Touch Screens: A Pressing Technology

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TOUCH SCREENS: A PRESSING TECHNOLOGY
Timothy Hoye (tmh60@pitt.edu) and Joseph Kozak (jpk54@pitt.edu)
Abstract - In today’s society, the way in which we physically
interact with electronic devices is changing how we focus
our technological research. This change has led to many
great advances, including the development of touch screen
technology. Through the use of touch screen technology, the
operator is given an alternative method of how he or she can
interact with a device. This technology operates in three
distinct ways: resistive systems, capacitive systems, and
infrared systems. This paper will investigate, discuss, and
compare these different technologies, focusing on the
differences in application, aspects of sustainability, as well
as the positive and negative qualities.
Key Words – Capacitive, Infrared, Multi-Touch, Resistive,
Sustainability, Touch Screens
A BRIEF HISTORY OF TOUCH SCREENS
Throughout the past century, technology has improved in
many ways. The way in which humans interact with
technology is one of the most important ways technology is
changing. By using touch screen technology, the user is able
to manipulate a digital environment by only the touch of
their finger, or another input device, on the screen.
Throughout this paper we will discuss the different
technologies that make this possible: infrared, resistive, and
capacitive touch screens, as well as their qualities in modern
devices.
Touch screen technology first entered the public eye in
1971, with the invention of the Elograph, by Elographics,
Inc [1]. This company was created to “produce Graphical
Data Digitizers for use in research and industrial
applications” [1]. This technology set the stage for many
devices to come. One of the next devices to be invented was
the HP-150, the first touch screen computer. Hewlett
Packard invented this device in 1983 [2]. This technology is
important because it “had infrared touch-screen capability,
allowing for creation of ATM-like applications” [2]. These
are two of the most important devices in the development of
touch screen technology. As time progressed, touch screen
devices have become increasingly more complex and
sustainable, providing the user with greater accuracy and
more features to improve the quality of life.
INFRARED TOUCH SCREENS
The first type of touch screen technology we shall discuss is
based upon infrared light. There are two main infrared
systems: a standard grid and an internal reflection system.
These systems are very accurate; however, they require more
space than other touch screen systems.
Dr. Andrew Hsu, an expert on touch screen
technologies, states that “IR (infrared) screens are among the
most durable surfaces and can handle hostile environments,
making them well suited for military applications”[3].
Although we will not be focusing on the technology in terms
of military applications, we can see that infrared touch
screen technologies, while being the most durable surfaces,
are also quite possibly the most versatile. This versatility
comes from having two variant systems of infrared touch
screen. The first system is similar to resistive systems, which
will be discussed later, in that it consists of a two
dimensional grid of infrared light.
In this technology, infrared LEDs (light emitting
diodes) are arranged on opposite sides of the unit underneath
the glass. The diodes project infrared light into sensors
located directly across from them. The sensors read the
strength of the beams, and “when a user makes contact with
the screen, the system measures the drop in the sensoroutput
signal; this measurement allows the system to
compute the location of the touch” [3]. This is to say that
when the finger touches the screen, the infrared beams are
obstructed by the user’s finger; however, some light
continues to pass to the sensor. The sensors send the
measurements of light to the operating system, which
analyzes the data and recognizes where the user touched.
This technology has multi-touch capability because the
beams of light are never fully obstructed by the user’s touch.
The second type of infrared system requires more
space than the first. This system is based upon internal
reflection; a beam of light is emitted from within the unit,
hits the glass, and part of the beam exits through the lens
while the other part goes back into the unit. Cameras are
placed inside the unit and are calibrated to the standard
reflection to the beams so that, “when objects such as fingers
touch the surface, the light diffuses at the contact point,
causing the acrylic’s internal-reflection pathways to change.
A camera below the surface captures the diffusion and sends
the information to image-processing software, which can
read multiple touches simultaneously and translate them into
a command” [4]. In the example stated, the screen is an
acrylic screen rather than standard glass, allowing for a
thinner lens that is just as durable, cheaper, and more
resilient.
Advances are being made in internal reflection systems
to make the instruments much smaller. One of these
advancements, called ThinSight, is “a thin form-factor
interactive surface technology based on optical sensors
embedded inside a regular liquid crystal display” [5].
ThinSight will allow a row of circuitry that possesses
hundreds of small infrared LEDs, similar to the emitters and
sensors, used in standard infrared systems. These sensors
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and emitters are much smaller and would cover a much
smaller area of the screen. Traditional internal reflection
system only required four or five emitters and cameras.
Thinsight technology is still in development; however, it is
believed that these advancements could make infrared touch
screen technology much smaller. The advancements would
also cause the system to be more available for future
consumer touch screen devices.
FIGURE 1
THINSIGHT TOUCH SCREEN INFRARED EMITTERS AND DETECTORS [5]
Microsoft Surface
As mentioned above, the Microsoft Surface is a great
example of internal reflection infrared touch screens. In
early 2001, “Steve Bathiche of Microsoft Hardware and
Andy Wilson of Microsoft Research began brainstorming
concepts for an interactive table”, beginning the
development of the Surface [6]. The process continued in
development until 2003 when the idea was presented to Bill
Gates, and 85 prototypes were created for developers on the
Surface Computing group, a group created in 2004 to solely
develop the Surface. In 2005, various prototypes were
created and analyzed, and eventually, in 2008, the Microsoft
Surface was on sale to the public for $12,500 or $15,000 [6].
The Surface’s “technology uses multiple image sensors
around one side of the touch surface and IR backlights on
the other side. When a user places his finger on the surface,
intercepting the infrared beam, the device projects a shadow.
Using multiple cameras, the unit converts this shadow into a
touch point through triangulation” [3]. As stated by Dr. Hsu,
the Surface uses a system of cameras and IR LED’s to
capture the image of the object touching the surface. The
Surface took a step forward with their technology,
developing a special motherboard to compute the data,
transmitted at 100 megabytes per second, from the cameras.
The board then divides the data into sections that decide
what is relevant and what is not. Relevant data is considered
to be any information from the cameras that has changed
since the last transmission. An example of that is the shadow
from the infrared light changing as someone touches the
screen.
This software is also revolutionary in that it detects and
distinguishes different types of touches. The Surface
categorizes touches into three sets: finger, blob, and tagged
objects. Fingers are given the ability to click on software
application buttons, paint across the screen, and further
interact with the Surface. Blobs are generic objects that are
given a circular or oval reading with a major and minor axis.
Blobs include inanimate objects like paper or other pieces of
technology like smart phones or cameras [7]. Microsoft
products, like the Microsoft Zune music player, can interact
with the Surface just by laying Zune upon the top of the
screen. Multiple Zunes can interact with one another using
the Surface as an intermediary, transferring songs from one
to another. Cameras and phones can also perform similar
tasks with contacts, pictures, maps, and many other pieces of
data. Microsoft has been “initially selling Surface to
Starwood Hotels for guest check in, Harrah’s Entertainment
for video gambling, and T-Mobile for providing customers
with technical information about the company’s mobile
phones” [4]. The Surface allows for the use of tagged items,
specific to where the device is located. Tagged items could
include gambling chips at a Harrah’s Casino or an electric
keycard at a Starwood Hotel. The tagged items give the
Surface very specific data, which includes the facing
direction of the object as well as electric data on the tagged
item itself [7].
FIGURE 2
MICROSOFT SURFACE [6]
Through previous advances in touch screen
technology, Microsoft took a step forward in infrared
systems with their Surface. The Surface’s starting price is
$12,500. This high cost makes the Surface inaccessible to
personal consumers, and is therefore marketed toward
corporations or companies.
RESISTIVE TOUCH SCREENS
Resistive touch screen systems are the most common type of
touch screen technology in today’s market. These devices
are used in many applications, such as cell phones, handheld
games, GPS navigation devices, and even some digital
cameras [3].
The resistive touch screen technology operates in a
very simple way. These screens are built using two layers of
the conductive material Indium Tin Oxide (ITO), separated
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by a small gap of air [3]. The bottom layer is generally on
glass, and the top on a flexible material, often plastic [8].
When the user presses down on the top ITO layer, it
physically bends to make contact with the bottom ITO layer,
changing the resistance of the two layers [8]. A typical
resistive touch screen uses 4 wires, 2 of them on each panel.
As seen in Figure 3, each panel corresponds to a different
axis. These perpendicular axes allow the computer to take
the measurements of the change in resistance from each
panel, and calculate the position of the touch point from its
X and Y components [3].
due to the slight electromagnetic charge contained in the
human body [8]. These changes in capacitance are measured
and calculated as touch points in a very similar way to
resistive touch screens, by using the X and Y components.
FIGURE 4
PROJECTED CAPACITIVE TOUCH SCREEN LAYERS [8]
FIGURE 3
TYPICAL 4-WIRE RESISTIVE TOUCH SCREEN [3]
CAPACITIVE TOUCH SCREENS
Capacitive touch screens are very important within the field
of touch screen technology. In the early 1990s, this
technology made its initial appearance into the touch screen
market in laptop computers, as touch pads [3]. Recently,
capacitive popularity has grown, as it has become one of the
leading technologies used in touch screen devices. In 2001,
it began appearing in consumer devices, such as MP3-
players and smart phones [3]. This increase in attention is
likely due to the effectiveness of its design, its use of multitouch
technology, and the popularity of Apple products
using this technology: iPod Touch, iPhone and most recently
the iPad. [9].
Projected Capacitive Touch Screens
The design of projected capacitive touch screens is
somewhat similar to that of resistive touch screens, in that
they both utilize 2 layers of ITO, with perpendicular
conductive measuring strips on the ends of each layer [8],
which are encased between two glass layers (See Figure 4).
This “grid,” formed by the perpendicular conductive layers,
projects the electric field through the top layer of glasshence
the name projected capacitive touch screens [8].
Because of this projection, when the user touches the top
layer of glass it “changes the measured capacitance values of
the electrodes closest to it” [3]. This change in capacitance is
Surface Capacitive Touch Screens
Surface capacitive is another form of capacitive touch
screen technology. The primary difference between surface
capacitive and projected capacitive is that surface capacitive
uses only one ITO surface [3]. This layer calculates touch
points using principles that are very similar to projected
capacitive touch screens, in that touch points are observed
by changes in capacitance if the ITO layer in the touch
screen. However, these touch points are measured in a very
different way. The computer measures the change in
capacitance from each corner of the ITO layer, and with
these 4 separate measurements, the X and Y coordinates of
the touch point are calculated [3].
Multi-Touch Technology
An important feature of capacitive touch screens is their
ability to recognize and calculate multiple touch points at
one time, commonly called multi-touch. “Multi-touch
technology has been around since early research at the
University of Toronto in 1982” [4]. The uses of this
technology are very vast, allowing for greater humancomputer
interaction. This technology is traditionally
associated with capacitive touch screens, but is not limited to
this technology. It can also be found in infrared touch
screens and is beginning to appear in resistive touch screens
[3]. Currently, multi-touch technology is being used with a
purpose similar to the function keys (Control, Alt, Option,
Command, etc.) on a standard keyboard. By adopting these
functions, the user is able to complete the same tasks as
before, but with one hand. With advances in hardware, multi
touch will allow multiple users to access the same device
simultaneously, like the Microsoft Surface’s capability of
300 plus touches. While the hardware is available to create
such devices, software implementation is holding back the
growth of multi touch.
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Apple
Apple Corporation is a pioneer in the field of consumer
electronics. With their innovative designs, Apple products
have been setting new standards to which other technology
is compared. In 2007, Apple revealed the iPhone, changing
the face of telecommunications and touch screen technology
[10]. This device was revolutionary because it incorporated a
cell phone, iPod, and Internet communications device,
making it the only electronic device you need [11]. Apple
also created a version of this device to be the next generation
of iPod, called the iPod Touch or the iTouch. It is very
similar to the iPhone, except for the difference that it does
not have cell phone capabilities. For our purposes, these
devices will be described as interchangeable, because they
both rely on the same touch technology and the same userinterface
[9].
manipulate documents, images, and other files with motions
that feel like the task actually being completed, the user is
able to work more efficiently and with less specific training
or knowledge.
Apple recently announced their next step in the field of
touch screen technology: the iPad. Little is known about this
device, aside from general technical descriptions of what it
can do, and its 9.7-inch multi-touch display [9]. It can be
assumed that it operates very similarly to that of the iPhone
and iPod touch. This device was designed to browse the
web, read and send email, view photos, movies, listen to
music, play games, and read e-books, among many other
things [13]. With these tasks in mind, the iPad is creating a
new market for touch screen devices, much as the iPhone
changed the mobile phone industry. By integrating
innovative hardware, and creating arguably the most
intuitive user interface available, Apple continues to raise
the bar for all of their competitors in the consumer touch
screen industry.
PROS AND CONS
FIGURE 5
APPLE IPHONE [9]
The iPhone and iTouch both use a projected capacitive
touch screen, and are often used as examples in comparing
projected capacitive technology to other touch screen
technologies [10]. The patent application for this system is
where most of our knowledge of the screen is from. It
describes two different types of touch technology, selfcapacitance
and mutual capacitance [12]. Self-capacitance is
“a simple passive array of 2436 sensing electrodes in a
single plane” [12]. Mutual capacitance, on the other hand,
works much more like other capacitive touch screens. By
setting two layers on top of each other, aligning the
measuring strips perpendicular to each other, and calculating
the touch point based on the X and Y coordinate of the
touch, greater accuracy is achieved [10]. Another
technologically relevant aspect of the iPhone is its use of
multi-touch input, which allows for 15 touches at a time
[12]. It is with this technology that the iPhone and iPod
touch revolutionized the touch screen industry.
These devices are important to the development of
touch screen technology, because of their superior user
interface (UI). There are six main functions of the iPhone’s
touch screen: “Single tap to select or activate something,
double tap to change the display format, drag and drop to
move something, a stroke (“swipe” or “flick”)
up/down/left/right to scroll, “pinching” two fingers together
to shrink something,” and “spreading” two fingers apart to
enlarge something” [10]. These intuitive controls have set a
new precedent for all user interfaces. By being able to
The two types of infrared systems are internal reflection and
infrared grid. Infrared grids systems are reliable and can be
manufactured inexpensively into appropriate sizes. Due to
the grid of lasers and sensors, users do not need to press
fully down on the screen putting less wear on screen
increasing the life expectancy of the unit. The grid also
increases the precision of the users touch.
Internal reflection systems are large systems because
of the space required for cameras to accurately measure the
shadow produced by the infrared LEDs. The large space
required for larger instruments does make internal reflection
devices the most accurate touch screen technology. Units
like the Microsoft Surface, the leading internal reflection
device, are also able to increase their multi-touch capabilities
to 300 plus touches.
Resistive touch screen technology is the cheapest of
the different types of touch screens; however, there are a few
drawbacks. The first problem with this technology is the
flexibility of the top layer of screen. This causes the ITO
coating to crack due to the continual stretching and
retracting of the flexible later [3]. This wear also relates to
the air gap between the ITO layers. This gap allows dirt and
dust to collect between the two conductive surfaces, making
the display appear dirty. Additionally, these devices are “less
than ideal for harsh environments,” due to the fact that they
are vulnerable to temperature and humidity changes, which
would affect the accuracy of the touch screen [3]. This is not
to say that they do not perform consistently under standard
conditions. One benefit of this technology is that the user is
able to use his or her finger or a stylus as input devices [3].
Overall, resistive touch screens perform very well,
considering that they are the cheapest of the different touch
screens.
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Capacitive touch screens are very common in many
consumer devices. Although there are two different types of
capacitive touch screens, their performance is very similar,
with the exception that projected capacitive touch screens
are a little more accurate than surface capacitive touch
screens, but this difference is relatively negligible. An
important feature of this technology is its use and application
of multi-touch gestures. This is because less force is required
to maintain a “touch point”, making dragging and zooming
items much easier. However, a drawback of this technology
is that you can only touch the screen with your finger. This
means that stylus and gloves, depending on their thickness,
will not work with this technology [8]. Another drawback is
the cost of the screen. These screens are more expensive
than resistive touch screens [3]. Overall, capacitive touch
screens are very effective in their current uses.
Resistive and capacitive technologies are very similar,
but also have several important differences. One of these
differences is in durability and the need for calibration. In
resistive touch screens, due to the deformation and warping
of the ITO layer, the screens performance will be changed.
This change creates a need to recalibrate the screen [8]. This
problem, however, is not found in capacitive touch screens.
This is because the ITO layers are less susceptible to
damage. Additionally, this is capable “because the system
can self-calibrate for environmental changes and is better
able to adapt to environmental issues than resistive
technology” [3]. Infrared devices also have the capability to
self-calibrate for scratches because the sensors only react
when something changes on the screen. A sensor would
initially react to a scratch however, once the device is in rest
it would read the new dimensions of the screen to be
standard.
This difference has a large impact on the use of each
of the devices. Since capacitive and infrared devices do not
need to be calibrated, they are more accurate than resistive
devices after some wear. This difference between capacitive
and resistive also increases because the ITO layers in
resistive devices deteriorate over time. Capacitive touch
screens are, therefore, more accurate and more durable than
resistive touch screens.
Another difference between the types of touch screens
is the type of input devices allowed. In both resistive and
infrared systems, you can use nearly any object to create a
touch point. The only limitation on resistive systems is that
the object needs to be somewhat pointed. This means that
these touch screens allow for the use of fingers, a stylus,
and, in infrared devices, just about any other object. This is a
great benefit over capacitive touch screens, where a user can
only use a finger to create touch points. The flexibility of
using a stylus or finger allows for greater accuracy, and also
allows the technology to be used in varied ways.
The main difference between touch screen
technologies is size and cost. Infrared touch screens are by
far the biggest of the touch screen technologies. They are
also more expensive than resistive and capacitive touch
screens [3]. The size of resistive and capacitive touch
screens is relatively similar, due to the similar nature of their
technologies. However, there is quite a difference in price
between these touch screens. Capacitive touch screens are
more expensive than resistive touch screens because the
systems of circuitry and measurement are more complex [3].
It is with these differences in mind that we discuss the
applications of these technologies. Infrared touch screens are
best suited to devices like the Microsoft Surface, which
require a very large touch screen. While resistive and
capacitive screens have the potential to be this size, they do
not perform as well as the infrared technology. This is
because infrared technology uses sets of infrared LEDs and
either cameras or sensors to detect the changes. Having a
large capacitive or resistive system would require a large
amount of wire throughout the screen; wires of that size
would increase the chance for malfunction in detection, as
well as wear and tear.
Capacitive touch screens are best suited for high end,
portable electronic devices, and devices that need to perform
consistently in many conditions. This is because of its
durability, accuracy, multi-touch support and ease, and its
aesthetically pleasing appearance.
Finally, resistive touch screens are best suited for
mobile applications, in which conditions will be consistent.
SUSTAINABILITY
One definition of sustainability is the improvement of the
quality of life by making life more enjoyable and less
burdensome. Touch screen technology fits within this
definition very well. Touch screen devices make life more
enjoyable by creating a fun and intuitive user interface. This
is a reason that the iPhone, iPod Touch, and similar devices
are so successful. By allowing the user to operate the device
in many different ways, the devices are more versatile and
create a better interface for many applications. With a better
interface, the devices become more enjoyable to use, and
allow for other applications of the device.
Sustainability also pertains to making life less
burdensome. Touch screens are very sustainable because of
the vast amount of applications that can be done on one
device. This can be seen very easily in the iPhone and
Microsoft Surface. Before the iPhone, many people carried
around a cell phone, iPod, and PDA. With the
implementation of a versatile touch screen, the iPhone and
other touch screen devices are able to do the tasks of all
three of these devices. This is because of the adaptability of
the interface. The Microsoft Surface is similar to the iPhone
because it makes many applications available to the user.
Users are able to transfer contact information, calendars,
pictures, etc. with just the touch of a finger. The
sustainability aspects of both of these devices show the
importance of these technologies. These devices improve the
quality of life for the user by simplifying everyday tasks and
making them more enjoyable for the user.
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THE FUTURE OF TOUCH
For now, the latest developments in touch screen technology
include what is considered as an interactive touch screen—it
is perceived that the screen shapes itself allowing a user to
click a button. A. Peshkin and J. Edward Colgate, professors
at Northwestern University, are at the forefront of this
innovation. The Tactile Pattern Display, TPaD, has a small
devise, piezoelectric ceramic disc that vibrates the outer
most layer of glass. The vibrations through the glass create a
small layer of air between the user’s finger and the screen
itself. This changes the coefficient of friction between the
finger and screen making it appear that the glass itself has
changed. Peshkin and Colgate explain that, “the entire plate
vibrates, so the amount of friction is the same all over the
TPaD’s surface at any given time. But because the
oscillations are modulated as your finger’s position changes,
the device fools you into thinking that there are varying
amounts of friction at different locations. The prototype uses
optical sensors to keep track of your finger’s position. The
friction reduction can be switched on and off so quickly
(within about 4 milliseconds on average) that the pitch of
virtual bumps or dips can be made far finer than what a
fingertip can discern” [14].
The TPaD is still in the prototype phase of
development; however, it is a step forward with human
interaction with technology. How far and how fast touch
screen technology develops is only limited to the funding
and resources available. Expect to be interacting with your
devices more and more as advances are made in technology.
TOUCH SCREENS AND CONSUMERS
Moving towards the future, consumers will continue to see
the growth of the touch screen industry, due to extensive
engineering advancements in user interfaces. The ability to
physically touch a screen is easier than searching for a
specific key in a sea of buttons. Society, for these reasons,
has found touch screens to be the future of many devices.
The social norm of today includes walking down the street
surfing the web on an iPhone or sifting through music on an
iPod Touch. No additional buttons are necessary, just the
small, portable device in one’s pocket until needed. Society
will continue to see the development of touch screen
technology as human-device interaction is perfected.
[5]Izadi, Shahram, et al. "ThinSight: A Thin Form-Factor Interactive
Surface Technology." Association for Computing Machinery.
[6]"The Origins of Microsoft Surface". www.microsoft.com.
http://www.microsoft.com/surface/Pages/Product/Origins.aspx. Accessed 3
March 2010.
[7]"The Microsoft Surface Vision System". www.microsoft.com.
http://go.microsoft.com/?linkid=9707395. Accessed 3 March 2010.
[8] Gray, Tony. “Projected Capacitive Touch Screen Technology”. Ocular,
Inc. Accessed 3 March 2010.
[9] “Apple.com”. www.Apple.com. http://www.apple.com. Accessed 3
March 2010.
[10] Walker, Geoff. “The Apple iPhone’s Impact on the Touch-Panel
Industry”. Information Display 5/07. Accessed 3 March 2010.
[11]"Apple Reinvents the Phone with iPhone". www.Apple.com.
http://www.apple.com/pr/library/2007/01/09iphone.html. Accessed 3 March
2010.
[12] Walker, Geoff. “Touch and the Apple iPhone”. Veritas et Visus.
Accessed 3 March 2010.
[13]"Apple Launches iPad". www.Apple.com.
http://www.apple.com/pr/library/2010/01/27ipad.html. Accessed 3 March
2010.
[14]Jones, Willie D. “Touch Screens with Feeling” IEEE Spectrum May.
2009: 15. Accessed 3 March 2010.
ADDITIONAL SOURCES
Aguilar, R.N., and G.C.M. Meijer. "Fast interface electronics for a resistive
touch screen." Proceedings of IEEE SENSORS 2002. 2002.
Hill, Anthony. "Touch screen technologies: Their advantages and
disadvantages; the guidelines offered will point you toward the best touch
technology for your application needs." Control Solutions, September 2002:
24.
Hoggan, Eve, Topi Kaaresoja, Pauli Laitinen, and Stephen Brewster.
"Crossmodal Congruence: The Look, Feel and Sound of Touchscreen
Widgets." ICMI. 2008. 157-164.
Kim, Hong-Ki, et al. "Transparent and flexible tactile sensor for multi touch
screen application with force sensing." Solid State Sensors, Actuators and
Microsystems Conference. 2009. 1146-1149.
Loviscach, Jorn. "Two-Finger Input with a Standard Touch Screen."
Fachbereich Elektrotechnik und Informatik, October 7-10, 2007: 169-172.
ACKNOWLEDGEMENTS
We would like to thank Luis Bon and Rowoli Scott-
Emuakpor for their guidance in this project. We also thank
those at the Engineering Library, Writing Center, and those
involved in the Freshman Engineering Writing Program who
have helped us with our research and in editing this project.
We would additionally like to thank our peers in the
Freshman Engineering Program, especially the SPACE
Floors, for their support and assistance with the editing and
revisions necessary to completing this project.
REFERENCES
[1]“Company history from Elographics to Elo TouchSystems, 1971 -
present - Elo TouchSystems - Tyco Electronics”. www.elotouch.com.
http://www.elotouch.com/AboutElo/History/default.asp. Accessed 3 March
2010.
[2]“The HP-150”. www.columbia.edu.
http://www.columbia.edu/acis/history/hp150.html. Accessed 3 March 2010.
[3]Hsu, Andrew. "Choosing a touch technology for handheld-system
applications." EDN, January 8, 2009: 40-44.
[4]Nichols, Steven J. Vaughan “New Interfaces at the Touch of a Fingertip”
IEEE Society August. 2007: 12-15.
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