How a Phone Touchscreen Senses Your Finger
Most phone screens detect touch by measuring tiny changes in an electric field. Here is how capacitive screens work, why gloves fail and how resistive screens differ.
In this article
Most phone screens sense your finger by measuring a tiny change in an electric field, not by feeling pressure. A grid of transparent wires under the glass is checked many times a second, and the spot where the signal changes is the spot you touched. This article explains that method, called capacitive sensing, and compares it with the older resistive design.
The two main designs
Wikipedia's overview of touchscreens describes two broad families. A resistive screen has two flexible, conductive layers separated by a small gap. Pressing the top layer makes it touch the bottom one, and the controller works out where. A capacitive screen has a transparent conductor on glass and notices when a conductive object, such as a finger, disturbs its electric field.
Resistive screens are cheap, work with gloves or any firm stylus, and cope with liquids and dirt. They need firmer pressure and are mainly used for single touches. Capacitive screens respond to a light touch and are the basis of most smartphone displays.
What capacitance means
Capacitance is the ability of nearby conductors to hold electric charge. Two conductive surfaces placed close together, with an insulator between them, form a capacitor. The human body conducts electricity, so a fingertip near one of these surfaces changes how much charge it can hold. A touch controller is built to detect that change.
Mutual capacitance: the grid
Wikipedia describes mutual capacitance as rows and columns of conductors that cross each other. The controller sends pulses along one set and listens on the other. Each place where a row crosses a column behaves like a small capacitor and acts as a sensing point.
When a finger is near an intersection, it lowers the capacitance there. The controller measures every intersection in turn and sees which ones changed. A grid with 16 rows and 14 columns would have 224 such points, according to one touchscreen maker's explainer, Nelson Miller. More intersections give finer resolution and let the screen follow more fingers at once.
Self-capacitance
The second method measures each trace on its own against the ground. Wikipedia says it is more sensitive and suits single touches, simple gestures and sensing a finger that hovers nearby, but it cannot pinpoint a location by itself when several fingers are down. Many products combine both methods.
Why multi-touch works
Pinching to zoom needs the screen to follow two or more fingers at once. Mutual capacitance does this well because every intersection is measured separately. Wikipedia names it as the main approach for tracking several fingers. Some resistive screens can do multi-touch, but they need many more connections.
Why gloves and water cause trouble
A standard capacitive screen needs a conductive touch. Wikipedia notes that gloves made of insulating material block the field, so the screen often ignores them. Workarounds include styluses with conductive tips and gloves with conductive thread. Some devices offer a high-sensitivity mode, which helps with gloves but can register touches that did not happen.
Moisture is another weakness. Nelson Miller says mutual capacitive screens are susceptible to moisture and suggests resistive screens for humid or outdoor use. A water drop can look like a touch to the controller, which is why wet screens sometimes behave oddly.
A short history
According to Wikipedia, E. A. Johnson of the Royal Radar Establishment in the United Kingdom described the first finger-driven capacitive touchscreen in 1965 and published a fuller account in 1967. At CERN, Bent Stumpe and Frank Beck developed a self-capacitance screen in 1972 and a mutual-capacitance one in 1977, and screens were in use in CERN's control room from 1973. A transparent resistive screen from Elographics and Siemens was shown at the 1982 World's Fair in Knoxville, and a University of Toronto team built an early multi-touch system the same year.
Surface capacitance
Wikipedia also describes surface capacitance, where only one side of an insulating panel is coated and the controller measures the change at the four corners. It is durable and has no moving parts, but its resolution is limited and it needs calibration, so it is mostly found in industrial controls and kiosks.
Summary
- Resistive: two layers pressed together; works with gloves; needs firmer pressure.
- Surface capacitive: corner readings; durable; limited resolution.
- Projected, mutual capacitive: a grid of crossings; light touch; multi-touch; used on phones.
- Self-capacitive: each trace alone; sensitive; used for simple touches and hover detection.
Phone makers do not all publish the details of their sensor layouts, so the exact grid sizes and signal processing in a particular phone are not public. The principle described here is the general one.
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