How Do Touchscreens Work?

How a resistive touchscreen works

A resistive touchscreen usually has at least three layers, built around a flexible plastic film (PET) suspended over a rigid sheet of glass or acrylic.^[4] Both facing surfaces carry a transparent conductive coating such as indium tin oxide (ITO), and tiny spacer dots keep them apart, so no current flows between them until someone presses the top sheet.^[4] In effect, the screen is a switch that closes under a force of about 0.1 to 1.5 newtons.^[4]

To find the touch, the controller applies a voltage across one layer, creating an even voltage gradient from one edge to the other.^[4] The second layer, now touching the first at the pressed point, picks up a voltage that shows where along that gradient the touch is.^[4] The controller then repeats this in the other direction to get the second coordinate.^[4] Four-wire designs can also estimate pressure, because the resistance between the two layers falls as the pressure or the pressed area grows.^[4]

Because a resistive screen responds to pressure rather than to electrical conductivity, it works with a stylus as well as a finger.^[1] Ars Technica describes resistive screens as cheap to produce and durable enough for restaurants, factories and hospitals.^[1] On early phones, Wired reported, users had to push hard with a fingernail or stylus to get a response.^[6]

How a capacitive touchscreen senses a finger

A capacitive screen does not depend on pressure; it responds to the electrical properties of the skin.^[6] The panel is an insulator such as glass coated with a transparent conductor such as ITO, and the human finger that touches it is a good electrical conductor.^[1] A touching finger disturbs the electric field of the conductive layer, and that change can be measured as a change in capacitance, the ability to store electrical charge.^[2][5][6]

In Ars Technica's 2013 description of a phone screen, a thin, scratch-resistant cover of glass or plastic sits on top, with the capacitive layer and a thinner layer of glass beneath it and the LCD panel at the bottom.^[2]

The change is small: in example figures from chipmaker Microchip Technology, an untouched electrode has a capacitance of about 100 picofarads, and a finger directly over it changes that by only 0.5 to 1.0 picofarad.^[5]

Why gloves and plastic styluses usually don't work

The iPhone's capacitive screen was designed to work with the electrical conductivity of a bare human finger.^[7] The New York Times explained in 2009 that anything that blocks or disrupts the finger's small electrical charge, such as winter gloves, long fingernails or wet hands, can make such a screen react erratically or not at all.^[7] For the same reason, it ignores a stylus made of non-conductive hard plastic; a stylus for a capacitive screen has to be made of conductive material.^[7] Resistive screens do not need a conductive touch, because pressure alone closes the circuit.^[1][4]

Projected capacitance and multi-touch

Wired reported in 2010 that phones usually used projected capacitive screens, whose transparent conductive layer is etched into a grid-like pattern.^[6] A single capacitive layer can accurately detect only one touch at a time.^[2] To register several fingers, the sensor uses two layers of electrodes, one acting as transmitters and one as receivers, running at right angles to form a grid.^[2][5]

Each crossing point of a top-layer and a bottom-layer electrode is called a node.^[5] A finger near a node provides a path to ground that draws away some of the current the transmitter sends toward the receiver.^[5] Measuring node by node lets the system track several touches at once.^[5] Ars Technica reported on April 17, 2013, that most phones and tablets handled between two and ten simultaneous touch points.^[2]

How the controller works out where you touched

The controller is a chip that turns these measurements into a location so the operating system can respond.^[2] Measuring each row and column electrode on its own, called self-capacitance, can locate one touch but does not easily separate several.^[5] Measuring every node, called mutual capacitance, tracks multiple touches but responds more slowly, so Microchip's design uses self-capacitance to find the touched rows and columns and then measures only their nodes.^[5]

Because the screen's own capacitance varies in ways that cannot be controlled, a touch is detected by measuring how far each reading differs from a stored "no-touch" baseline and comparing that difference with a threshold.^[5] If the electrodes are 5 mm apart, the coarse resolution is 5 mm, so the controller interpolates, estimating positions between neighbouring electrodes from the relative strength of their signals.^[5] It then sends the coordinates to the host device.^[5]

A brief history of the touchscreen

Historians generally credit E.A. Johnson of the Royal Radar Establishment in Malvern, United Kingdom, with the first finger-driven touchscreen, in 1965.^[1] It used what is now called capacitive touch, could handle only one touch at a time, and was later adopted by UK air traffic controllers, who used it until the late 1990s.^[1]

At CERN, Bent Stumpe proposed a capacitive touch screen in a handwritten note dated 11 March 1972.^[3] When CERN's Super Proton Synchrotron started up in 1976, its control room was fully equipped with these screens, which CERN Courier called apparently the first application of the capacitive touch screen in the world.^[3] The accounts differ: Ars Technica says Johnson's earlier screen went into UK air traffic control, without giving a date.^[1][3]

Resistive screens grew from work by the American inventor G. Samuel Hurst and colleagues at the University of Kentucky.^[1] After returning to Oak Ridge National Laboratory in 1970, Hurst and nine friends refined the idea after hours in a venture they called Elographics.^[1] Although capacitive touchscreens came first, resistive ones dominated the early years.^[1] Ars Technica reports that the first multitouch screen was developed at Bell Labs in 1984, using a transparent capacitive array of touch sensors over a CRT display.^[1] A second Ars Technica article credits the iPhone's 2007 introduction with turning smartphones and tablets from niche products into billion-dollar industries.^[2]