Touchscreen - Professional Tablet PC, industrial rugged Panel PC and monitors Specialist

Touchscreen

TOUCHSCREEN TECHNOLOGIES
 
There are multiple touchscreen technologies available for LCD monitors, Panel PC or tablet PC. The choice depends on the usage and environment. Here is a simplified presentation to compare the main caracteristics of touchscreen technologies. This document does not intend to present all technologies available.
Yellow background indicates the most common touchscreen solutions used in professionnal solutions

  4/8 wire resistive 5,6,7 wire resistive Capacitive Projected Capacitive multi-touch
Projected Capacitive
surface wave Infra-red
Detection pressure pressure finger touch finger touch finger touch detect
"x, y"
detect
"x, y"
Sensitivity ++ ++ +++ +++ +++ ++ ++
Precision ++ ++ +++ +++ +++ + +
Multitouch No No No No Yes No No
Brightness ~ 80% ~ 80% ~ 90% ~ 90% ~ 90% ~ 100% ~ 100%
Calibration stable stable not stable stable stable very stable very stable
Scratch - - - - - - +++ +++ +++ +++
Vandal proof no no no option tempered glass no option tempered glass option tempered glass
Screen size < 12'' < 24''   all < 16'', growing > 19'' > 19''
Water proof +++ +++ +++ +++ +++ + +

1- Resistive Technology
It is the most common and cheapest. It consists of 2 thin films that touch each other when pressure is applied. Any pressure above the minimum required can be detected : finger, plastic made passive stylus. Without pressure, the 2 films do not touch - they are separated by small balls. The touchscreen surface must be aligned with the display image - this is called "calibration". Calibration is fairly stable with resistive technology. Over time, re-calibration may be required - it is done with a calibration software. The resistive films absorb around 20% of the light emited by the LCD panel, the display brightness is reduced up to 20% compared to the same display without resistive touchscreen.
Resistive touchscreen can be damaged, i.e. scratched or cut. They cannot be used in public environment with the risk of vandalism.
The pressure needed to be detected varies:
- 40 grammes high sensitivity,
- 80 grammes sensitive to finget touch,
- 120 grammes hardly sensitive to fingers, need a stronger pressure such as stylus pens.
4 and 8 wires resistive touchscreen are less durable than 5, 6 or 7 wire versions that are recommended for frequent usage.
 
Multitouch
Multitouch has been widely deployed using capacitive touchscreen.

2- Capacitive Technologies:
Surface Capacitive
This technologie relies on a conducting film on the screen. The change of electrical capacity is detected by the film; the contact of a finger is detected since the human body has an electrical capacity. A finger in a glove is not detected, nor is a passive stylus.
The surface capacitive film is sensitive to scratch, and absorbs a small part of the LCD light. The
calibration must be redone from time to time to maintain the alignment touchscreen / display.
 
Projected Capacitive
The projected capacitive is between 2 glass layers. Again, it detects fingers not inside gloves. It is not sensitive to scratch and can be used on vandal-proof tempered glass.
 
Thick Glass
Some versions are available to be sensitive through thick galsses, and provide the capability to place a touchscreen behinb a storefront window up to 8mm thick. 
 
Limitation
Capactive touchscreen must not touch metal, therefore, joints are applied to integrate touchscreen into metal chassis such as industrial panel PC or LCD.
 
Multitouch
Capacitive is known to provide multitouch version, which is also available in some resistive touchscreen.

3- Surface Accoustic Wave (SAW):
This technology uses the sides to detect the touch location. The glass surface is inactive; the contact location "cuts" accoustic waves circulating on the glass surface. The contact must be large enough : a finger is detected, a stylus is too thin to be detected. This is suitable to touching navigation buttons; handwriting recognition is not possible.
Since the surface is a simple glass, it does not absorb light emitted by the display, it is scratch resistant (like glass) and can be vandal proof if tempered glass is used.
Finally, calibration is stable.

4- Infra-red :
This technology uses the sides to detect the touch location - the same way as SAW. The glass surface is inactive; the sides detect the "X,Y" location when IR rays that are "cut" - LED are situated inside the frame (IR emission LED, reception). The contact on the surface must be large enough to be detected: a finger is detected, a stylus is too thin to be detected. This is suitable to touching navigation buttons; handwriting recognition is not possible.
Since the surface is a simple glass, it does not absorb light emitted by the display, it is scratch resistant (like glass) and can be vandal proof if tempered glass is used. Calibration is stable like for SAW.
Intense light can conflict with IR LED - it is not suitable to touchscreen in direct sun light .
 
5- Digitizer
Digitizers were originally in every tablet PC as standardized by microsoft in 2001. The pen is active and detected by the digitizer. No pressure is detected, therefore you can lay the hand on the display while writing.Digitizers can be combined with resistive touchscreen for instance - either one is active at one point in time.

6- Water / Dust resistance
Capacitive touchscreen are not well suited to be in contact with water. All touchscreen can be sealed to provide IP protected display. IR and SAW can also IP protected, not as easily as resistive touchscreen.