Comparative Touchscreen Technologies

NextWindow’s Optical Imaging Technology

NextWindow’s optical imaging technology uses line scanning cameras to detect the touch point. Using this unique technology, the touch actually registers just before the physical touch on the screen. NextWindow’s solution scales to touch-enable very large displays. Surface coating overlays are not used on the touch screen surface, therefore excellent image clarity is preserved. In addition, scratches on the touch surface will not affect the touch screen operation. Optical imaging provides a solution without calibration drift, therefore, once the touch screen has been calibrated it does not require any further adjustment.

Comparison of Touch Screen Technologies

Resistive

Resistive is the most
common type of touch screen technology. It is a low-cost
solution found in many touch screen applications, including
hand-held computers, PDAs, consumer electronics, and
point-of-sale-applications. 



A resistive touch screen uses a controller and a specially
coated glass overlay on the display face to produce the touch
connection. 

The primary types
of resistive overlays are 4-wire, 5-wire, and 8-wire. The
5-wire and 8-wire technologies are more expensive to
manufacture and calibrate, while 4-wire provides lower image
clarity. 



Two options are generally given: polished or
anti-glare. 

• Polished offers clarity
  of image, but generally introduces glare. 
  

  
  



• Anti-glare will minimize
  glare, but will also further diffuse the light,
  thereby further reducing the clarity.
  

  
  
  
  

One benefit of using a
resistive display is that it can be accessed with a finger
(gloved or not), pen, stylus, or a hard object. 

However,
resistive displays are less effective in public environments
due to the degradation in image clarity and the need for periodic
recalibration caused by the layers
of resistive film deteriorating, and its susceptibility to scratching. 
Resistive displays are susceptible to vandalism and touches will not
register if the resistive sheet is cut or scratched.



Despite the trade-offs, the resistive screen is the most
popular technology because of its relatively low price (at
smaller screen sizes), and ability to use a range of input
means (fingers, gloves, hard and soft stylus).

Capacitive

Capacitive touch screens
are all glass and designed for use in ATMs and similar kiosk
type applications. A small current of electricity runs across
the screen with circuits located at the corners of the screen
to measure the capacitance of a person touching the overlay.
Touching the screen interrupts the current and activates the
software operating the kiosk.

Because the glass and bezel that mounts it to the monitor can be sealed, the
touch screen is both durable and resistant to water, dirt and dust. This
makes it commonly used in harsher environments like gaming, vending retail
displays, public kiosks and industrial applications.



However, the capacitive touch screen is only activated by the touch of a
human finger and scratches in the coatings can cause dead spots on the
screens. A gloved finger, pen, stylus or hard object will not work. Hence,
it is inappropriate for use in many applications, including medical and food
preparation.  The technology was originally created for small screens
and will not scale to larger screens easily and can require periodic
recalibration

Surface Acoustic Wave
(SAW)

SAW technology provides better image clarity because it
uses pure glass construction. A SAW touch screen uses a glass
display overlay. 



When sound waves are transmitted across the surface of the
display:

• Each wave is spread
  across the screen by bouncing off reflector arrays
  along the edges of the overlay; 
  

  
  



• Two receivers detect the
  waves; 
  

  
  



• When the user touches the
  glass surface, the user’s finger absorbs some of
  the energy of the acoustic wave and the controller
  circuitry measures the touch location.
  

SAW touch screen technology is
used in ATMs, amusement parks, banking and financial
applications and kiosks. The technology is not able to be
gasket sealed, and hence is not suitable to many industrial
or commercial applications as it can be adversely affected by surface
contaminants and water.  The contaminants can cause dead spots on the
screen requiring  periodic cleaning of the sensor and sometimes also
recalibration. Due to the way the technology works it can also be
susceptible to "noise".



Compared to resistive and capacitive technologies, it
provides superior image clarity, resolution, and higher light
transmission.  However, it was again originally
designed for smaller screens and may not scale easily to screens sized over
30".

Infrared

Infrared technology relies
on the interruption of an infrared light grid in front of the
display screen. The touch frame or contains
a row of infrared LEDs and photo transistors, each mounted on
two opposite sides to create a grid of invisible infrared
light.



The frame assembly comprises printed wiring boards, on
which the electronics are mounted, and is concealed
behind an infrared-transparent bezel. How it functions:

• The bezel shields the
  electronics from the operating environment while
  allowing the infrared beams to pass through; 
  

  
  



• The infrared controller
  sequentially pulses the LEDs to create a grid of
  infrared light beams;
  

  
  



• When a stylus, such as a
  finger, enters the grid, it obstructs the beams;
  

  
  



• One or more
  phototransistors detect the absence of light and
  transmit a signal that identifies the x and y
  coordinates.
  

  
  

Infrared touch screens are
often used in manufacturing and medical applications because
they can be completely sealed and operated using any number
of hard or soft materials.

The major issue with infrared is that the seating
of the touch frame is slightly above the screen.
Consequently, it is susceptible to early
activation before the finger or stylus has actually
touched and surface. Contaminants can also cause false activation on the screen inside the thick border
that is required for the frame. The cost to manufacture the infrared
bezel is also quite substantial.