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Technologies
There are a number of types of touch screen technology:
Resistive
A resistive touch screen panel is coated with a thin metallic electrically
conductive and resistive layer that causes a change in the electrical current
which is registered as a touch event and sent to the controller for processing.
Some resistive panels can estimate the area (and hence the pressure) of a touch
based on calculations from the resistances.
Resistive touch screen panels are generally more affordable but offer only 75%
clarity (premium films and glass finishes allow transmissivity to approach 85%)
and the layer can be damaged by sharp objects. Resistive touch screen panels are
not affected by outside elements such as dust or water and are the type most
commonly used today.
Surface wave
Surface wave technology uses ultrasonic waves that pass over the touch screen
panel. When the panel is touched, a portion of the wave is absorbed. This change
in the ultrasonic waves registers the position of the touch event and sends this
information to the controller for processing. Surface wave touch screen panels
can be damaged by outside elements. Contaminants on the surface can also
interfere with the functionality of the touchscreen.
Capacitive
A capacitive touch screen panel is coated with a material, typically indium tin
oxide that conducts a continuous electrical current across the sensor. The
sensor therefore exhibits a precisely controlled field of stored electrons in
both the horizontal and vertical axes - it achieves capacitance. The human body
is also an electrical device which has stored electrons and therefore also
exhibits capacitance. When the sensor's 'normal' capacitance field (its
reference state) is altered by another capacitance field, i.e., someone's
finger, electronic circuits located at each corner of the panel measure the
resultant 'distortion' in the sine wave characteristics of the reference field
and send the information about the event to the controller for mathematical
processing. Capacitive sensors can either be touched with a bare finger or with
a conductive device being held by a bare hand. Capacitive touch screens are not
affected by outside elements and have high clarity.
Infrared
An infrared touch screen panel employs one of two very different methodologies.
One method used thermal induced changes of the surface resistance. This method
was sometimes slow and required warm hands. Another method is an array of
vertical and horizontal IR sensors that detected the interruption of a modulated
light beam near the surface of the screen. IR touch screens have the most
durable surfaces and are used in many military applications that require a touch
panel display.
Strain Gauge
In a strain gauge configuration the screen is spring mounted on the four corners
and strain gauges are used to determine deflection when the screen is touched.
This technology can also measure the Z-axis. Typical application include
protecting new touch-screen railway ticket machines from vandalism.
Optical Imaging
A relatively-modern development in touch screen technology, two or more image
sensors are placed around the edges (usually the corners) of the screen.
Infrared backlights are placed in the camera's field of view on the other sides
of the screen. A touch shows up as a shadow and each pair of cameras can then be
triangulated to locate the touch. This technology is growing in popularity, due
to its scalability, versatility, and affordability, especially for larger units.
Dispersive Signal Technology
Introduced in 2002, this system uses sensors to detect the mechanical energy in
the glass that occur due to a touch. Complex algorithms then interpret this
information and provide the actual location of the touch. The technology claims
to be unaffected by dust and other outside elements, including scratches. Since
there is no need for additional elements on screen, it also claims to provide
excellent optical clarity. Also, since mechanical vibrations are used to detect
a touch event, any object can be used to generate these events, including
fingers and styli.
Acoustic Pulse Recognition
This system uses more than two piezoelectric transducers located at some
positions of the screen to turn the mechanical energy of a touch (vibration)
into an electronic signal. This signal is then converted into an audio file, and
then compared to preexisting audio profile for every position on the screen.
This system works without a grid of wires running through the screen, the touch
screen itself is actually pure glass, giving it the optics and durability of the
glass out of which it is made. It works with scratches and dust on the screen,
and accuracy is very good. It does not need a conductive object to activate it.
It is a major advantage for larger displays.
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