uoıʇɔǝןןoɔ uǝʇ-ǝʌıɟ ǝɥʇ: How Touch Screen Monitors Work

Brief History

In 1972, a scientist at CERN - The European Organization for Nuclear Research - proposed the idea for the world’s first capacitive touch screen. A touch screen display would solve the complex interface required to operate CERN’s particle accelerator.

The proposed touch screen had a fixed number of programmable buttons on the display. Four years later in 1976, CERN produced its first capacitive touch screen terminal.

The early version of the capacitive touch screen featured fixed buttons on a display. This gave the flexibility to
re-map the buttons on screen as required.

Key technologies

Modern touch screen technology evolved from the capacitive touch screen. To understand how current generation touch screen technology works, we must understand how the most basic touch screen monitor - the capacitive touch screen - and its functions.

There are several core touch screen technologies:

Surface Capacitance
Projected Capacitance
- Mutual Capacitance
- Self Capacitance
Resistive
Infrared
Surface Acoustic Wave (SAW)
Optical Touch Screens

Capacitive Touch Screens

Capacitive touch screens rely on conductors that are imbedded onto a sheet of glass. When the display turns on, the conductors create a electrostatic field. Since the human body naturally holds an electrical charge, touching the display distorts the electrostatic field. From the distortion of the electrostatic field, the image processor, much like a CPU, calculates where the touch occurred. Derivations of capacitive touch screens include: Surface Capacitance Projected Capacitance Mutual Capacitance Self Capacitance

Surface Capacitance

One side of the insulator coating has a
conductive layer. The conductive layer creates an electrostatic field on the glass
The electrostatic field changes when a human finger touches the surface
A sensor detects the change in the electro static field, and calculates the location of the touch
Common in Kiosks/Point of Sale (POS) systems
Prone to false signals

Projected Capacitance

Two types of Projected Capacitance: Mutual Capacitance and Self Capacitance
Used in iPhone
More accurate than surface capacitance
Relies on an X-Y axis grid, etched onto the electrodes on the conductive material behind the glass
Operates under screen protectors / vandal-proof glass
Common in POS systems that require signatures (i.e self-checkout counters that require a digital signature)
Sweaty finger tips (i.e from humid environments) decreases the touch screen accuracy

Mutual Capacitance (Derivative of Projected Capacitance)

Used in multi-touch applications (i.e iPhone)
Requires two distinct layers of material to operate
Capacitor on every row and column, creating a grid pattern across the screen (i.e. a screen with a 10-by-10 layout will have a total of 100 independent capacitors.
The capacitors operate in unison, with the ability to register multi-touch
Each capacitor can register its own touch points

Self Capacitance (Derivative of Projected Capacitance)

Similar to Mutual capacitance, but does not register multiple touch points
Each column and row on the screen operates independently

Resistive Touch Screens

Made with two flexible sheets, separated by a tiny air gap
When contact is made on the screens, the two sheets touch, and registers the location of the finger
Allows a stylus to be used
Can be made to support multi-touch
A stylus must be used to perform motor-intensive tasks (i.e. manipulating a small portion on the screen, writing on the screen)
Usually made of glass or acrylic
Acrylic-based screens prone to scratches
Limited clarity

Infrared Touch Screens

Uses infrared LEDs that are arranged around the perimeter of the screen to detect changes in the infrared beam
Good for outdoor use. Common in POS (point-of-sale) systems
Does not rely on a bare finger to activate the touch screen
Can detect touch from a stylus, or a gloved hand

Surface Acoustic Wave (SAW)

Uses ultrasonic waves that pass over the screen. When a finger touches the screen, part of
the wave is absorbed. The change in the waves determines where the touch occurred
Good for large surfaces
Can recognize "touch" gestures with a finger, stylus, or gloved hand

Optical Touch Screens

Rely on imaging sensors positioned on the corners of the screen
Infrared backlight captures actions on the screen
A touch registers as a shadow, and uses the imaging sensors to triangulate the position of contact
Gaining popularity due to lower cost and scalability
Suitable for large screens
Lorex uses optical touch screens with touch-enabled DVR systems

Single Touch vs Multi Touch Gestures

Lorex optical touch screens recognize single and multi touch gestures. Touch gestures are available on touch-enabled DVRs.

Single touch gestures require one finger (i.e. panning, dragging, flicks). Multi touch gestures require two fingers (i.e. Zoom, rotate, two finger tap).

Popular Touch Screen Technologies

The iPhone uses both Mutual Capacitance and Self Capacitance technology to achieve multi touch gestures. Mutual Capacitance requires two layers of material: one layer for the Driving Lines, one layer for the Sensing Lines. Driving Lines provide current for the touch screen. Sensing Lines detect the current for the electric node layer. When the finger touches the screen, the Sensing Lines determine the location of where the touch occurred. Every point on the grid generates its own signal. From the grid, touch information is relayed back into the iPhone’s processor. The iPhone’s processor is integral in how the system interprets a touch on the screen. The Sensing Lines and Driving Lines creates a coordinate grid for the iPhone CPU. This allows the phone to locate the specific area where the touch occurred. Because iPhone relies on a capacitive touch screen, the screen operates only when a bare finger touches the screen. A gloved hand, or stylus will not activate a capacitive touch screen. The combination of Mutual and Self Capacitance technology allows the iPhone processor to register multi-touch functions.
		Mutual Capacitance screens rely on a grid system to determine where the touch occurs. Note that the diagram is not to scale. Image courtesy of HowStuffWorks, 2007

		Self Capacitance screens use a transparent electrode layer to register screen contact. Note that the diagram is not to scale. Image courtesy of HowStuffWorks, 2007

How the iPhone interprets touch signals

After collecting touch signal data from the screen, the iPhone processor must interpret the raw touch data into a gesture. This instantaneous process is broken into several steps: The screen interprets a touch on the screen as electrical signals. These electrical signals are sent to the iPhone processor. The iPhone processor software analyzes the size, shape and location of the touch. If you swipe your finger along the screen, the software determines the start and end points of the touch. The processor uses ‘‘gesture-interpretation‘‘ software to determine the type of touch on the screen (i.e. swipe, scroll, flick etc.) The processor relays the data to the program that you are using. The simplified explanation above explains the key concepts of a multi-touch platform like the iPhone. Touch screens may interpret touch data in a different way.

		Images courtesy of HowStuffWorks, 2007