One of the big questions I have to deal with after showing someone an iPhone, iPad, or iPod touch is how does this touch screen work.  Yes, they are use to their microwave ovens, and the basic touch screens, but I watch people push down on the screen as if they were trying to break the glass….

Here’s a secret, there are different types of touch screens, and they all behave differently.

Types of Touch Screens:

• Resistive –  The touchscreen is composed of several layers, including typically two thin, metallic electrically conductive layers.  When an object presses down on the outer layer, the two metallic layers are pressed together which is detected by the touchscreen controller.  This is actived purely by pressure, so any type of contact will work, as long as there is enough pressure.  This is less likely to support multitouch.
• Surface Acoustic Wave –  This uses ultrasonic waves that are detected by a series of sensors, and any break in the signal triangulates the position of the “break”.  Drawbacks include that this method is exposed (potentially) to the elements, and contaminants on the screen can affect the signal.  This is also material neutral.
• Capacitive –  The display has been coated with a transparent conductor, and when the surface is touched by a conductive object (eg a finger), causes a disruption/alteration in the electrostatic field.  This disruption is localized, and the position is generated from that.  This technology easily supports multitouch, and continous input.
  • Surface Capacitance only has one side of the display coated, and a small charge is applied to this layer creating a uniform electrostatic field.  The disruption is located by calculating the distance from the corners of the display.  This has no moving parts, is durable, but has a limited resolution, has to be calibrated and can be prone to false signals.
  • Projected Capacitance uses two different capacitance layers and is used quite widely, including in most high end cell phones, including the iPad, iPod Touch, iPhone, etc.  The improved sensitivity allows the capacitive grid to be covered with additional protective layers…  And protective layers means that it will generally last longer and be more durable.
• Infrared – I know this from older CRT based units, but the same technology can be used with LCD, etc.  There is a grid of infrared (invisible) LED and photodetector units around the edge of the display.  This forms an invisible mesh that when an object breaks the beam, it will break an X & Y beam, giving the system the “coordinates” of the break.
• Strain Gauge (force panel technology), the display is spring mounted at the corners, and strain gauges are used to determine where the screen is touched.
• Optical imaging – Two or more image sensors are placed around the edges of the screen, an infrared back light is placed in each camera’s field of view, and the shadow that is created by an object touching the screen is used to triangulate the touch and/or measure the size of the object.  I believe this basic technology is used by Microsoft’s surface.
• Acoustic Pulse recognition – Small piezoelectric transducers are located at various positions around the screen, when the screen is touched a small vibration is caused that is then transformed through the transducers into an electrical signal.  But this can only detect the action of being touched, so an finger left on the display would be “invisible”.

All of the significant touchscreen technology patents that were filed during the 1970’s and 1980’s are expired and for that reason royalties and patent licenses are virtually non-existant….