a type of photoreceptor known as a
'
cone
'
. In simplified terms, the
HVS uses a trichromatic process

in which there are three types of cone (blue, green, red),
each of which detects a different range of wavelengths of light
4
.

The red cone detects light
around 564nm, green at 533nm and blue at 437nm.


4.5
C
olour gamut

(chromatic range)


Colour can help c
onvey a tremendous amoun
t of information in an image so

a displa
y's
capacity

to generate colour is important. Colour monitors
emit or reflect light of varying
strength for each of three primary colours


red, green and blue


and then mix them to
generate
a colour image
. Colour gamut is

the measure of a monitor's ability to generate and
display such a range of colours. The gamut possible from a particular display is dependent on
the mechanism used to generate colour and is bounded by the degree of saturatio
n possible
for each of the three primary colours (Wisnieff

and Ritsko
, 2000).




4

See:
http://webvision.med.utah.edu/kallcolor.html

for

a

fuller discussion of
the models used
for

HVS colour vision
.

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The 1931 Commission Internationale de l'Eclairage (CIE) developed a three dimensional colour
'space' that allows any visible colour to be mapped. It uses three, standard
ised

col
ours that
are derived from red, green and blue, but

which are represented

by x, y, and z. This colour
space is bounded by the purest colours i.e. those which consist solely of a dominant
wavelength. Any colour can be located within the colour space and its

composition from each
of the three primaries can also be determined.


The 1976 Uniform Chromatic Space (UCS) standard created a two dimensional derivative of
the CIE standard, which uses u’, v’ to plot colour gamut

(figure 2). The detail of this is not
re
levant here (interested readers are pointed to Foley (1990, page 408), but the key point, as
we see in figure 3, is that if we plot the colours that can be ge
nerated by various displays

onto
the UCS

diagram
, we can see that the colour gamut provided by dis
plays is much smaller than
the range the HVS can process. This means that images displayed on a screen will always
seem less rich than they appear in real life.




Figure
2
:
CIE 1976 UCS Chromaticity Diagram.
Source: Joe Kane Prod
uctions, California.


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Figure
3
:
UCS Diagram showing chromaticity coordinates for different display technologies
.
Source: DisplayMate Technologies Corp.

(www.displaymate.com
)

(In
:

Soneira, 2005).


However, t
here is a further compl
ication with colour in that the background and ambient
lighting further alter the colours perceived on a display.
Colours from the ambient light will
reflect off the front of the display and supplement the colours being transmitted.
This is known
as the
me
tameric effect
.


4.6
Perception of motion, refresh rate and

CFF


The time required for the HVS to detect and process an object and its motion within a scene is
around 120 milliseconds (Burr and Morgan, 1997). The ability to see motion is also directly
affe
c
ted by human visual persistence:

the amount of time taken for the image on the retina to
decay gradually. The HVS can take in about 20 different images per second before they begin
to blur together
-

the effect that is used when showing a film or video, a
s the individual frames
blur to form a smooth image.


Refresh rate is the frequency at which a CRT display redraws the image on the screen (the
raster scan, see appendix A).

If the refresh rate is too slow the human eye will be able to
discern breaks betw
een the pictures and will perceive this as a feeling of ‘flicker’.


The critical fusion frequency (CFF) is the point at which

the

eye stops seeing the individual
picture
s as they are refreshed and

‘fuse
s
’ it into a single image. The CFF is affected by sev
eral
factors including the refresh rate, ambient light levels, brightness of the display and angle of
viewing (Sidebottom, 1997). It is also dependent on the individual, varying from person to
person. For comfort, a display should be running at least 72Hz
(Dillon, 2004) and, for at least
95% of a given population to be able to view flicker
-
free, it should ideally run at 95Hz (Bauer,
1983).




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4.7

HVS Variations and accessibility


Obviously, as with all human systems, the HVS has many variations and there ar
e many
issues around accessibility for people with different visual levels and problems. Visual acuity,
for example, is one of the factors that varies widely (even between an individual's two eyes)
and can be severely affected in people with visual problem
s.
Colour vision is anot
her major
variable, with, for
example, 10% of the male population being colour deficient.
The display
technology used can also have an impact on people with specific visual impairments or
medical problems, for example, CRTs can be a

problem for those with visual sensitivities such
as epilepsy because of their propensity to flicker (JISC TechDis, 2005). For those who rely on
screen magnification to improve their visual abilities the increasing availability of cost
-
effective, larger di
splays can increase their ability to work and study with screen
-
based
information (Neumann, 2004).



5
. Display Technologies: the state of the art


We have seen that several key factors in a display technology are determined by the need to
match the techno
logy to the information capacity of the human visual system (HVS) and that
these factors include brightness, contrast, resolution and colour gamut.
In general, the
development of display

technologie
s
is characterised by continued att
empts
to match the
capa
bilities of the HVS
and one would expect the state of the art to reflect this. However, as
with any product
,

cost and
the
general 'look' and design are important

factors,

and it is the
latter that has had by far the most bearing on the re
cent history of di
splays due to the desire
for flatter, thinner displays
.


The current state of the
art is thus characterised by the recent and continuing rapid evolution
from bulky cathode ray tube (CRT) displays to flatter screens known as Flat Panel Displays
(FPD) based
around two newer technologies: Liquid Crystal Display (LCD) and Plasma. The
key to understanding the development of the market to date is to appreciate the different
drivers operating in the two key market segments: computer displays and TVs. In the former
,

a key determinant was the requirement for space
-
saving, lower power
usage
and port
ability.
In the latter, the desire for space
-
saving and

general attract
iveness
,

and the need for larger,
brighter
screens capable of handling new, higher definition content
, were important
. The CRT
is inherently bulky as its operation requires a relatively large space for the vacuum tube
,

and
FPD
s

have challenged CRT

displays

in both market segments by exploiting this key weakness.

Initially, LCD, which is not only thin but
also lightweight, was seen as the only viable
technology for mobile computing platforms such as the laptop and mobile phone.

Simple LCD
displays have been available since the 1970s, but
with the
introduction in the Sha
rp PC
-
7000
in 1985, LCD has

made stead
y and increasing in
-
roads into the
general computer display
market.

I
n 20
04, for the first time, the number of LCD sales was higher than equivalent CRT
displays for computer monitors (Cranton, 2005).


Plasma has taken a rockier road since it was first inve
nted in 1964 at the University of Illinois,
USA. Its original inventors were driven by the desire to develop a better display screen for
educational use, which could present high quality graphics and media (although Hutchinson
(2003) shows that initially,
the main driver during the late 1960s/70s was from the Japanese
who needed to develop an alternative display to CRT that could reliably reproduce kanji
script). The large investment required for the mass
-
production of Plasma proved a difficulty
and increas
ing competition from LCD meant that most of this work had been abandoned by
the end of the 1980s
,
although there was still

interest for

military
applications
.
However, t
he
forthcoming introduction of High De
finition TV (HDTV)
was the
driver to st
art the
co
mmercial

redevelopment of Plasma

in the 199
0s,

since t
he clarity

of HDTV can

not be fully appreciated
on CRT TVs and, at that time, LCD could not be manufactured with a large enough screen size.
Since then, developments in
the
manufacturing of LCD have all
owed larger size TVs to be
manufactured and this has led to increasing competition between the two FPD technologies in
the TV market, particu
larly in the size range of 32 to
42 inches (Putman, 2005).


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Before we discuss the various pros and cons of these te
chnologies, it is worth reviewing briefly
the technologies involved in current displays. Appendix A contains a more detailed outline of
each of the three, but the following table (table 1) summarises:



Table
1
: Summary of the three

current display technologies

Display

Type

Main Uses

Mechanism

Notes

LCD

Passive:

Transmissive


o
r

Passive:

Reflective



or

Passive:

Transflective

Mobile phone

Laptop screens

Computer displays

TVs up to 42"


Digital watches and
calculators



Mobile/laptop
s

Liquid crystal light gates
control
transmission of a
backlight through polarised
light filters


Liquid crystal light gates
control reflection of ambient
light
through polarised light
filters

A mixture of both


Plasma

Emissive

Large TVs

Public informatio
n
displays

Cells of neon gas are ionised
by high voltage to release UV
pho
tons which hit a phosphor
screen

Can only be
manufactured in
large sizes
.

CRT

Emissive

Traditional
TV

Traditional computer
monitor

Electrons generated by
thermionic emission by high

voltage at a cathode are
swept by magnets into a
focused beam which strikes a
phosphor screen

Sweeps in a
raster pattern,
right to left and
up to down
across screen



has implications
for flicker.



Passive

versus

emissive displays


The above table ca
teg
orises displays as emissive or passive
. An emissive display is one that
produces its own
light;

a
passive display
modulates light that passes through it
. LCDs
are
passive and
can be
further categorised into
transmissive

(
allows

the passage of photons from
a backlight
),
reflective

(simply reflects external, ambient light) or
transflective

(a mixture of
both)
.

The standard LCD thin film transistor (TFT) display on a laptop computer is
transmissive, whereas a calculator would use a reflective liquid crystal. A
dvantages and
disadvantages are argued

for these two key properties. F
or example, it is argued by some that
emissive displays are not as easy to read in direct daylight,
as
for example, the
transmissive
LCD TFT display on a laptop. As mentioned in
section
4

the context and environment in which
a display is to be used is important: see for example, a discussion on the use of displays in
mobile phone technology (Kimmel, 2002) and for one manufacturer's viewpoint on the
benefits of r
eflective and
emissive disp
lays
,

see

Eink (2005).



5
.1 Discussion of strengths and weaknesses of current technologies


Before moving

on to a discussion of

the future trends in display technologies it is useful to
review the situation with regard to the strengths and weaknesses of t
he classes of product in
the current market place, particularly in the context of our previous discussion of the HVS.
Table 2
, at the end of the discussion,

outlines a series of characteristics of the HVS and
identifies the display technology factors that
are of relevance. For each characteristic the table
outlines the relative position and strengths/weaknesses of each class of product.

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5.1.1
Discussion of display characteristics relating to HVS


Brightness


In display terms, brightness is formally defined

as the luminosity or luminance of white colour
in the centre of the screen and is measured in candela per square metre (cd/m
2
) or
'
nits
'

(Da
-
Lite,
1995). The brightest,
commercially available, “off the shelf”
5
,

displays are Plasma
-
based,
which offer outst
anding luminance in the range 500 to 1,200 cd/m
2
. In comparison, CRTs
typically offer an operation
al brightness of around 90 to 15
0 cd/m
2
.
Commercial
LCDs offer a
mid
-
range, with typical values of 400 to 500 cd/m
2

being recently quoted by commentators
(Wha
t Plasma, 2005). Generally, brightness levels of 150
-
250 cd/m
2

are considered suitable
for typical office work (Wisnieff

and Ritsko
, 2000) whilst public displays and TVs benefit from
higher levels.

CRT’s inability to deliver high levels of brightness is p
articularly an issue when
strong ambient light is present (for example, in the case of a screen
used at a bank's outdoor

cash point machine) (Hopper, 1999).


Dynamic Range and Contrast


Dynamic range

is the luminance ratio of pure white to pure black, agai
n as measured in the
centre of the screen, but not in same image (Foley, 1990). The ratio is driven by the
maximum luminance intensity levels (white brightness) that can be generated and by the
ability to deliver a 'pure' black.
Contrast

(or
contrast ratio
) is very closely related to dynamic
range, being the maximum ratio of white to black luminance that can be generated in the
same

image. These two figures differ because when measured in the same image there may
be internal reflection from the light areas
of the picture across to the dark areas caused by the
glass of the screen. It is not always clear which type of f
igure a manufacturer is
quoting and it

is worth bearing in mind
that such formal measurements bear little relation to the contrast
levels avail
able from a display in a real office environment where ambient light and reflection
from the screen will significantly affect the values.



A display may not be able to deliver a pure black because the technology leaks reflected light
(in the case of LCD)
or reflects ambient light. A poor ability to generate black will also have a
knock
-
on effect throughout the colour range in the darker colours. As an example of the
different levels, when displaying black a typical CRT still emits light of luminance of 0.0
1
cd/m
2
, LCD of 0.72 cd/m
2

and Plasma of 0.42 cd/m
2

(Soneira, 2005).
A good CRT monitor will
offer a contrast that exceeds 1000:1, and will be close to displaying a pure black (i.e. the best
dyna
mic range)
. However, d
espite not being able to display the pu
rest black, Plasma displays
exhibit the best contrast, offering a 1000
-
3000:1 ratio, due to their high brightness
capabilities.
N.B.
The
importance

of dynamic range is also partly governed by the type of
content a display is designed for (for example, movi
e makers prefer a lower dynamic range
with blacker blacks and more grey shades).


Compared to the other technologies, LCD suffers from contrast problems
,
although
as a
comparator
this is somewhat mitigated by the general loss of contrast all displays
types

suffer
in ambient light situations
.

The pixels of an LCD display operate like light 'gates' and these
gates are not perfect whe
n turning
the light on and off,

and LCD monitors currently on the
market typically have a ratio of only 400

600:1.
To counter th
is
, LCDs are being produced
with additional white sub
-
pixels which increase the luminance that can be delivered (see
Colour Gamut section, below).

By way of a comparison, the contrast ratio of a photograph is
100:1 and a photographic slide is 1000:1.


Fove
al field of view (Viewing angle)


This is the angle at which th
e viewer must be positioned in relation

to the screen, in order to
clearly see the image on a display (
where zero degrees is

directly in front

and

180 degrees
is



5

Limited edition LCD display units for public, outdoor display can reach 6,000+ cd/m
2

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directly to the left or right h
and side). In the LCD display industry this angle is more precisely
defined as the angle at which the brightness is halved for the viewer.


In general, LCD screens suffer from a relatively small viewing angle compared to other
technologies, as the light mu
st pass through a series of materials
before reaching the viewer
(NEC, 2005). This means

that

if the viewer is si
tuated to one side of a display

he/she will
experience a loss of brightness, contrast and colour

(particularly with respect to the vertical
asp
ect)
. Since first introducing the technology, LCD manufacturers have put considerable
engineering effort into finding solutions to this problem, adding, for example, light dispersion
films to the front of a display (Funada and
Hijikigawa,

1997
). Manufactur
ers now claim LCD
viewing angles of 150 degrees in

the horizontal (and even 170 degrees

for more expensive
proprietary technologies
)
, but some industry commentators remain unconvinced, with, for
example, What Plasma & LCD TV magazine (2005b) recently maint
aining that on viewing
angle, "Our experience suggests that these figures are very optimistic." In a recent Which?
report (2005) several leading LCD computer displays were reported to have achieved a
horizontal angle of only 140 degrees.


In educational s
ettings this restriction is most likely to matter when multi
-
person viewing is
taking place in, say, a small tutorial group or a lecture theatre. Plasma and CRT displays do
not suffer from this restriction, with the former allowing angles up to 160 degrees

and the
latter up to 180 degrees.

However,
t
o
a
certain extent these maximum figures are not
particularly helpful

in real life (nobody can actually see a screen set at 180 degrees to their
eyes) and what is important for educational use is what level of d
istortion of text and graphics
there is at a particular angle and how this would affect a viewing audience.


Colour Gamut


As we have seen in the previous section on the HVS, the ability to display a range of colours is
kno
wn as the colour gamut.
Figure 3

illustrated the different technologies and their range in
relation to the colour space which the HVS can perceive. No display technology can, to date,
render the entire range of colours that the HVS can perceive.


The degree

of saturation (or strength) of

each of the primary colours is determined by the
type of
technology used in the display

and
to a certain extent the quality (and cost) of
materials involved
. T
he actual number of colours that can be displayed is driven by the
number of different luminance

levels (or
'
grey
-
scale
'
6
) a device can generate. A typical LCD
computer monitor can generate an eight
-
bit luminance level, providing 256 different levels.
Conventional LCDs have
sub
-
pixel
s for
the
three primary colo
urs


red, green and blue

(RGB),
but s
om
e manufacturers are
now
adding a fourth primary
-

a
white
sub
-
pixel
. This increases
brightness and
facilitates

a
n

enhanced display of non
-
saturated colours since a white pixel is
essentially a
completely
transparent filter which allows the backlight’s ligh
t to pa
ss through
unhindered (Mokhoff, 2003)
.


It is worth noting that, for historical reasons, many display manufacturers give figures for

their
displays based on the percentage of colours used by

the US National TV Standards
Committee's (NTSC) colour spa
ce within the standard CIE. For example, a CRT display can
del
iver perhaps 65
-
75% of
the NTSC standard (Goodart, 2003).
However, it should be noted
that colour gamut is

quite a complex area, with a number of standards

in existence (for
example, ICM, ICC, s
RGB). See
Stone
(
2001) for more information.


In general, Plasma offers the widest gamut and CRTs are able to provide a wider colour gamut
than LCD. Plasma and CRT also offer a more consistent illumination level across the screen,
which helps when colour
calibration is undertaken.




6

The grey
-
scale is a
logarithmic

scale of the l
uminance a display can deliver

for a given input signal e.g.
f
or an 8
-
bit scale, 0 is black and 255 is white,
giving 256 points in the scale (
Foley
, 1990)
. The eye is
sensitive to ratios of intensity levels and so the scale is spaced logarithmically rathe
r than linearly to
achieve perceived equal steps in brightness. The scale is stepped based on a power
-
law function, with an
exponent of
gamma.
Standard gamma for a CRT is 2.
5

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Visual acuity (resolution)


A screen's resolution determines the amount of visual detail that can be displayed
and can be

expressed using
a number of factors. Commonly
,

the resolution is expressed as
the number of
pixels in the

horizontal and vertical planes

(or in the case of a CRT the number of lines)
.
The
upper limit

on the
resolution of

a given display
is obviously determined by
the
size of the
screen and the number of pixels that
are
displayed in a given area


the

pixel de
nsity



which
is me
asured in pixels per inch (ppi))
.
Clearly the pixel density is also
related to "dot pitch" or
"pixel pitch", which is the size, expressed in millimetres, of each individual screen pixel.
Different technologies offer different physical pi
xel sizes and therefore offer a range of
resolutions a
t a range of screen sizes. A 42
-
inch

Plasma TV might offer the same
maximum
resolution

(HDTV) as a 17
-
inch LCD TV, but will have a larger pixel pitch
.


As an indication, for computer displays, a

typica
l 15
-
inch extended graphics array (XGA)
display
of
1024

×768

pixels
ha
s a pixel density of 85

ppi, and
a U
XGA
of 1600 x 1200
has a
density of 133

ppi.
T
he pixel density
factor is particularly important in small displays

for
phones and handhelds as they
hav
e a limited diagonal screen size and are viewed at short
distances.
In these

use

cases

pixel pitches of
300 ppi

are
becoming available
.
In general
terms, the smaller this pitch, the sharper
the image displayed (NEC, 2005)
. See (Dell, 2003)
for a table of t
ypical sizes of pixel pitch from a leading computer manufacturer.


LCD and Plasma displays have a fixed
,

maximum number of pixels that th
ey can display (since
there is
a fixed number of cells of plasma or liquid crystal light gates) and this is known as t
he
native

or
natural

level of resolution. Such displays are best operated at this natural resolution,
otherwise an image must be scaled up using interpolation algorithms (NEC,

1999) and such
scaling can degrade the quality of the image.
As a general point,

many LCD
s

in common office
and educational usage have not been configured correctly and
are
being used at their non
-
native resolution.


A CRT's resolution is a bit more complex, involving a number of factors such as the dot pitch
of the phosphor, focus o
f the electron beam an
d the scanning frequency rate.
A CRT has a
preferred resolution, rather than a native one, and can scale up to a maximum resolution
whic
h is typically, for a
computer monitor, 1280 by 1024 or higher.
It can be argued that
CRT
displays

therefore offer more flexibility over display resolution rates (Goodart
,

2003).
However, although there are no fixed physical pixels or sub
-
cells, it is unusual for a CRT to
have a phosphor of dot pitch
less than 0.25

mm which equates to around 100

ppi
,

a
nd this
relatively low and bounded level of resolution is one of the limitations of a CRT (a typical LCD
laptop will have, say, 142 ppi).



How do these resolution rates compare with the HVS, or for that matter with film and
photography? In typical compute
r display work t
he viewer is normally around 12 to
24 inches
or so in front of the screen. The following diagram
(
figure 4
),

courtesy of IBM, maps va
rious
display systems

based on their pixel count and their image size (diagonal size of the screen)
and com
pares these systems with the limits of the human visual system (Alt

and Noda
, 1998).
The area that is shaded represents that served by conventional dis
play technologies such as
CRT, P
lasma and LCD screens (these are shown as blue diamonds, red circles,
and

black
squares respectively). The chart also shows alternative conventional systems such as
photographs, professional 35mm film etc
.

for comparison (green triangles). The red lines show
the limiting resolution of the HVS at viewing distances of 12, 18 and
24 inches based on ocular
resolution. Anything above the red line provides more information than the HVS can absorb
through visual acuity. The key point of this diagram is that conventional display systems lie
below the red line and are therefore a) not pr
oviding as much detail as the HVS could cope
with and
are
therefore conveying less information than they could,

and b) create

a display in
which people can actually perceive the individual pixels.


This diagram shows that with conventional technology the v
ast majority of display systems are
operating below the capacity level of human visual system (as opposed to conventional film or
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laser
-
printed paper text). At such levels our visual system can discern individual lines and
pixels and th
is affects readabili
ty compared

to
,

say, paper.



Figure
4
:
Display content versus display diagonal.
Source: IBM Research and Development Journal,
2000. (In: Wisnieff and Ritsko)


Motion and response
-
time


The time an individual pixel or cell in a di
splay screen takes to change from white to black is
known as the
response time

and is measured in milliseconds (ms). The response time affects
the ability to change an image rapidly on the screen, for example, during movement in a
sports programme, a fast
action video or a computer game. If the response time is not
adequate the HVS will detect a blurring or 'ghosting' in the image. It is generally thought by
commentators that
the minimum

response times for
different
types of content are as follows
(CNet, 20
05):


25ms for general computer applications

12
-
15ms for TV, sports and gaming

16ms for 30 frames
-
per
-
second DVD video


LCD has an inherent latency time due to the switching of the liquid crystal gates and this
introduces a longer response time than is
req
uired by

some content types. There is a further
complication in that dark images with
less contrast can also produce

longer response times
due to the lower intensity of the applied electric field required to generate darker colours. For
this reason many ma
nufacturers now quote
figures for the average time of

transition betwee
n
sets of randomly grey levels,
referred to a
s 'grey
-
to
-
grey' response times

(Monckton, 200
5).
This is not an issue for Plasma

or CRT,
which have response times

of
around
one to three m
s.


Again, as with the viewing angle, LCD manufacturers have attempted to engineer solutions to
the response time problem and LCD TVs and monitors are
be
coming
available

(
in mid 2005
)

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with typical response times of 16ms, with some offering times as low as

5ms (What Plasma,
2005c). A recent Which?
(2005)
computer report stated: "Today, LCD response times are so
fast that yo
u won't notice much difference [compared to CRT]

unless you play high
-
resolution
games or watch action packed DVDs onscreen."
In their o
wn tests (as opposed to
manufacturer's figures) Which?

f
ound

that
the longest response time of
a standard LCD
monitor was 17ms, whilst

some were a
s low as

7ms.


LCDs have another issue with regard to the display of motion. Not only do

the

individual
pixel
s
have

a response time
,

but
they
can

also exhibit

‘sample and hold’ when displaying motion.
This occurs because each horizontal line of pixels in the display is set
, in turn,

to the

image
and then held whilst the display sets the other lines. Only once
all

the lines of pixels have

been set can the process start

again, with the next frame of the image. This means the next
frame’s worth of information cannot start to be displayed until all the pixels have been set.
Unfortunately, if motion has been present in

the image being displayed then the eye will be
expecting a certain ‘flow’ of such movement and
will
already
have started

some pre
-
processing in anticipation. When the next frame is drawn the movement shown may not
concur with the expectation of the eye an
d will present visual affects known as
artefacts
.
Some, but not all, people can detect such artefacts on the screen. In order to avoid this
problem entirely a
n

LCD would need to run at around 400Hz



at least four times

the

typical
current rates.


Refresh
rate, response time
and CFF


Flicker is only considered a
serious
problem with CRT
displays
and is a
consequence

of the
need to repeatedly
refresh
the phosphor

dots on the screen (as their light decays rapidly).
This refresh
rate
is
typical
ly

75

Hz or 85 H
z, slightly less than that
what is really needed to

ensure that the

vast majority of people do not see flicker (see section 4.6)
.
It is less of an
issue with LCD
-
based displays as they do not have
such a cyclical refresh

process
,

but
flicker
can be a probl
em if the rate at which the image itself is changing (the frame rate) happens to
match the
LCD
backlight’s drive frequency.
PDP displays
a
re able to use a

different scanning
refresh process

(
as the phosphors do not
fade as quickly
), and therefore

they

have

little issue
with flicker.
















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Table
2
: HVS
-
related issues with respect to current display technologies (Dupont, 2005), (Mann 2004)

HVS Factor

Importance

Display
Characteristic

CRT

LCD

PDP

Comments

Brightness

Affects v
isual
acuity

luminance (cd/m2)

100 cd/m
2

400
-
6
00 cd/m
2

500
-
1200cd/m
2

No display anywhere near to
matching HVS
.

Plasma has s
ignificant
advantage
.


Contrast

(incl.
dynamic range)

Affects visual
acuity

Contrast or contrast
ratio (white to black)

1000:1


(Bla
ck:
0.01 cd/m
2
)

100
-
600:1


(black: 0.7 cd.m
2
)

1000
-
3000:1


(black: 0.42
cd.m
2
)

Plasma

offers excellent
contrast
, but CRT offers better
dynamic range.

LCD has significant
disadvantage.


F
ield of view

Could affect
number of people
viewing
simultaneously

Vi
ewing angle limits
(
absolute
theoretical
maximum is
180
degrees
)

180


140
-
150


160


Displays close to matching HVS,
but note that 180


is not really
possible. Generally viewed as

a
n issue for LCD (see discussion)
.


Colour gamut

Range affects
pal
et
te of

co
lours

Satur
ation level of
primary colours and

grey
-
scale levels

wide

(
65
-
70%
NTSC
)


wide

(
65
-
75% NTSC
)



widest gamut,
(>75%

NTSC
)



Displays not able to match
HVS
.


Visual acuity

Determining
factor in amount
of information
conveyed

Resolution (number
of

pixels or lines)
.






P
ixel
pitch (distance
between pixels
-

affects

'
sharpness
')

720 x 576
(PAL)







pixel pitch:
< 0.28mm

XGA (1024 x 768)
is typical

in
monitors

(M
ax: 3840x2400
)


HD
TV
: 1920 x
1080



pixel pitch: 0.2


0.5mm
.
Highest
ppi:
403
ppi


So
me at 1
024

x

768, newer ones
at HD
TV

rate of

1920 x 1080





pixel pitch:
0.3
-

0.5mm

Still below HVS thresholds
.







The smaller the pixel pitch the
'sharper' the image will appear

Motion perception

Slow response time
c
an result in motion
blurring whe
n
showing movement
or fast
,

video game
action

grey
-
to
-
grey
response time in
millisec
ond
s

1
-
3ms

Typical: 16ms


Lowest: 5ms

1
-
3ms

A disadvantage for LCD where
it is an issue. Not so for CRT or
PDP (see discussion)

Critical Fusion
F
requency (CFF)

Determines
the
point at which
display may
exhibit flicker

Refresh rate

80
-

100
Hz

70
-

75

Hz

60


72 Hz,

Refresh frequency (Hz) for CRT

needs to be at least 72hz


Not really relevant for LCD and
Plasma although latter can cause
eye fatigue at close distances

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5.1
.2
Discussion of non
-
HVS related display characteristics


Table 3

outlines

other characteristics of state of the
art displays such as size and weight which
are not related to the human visual system.


Display Size


The display size is the physical size of
the screen or projected image, measured diagonally.
Due to the size of an individual plasma cell in a
Plasma Display Panel (
PDP
)


around 0.3 to
0.5
mm
-

it has been difficult to manufacture smaller plasma displays and
none have been
commercially successful

below 32 inches
. Plasma
displays are

therefore mostly used in
situations where the viewer is some distance from the screen
, and they are

not generally
considered suitable for close
-
up computer work. For this reason, PDP manufacturers have
focused on
the l
arge TV

and public displays
segment of the market
with a plasma TV typi
cally
being at least 42 inches.
Competition on size is intense, with, for example, Samsung
announcing a world
-
beating 102
-
inch
Plasma

display at CeBIT in March 2005.


Until recently it

was difficult to make LCD panels at sizes greater than 17 inches, but new, so
-
called sixth and seventh generation manufacturing processes are rapidly leading to the
production of 32
-
inch and larger LCD displays (Fihn, 2004). Sharp, for example, recently
a
nnounced a 65
-
inch d
isplay (Associated Press, 2005) and Samsung anno
unced an 81
-
inch
LCD at SID, Boston
,

in June.


The depth of the display has been the key issue with the changeover from CRT to flat screen
technologies. A typical CRT has a depth of 17 inc
hes o
r more, compared to a depth of two to
four inches for

LCD or PDP.


Aspect Ratio


Aspect ratio is the ratio of the

width of screen to its height

and is an important characteristic
of a display
.
The choice of a display

should be
partly driven by the
as
pect
ratio of the

desired

available content (TV, movies, video, multimedia etc
.
)
likely to be viewed
and this has had a
long and chequered history

(Display Search, 2003)
.


The first CRTs had an aspect ratio of 5:4, but by th
e

1950s had
start
ed

to be
stand
ardised on
the National Television Standards Committee
's

(NTSC)

4:3 ratio. The Hollywood

movie studios
felt considerably threatened by TV in
the
1950s and, as a defensive measure, adopted a range
of wide
-
screen aspect ratios such as Cinemascope (2.55:1) an
d Panavision (2.39:1) which
offered a higher entertainment value in which the viewer felt more immersed.
More recently,
High Definition Television (HDTV)
has been

defined as 16:9, a compromise ratio that was
chosen because it could contain within it all th
e

different movie studio formats, but

d
espite
this standardisation many TV manufacturers are producing displays that have a ratio of
15:9
(WXGA format of 1
280 x 768 pixels) as this has

less of a visual impact on current TV output of
4:3.


Since early compu
ter displays were CRTs and, in fact, were generally made on behalf of the
computer companies by TV manufacturers, the dominant aspect ratio in
CRT or LCD computer
display
s

is
4:3
.
Computer manufacturers have also introduced models with wider screen
ratios,

the dominant form
being
16:10
, and these are expected to capture significant market
share
, especially amongst people who want to edit

and

play

DVDs

(Display Search, 2003
)
.


Electromagnetic emissions


LCD and PDP emit
fewer electromagnetic emissions
at the

range consider
ed

harmful,

namely

X
-
rays. Although to date, little
,

if any, measurable risk to human health
has been identified
from sitting in fr
ont of a CRT,
this may be a factor in a purchasing decision (PC World, 2003).




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Degradation and screen burn
-
i
n


The scintillators or phosphors present in PDP and CRT technologies ag
e over time and this
degrades

the

quality of the display
. Also, when the same image is displayed on the same part
of the screen for a long time, perhaps the logo of a cable channel, th
e ageing takes place at a
faster rate and the screen exhibits 'burn
-
in'. This has been seen as a particular issue with
Plasma display
s rather than CRT (Fihn, 2004)

although
Plasma is being improved with
the
introduction of improved phosphor mixes
.

CRT disp
lays

have an expect
ed operational lifetime
of 15 to
20 years.


Power

and
environmental

considerations


Power consumption is not only an issue of expense, but also, in a more energy conscious
world, an issue for environmental sustainability. In addition, di
splays meant for portable,
battery
-
operated devices such as laptops need to minimise power consumption. Generally,
PDPs are pow
er 'guzzlers' with a typical 42
-
inch

(107 cm) Plasma display cons
uming perhaps
250W

(joules/second)
, while a CRT

display with the

same diagonal measurement will consume
only 150W. LCD computer displays have even lower power consumption rates, perhaps 40W
for a 19
-
inch, compared to 130W for a similar sized CRT.
However, larger sized LCD TVs
currently have power consumption comparable

to that of Plasma.


As it currently stands displays built using
LCDs are also expected to have a
similar or
longer
lifespan than the average CRT (see above) and are easier to dispose of in an environmentally
sustainable manner.
However, there may be a lif
etime issue with the backlights used in LCD
displays which may ne
ed replacing after

a few years.

In general, it can be argued that the
total cost of ownership of an LCD display is
now significantly
lower than the other
display
technologies

(Fihn,
2002).
Th
is is an important counter
weight to some of the concerns about
LCD performance, such as viewing angle, as outlined in this section.


5.2 Projection


an alternative for multi
-
viewer scenarios


The alternative to CRT and FPD displays, particularly in scenar
ios involving an audience, is to
use some form of projection. There are two basic forms
:

rear and front systems. Both forms
use projection systems based on either CRT or one of a range of the newer
microdisplay

technologies in which a specialist silicon ch
ip is illuminated by a lamp to produce the image
(
Katzmaier
,

2005). There are three main microdisplay systems: LCD,
DLP


(proprietary
system belonging to Texas Instruments)
and liquid crystal on silicon (LCoS). A detailed
discussion of projection is beyond

the scope of this report, but, appendix B gives some further
technical details of the different technol
ogies. The important point

to bear in mind is that these
projection systems are increasingly based on microdisplays and that this alternative technical
solution will, as we shall see in
section 6
.6
, have a role to play in future display systems.


5.3 CRT


getting thinner and still in the market


Despite the rapid rise of display solutions based on variou
s flat screen technologies, CRT
displays

are still

in the market and new products are being launched. Arguably, CRT
technology
continues to offer a superior quality of picture and high
er

contrast ratios than FPD
s

and at highly competitive costs (although see the discussion of total cost of ownership when
power consumption is co
nsidered
) and
i
t is likely that CRT will continue to dominate the very
low end of the TV market for some time.

They are also challenging the 'thinness' issue.
Samsung recently announced plans to produce a
CRT
32
-
inch TV display with
a depth
that is
half that of existing solutions (Mann, 2004) based on its "vixlim" tube technology
(
www.vixlim.com
) and the trademark registration indicates potential plans for computer
monitors (Patent Office, 2004).



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Table
3
: Non HVS
-
related factors with respect to current display technologies

Factor

Factor
measurable?

CRT

LCD

PDP

General


Traditional, reliable, known
technology

Beginning to dominate

Huge manufacturing
momentum

Mainly used
for high
-
end TV
and

larger public displays

Size

Weight in lbs

Diagonal in inches

Depth in inches

Heavy: typically 40
-
50lbs for
a computer monitor, 120
-
160lbs for TV

Bulky: 17
-
20"

deep

for
monitor, 30
-
40"

for a TV (but
16
"

for newer 'thin' tube TVs)

Light
: appr
ox
. 11lb

20lb

Thin: 65

200mm

Flexible size o
f
manufacture: diagonal
size 12

65
"

Around 60

120 lbs

Thin: 3

4"

Possible to make large displays
due to manufacturing process

Dif
ficult to make smaller
displays: diagonal size >32",
range from 32

8
0"

Aspe
ct Ratio

Height x Width

4:3

4:3,
16:9, 15:9 and

a range in laptops although
16:10 may dominate

16:9

Screen/Glass


Traditionally c
urved,
reflecting ambient glare
, but
now increasingly flatter

Flat screen

Flat screen

Electromagnetic
emissions


Possible iss
ues with x
-
ray
and other emissions

No
X
-
ray
emissions

No
X
-
ray
emissions

Degradation and
screen burn

Manufacturers
quote half
-
life (the
time unti
l

screen is
half
its original
brightness)

Phosphor ages over time
, but
accepted
lifetime
of CRT
is
15
-
20 years

Not an issue
, but LCD
backlights may have
relatively
short lifetime

Exhibit screen 'burn
-
in'
.

Degradation of phosphor
screen
may be an issue
(typically quoted as 60,000
hours half
-
life)

Power

consumption

Watts

Medium

Typically 130W

Low

for computer
displ
ays(t
ypically 40W
)


High for large TVs

Heavy

Typically 250W, can be 400W
or more

Cost

£, €, $

Offer excellent value for
money

TV
s
: £50

£1000

m
onitor
s
: £100
-
£
700

Falling costs:

TVs: £2
00
-
£3000

monitors: £200
-
£600

Generally expensive £1
000
-
£4000 or more

E
missive


Yes

No, either reflective,
requires a backlight
(transmissive)

or both
.
Backlight consumes more
power than display.

Yes

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6
. The future of display technologies


the next 10 years


In the previous sections we have seen how the human visual system
(HVS) interacts with
display technologies, looked at some of the current trends in the state of the art, and
reviewed efforts by display manufacturers to match the capacity of the HVS. The older,
bulky CRT technologies are being replaced by flat screen sol
utions and although some
engineering effort has gone into developing displays that provide more of a match to the
HVS, the key driver has been the development of thinner display devices.


In the following section we review trends in the future development
of display
technologies. As we shall see, the drive to develop thinner displays that offer more of a
match to the HVS will continue, with new developments in materials and display
technologies. Often the uptake of these will be driven by the availability o
f new types of
content, but, a little further down the track, we will also see some completely different
approaches to the nature of display.


Key trend: Content as a driver for display choices


A key trend in the development of displays is that, in genera
l, we are moving to a more
multimedia
-
driven culture and that content itself is evolving. We are witnessing a
convergence of TV
,

computers and developments in bandwidth
,

and these are
paving the
way for
the introduction of High Definition content (HD video

and HDTV) accessed via the
Web, satellite, cable and DVD discs (see section 5.3).
For more information on the use of
digital video in teaching and learning, see BECTA's
(2003)
report
What the research says
about digital video in teaching and learning
.


T
hese

developments will tend to drive people's awareness of the display technology they
are using. In particular, the move to HDT
V is driving development of
large, flat screen
display technologies in those markets (USA, Japan) where thi
s format is becoming
widespread
. This means that
the
1920 x 1080 resolution

will become the base standard for
disp
lay devices.

The same will apply to the forthcoming introduction of 3
-
D: educational
content that becomes available, through
,

for example, games and 3
-
D virtual mo
dels of
museum exhibitions and buildings, will build awareness of what these new technologies are
capable of and drive expectations about what can be delivered. In this context it is worth
noting that often the software and the content last a lot longer th
an the display hardware
(Brodlie, 2004).


6
.1 Specific Trends: F
PDs
-

Flattening of the screen


Trend summary:

LCD versus t
he rest
-

thinner, lower power consumption
, better images


As we have seen in previous sections, recent years have witnessed a move f
rom bulky CRT
displays towards thinner, lighter Flat Panel Displays (FPDs). In 2004, for the first time,
sale
s of LCD
-
based computer screens

were higher than those for the mo
re traditional CRT
-
based displays
. At the same time, in the consumer market, home
owners are increasingly
opting for the enhanced design looks and space
-
saving potential offered by flatter Plasma
and LCD TVs and displays. The overall goal is to develop cost effective flat panel displays
that offer a picture quality comparable to today's

CRT, and, in the longer run, increasingly
match the human visual system param
e
ters discuss
ed in section four
.


At the moment the momentum is behind LCD which has a large market share, the ability
to deliver display solutions

for both small and large
devi
ces and which is being
manufactured on a large scale. Although LCD has some issues, such as motion blur,
engineering effort has gone into tackling these problems through, for example,
improvements in backlighting (Cranton, 2005)

and the

introduction of add
itional
sub
-
pixel
s
to improve luminosity. As the dominant technology
,

LCD will undergo furt
her
improvements, e.g. the introduction of new
sub
-
pixel

layouts to enhance colour gamut

(
Brown Elliot, 2005
)

and th
e addition of extra primary
sub
-
pixel

colours.
Ho
wever, in the
next few years a number of alternative FPD technologies w
ill attempt to improve on LCD’s
match

of the HVS, by improving

picture quality and lower
ing

price
s

in an effort to gain
some of LCD's market share. Technologies of particular interest a
re Field Emission Displays
(FED), Organic LEDs (OLED), Polymer LEDs (PLED) and Surface
-
conduction Electron
-
emitter Displays (SED). The following sections provide further details of these technologies
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and table 4

summarises some of the key characteristics o
f these displays with respect to
features of the HVS. Firstly, though a caveat: This is a very fast moving field and many of
the
se technologies are at a prototype
stage and therefore, by necessity, the figures given
in the table are often tentative and sho
uld only be taken as an indication.


6
.1.1 Field Emission Displays (FED): Carbon nanotubes


Expected: End 2006 onwards


"The concept of a nanotube TV will give you image quality similar to CRTs. All the major
display manufacturers are looking at nanotube
TVs."


Tom Pitstick, Carbon Nanotechnologies (In: Kanellos, 2005)


Field emission devices use electrons to directly fire
-
up a phosphor screen, in the same
manner as traditional CRT
s
. In a CRT
device
a single 'gun' (the cathode) fires the
electrons
,

and
has to be heated to achieve
this (a thermionic process). However,

with field
emission devices the mechanism used to generate the electrons i
s completely different. It
is
non
-
the
rmionic and
uses the physical properties of 'field emission effect' and 'quantu
m
tunnelling', whereby a low voltage is applied to a very large number of tiny, highly pointed
cathodes in order to release electrons.
These cathodes can be
made from
a number of
possible materials including carbon inks, diamond
-
based structures,
Spindt

ti
ps
(
mol
ybdenum) and
carbon nanotubes. Carbon nanotubes are

one of the new products
emerging from the field of nanotechnology

(Amaratunga, 2003)

and it is these that are
generating particular interest.



With the use of carbon nanotubes FEDs will, in theory
, solve many of the pr
oblems
associated with CRT and F
PD

technologies
(Mann, 2004). As with CRT, electrons will be
shot across a vacuum at a phosphor screen, but due to the nature of the electron emission
a display built with this technology will use less
power, be
substantially

thinner (currently
2mm in prototypes), will have a significantly faster response time than LCD, be viewable
from any an
gle and will not suffer from

plasma screen burn
-
in. Power consumption in
technical prototypes is 100W
,

an improve
ment on the 250
-
300 or more Watts required for
a Plasma TV, and
a
similar figure to that required by CRT (Mann, 2004)
,

thanks to the low
voltages used in the electron guns (Brandon, 2005). Costs are expected to be low,
certainly compared to Plasma, as some

of the manufacturing techniques will use existing