UK How a touch screen works
A touch screen is a display with a transparent sensor in front of it, or around its edges, that detects where it is touched. Every touch screen has three parts. The sensor turns a touch into an electrical signal. The touch controller, a small chip on a ribbon cable or circuit board, measures that signal many times a second and converts it into X and Y coordinates. The driver in the operating system then treats those coordinates like a mouse click, or as several fingers at once for gestures.
The sensor is where touch technologies differ. Some sense the pressure of a finger, some the electrical charge it carries, and some the way it blocks sound waves or light. Each method has different strengths for clarity, gloves, water, size and cost, which is why industrial displays still use several of them.
The idea is older than most people expect. E. A. Johnson at the Royal Radar Establishment in Malvern described a capacitive touch display in 1965, and it was used for UK air traffic control. Resistive touch followed in the early 1970s, from work by Samuel Hurst in the United States.
The main types of touch screen
Five sensing methods cover almost every touch screen in use today.
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Resistive. Two layers coated with a transparent conductor are held apart by tiny spacer dots. Pressing the top film makes them touch, and the controller reads the position from the voltage at that point. Anything that presses works: a finger, a thick glove or any stylus.
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Surface capacitive. A conductive coating on glass has a small voltage at each corner. A finger draws a tiny current, and the controller locates the touch from how much flows from each corner. Single touch only, and now mostly found on older kiosks.
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Projected capacitive (PCAP). A fine grid of transparent electrodes sits behind the cover glass. A finger changes the capacitance where the rows and columns cross, so the controller can track ten or more touches at once. This is the touch on phones, tablets and most new panel PCs.
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Surface acoustic wave (SAW). Transducers send ultrasonic waves across the surface of a plain glass panel. A finger absorbs part of the wave, and the controller finds the touch from the timing of the dip. The glass front gives a very clear image.
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Infrared (IR). LEDs and light sensors in the bezel make a grid of invisible beams just above the screen. Any object that breaks the beams is detected, so IR scales to very large displays without a sensor layer over the panel. Camera-based optical touch works on a similar principle.
Touch technologies compared
The table shows how the technologies behave in practice. The figures are typical; the cover glass, coatings and controller can change them.
| Technology | What it senses | Works with | Multi-touch | Front surface | Usual sizes |
|---|---|---|---|---|---|
| Resistive | Pressure | Finger, any glove, any stylus | Usually single touch | Plastic film | Small to around 22″ |
| Surface capacitive | Current drawn by a finger | Bare finger | No | Coated glass | Kiosk sizes |
| Projected capacitive | Change in capacitance | Finger, thin gloves, capacitive stylus | Yes, typically 10 points | Glass | 7″ to 86″ and more |
| Surface acoustic wave | Absorbed ultrasonic wave | Finger, soft-tipped stylus | Limited, often two points | Plain glass | Around 10″ to 32″ |
| Infrared | Interrupted light beams | Any object | Yes, on most current frames | Any, with a raised bezel | Large format, 32″ to 100″+ |
What the touch controller does
The controller is where raw sensor readings become usable input. It scans the sensor, typically 100 times a second or more, filters out electrical noise, works out the position of each touch and reports it to the computer. Most controllers connect over USB, or over I²C on embedded boards, and present themselves as a standard HID touch device, so Windows and Linux recognise them without a special driver in most cases.
The controller firmware also holds the settings that matter on an industrial screen: sensitivity for gloves, rejection of water droplets, palm rejection and how many touch points to report. These are agreed when the screen is specified and set before it is installed.
Touch screen calibration lines up the touch coordinates with the image underneath. Resistive screens can drift as the film ages, so they may need recalibrating with a few taps on target points. Projected capacitive and SAW screens are calibrated at the factory and rarely need it, so if one starts registering touches in the wrong place, the cause is usually the display settings or a fault rather than drift.
Choosing touch for industrial use
The environment usually decides the technology before price does. Start with what operators wear, what lands on the screen and how big it needs to be.
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Gloves and tools. Resistive works with any glove. PCAP handles thin gloves as standard and thicker ones with a glove mode in the controller.
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Water and washdown. PCAP with water rejection, or resistive, suits wet areas. SAW is the least tolerant of water and dirt on the glass.
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Impact and vandalism. Thick strengthened glass in front of a PCAP sensor survives knocks that would tear a resistive film.
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Very large screens. Infrared and PCAP both scale to video-wall and meeting-room sizes; IR adds a raised frame around the glass.
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Sunlight. Fewer layers and an optically bonded cover glass keep reflections down, which favours PCAP outdoors.
Touch screens in our range
Our PCAP touch panel PCs cover most new HMI and operator-station designs, with resistive touch monitors for glove-heavy and legacy single-touch applications and SAW touch monitors for clean, indoor kiosk-style use. The full spread of sizes and mounting styles is in our industrial touch screen monitors.
For a deeper look at matching the technology to the application, read our guide to choosing the right touch technology for an industrial display.
Quick answers on touch screens
Touch screens: common questions
How does a touch screen know where you touch?
The controller measures it. It reads the sensor many times a second and calculates the X and Y position from the change in voltage, capacitance, sound or light at each point, then sends those coordinates to the computer.
Do touch screens detect heat?
No. Capacitive screens respond to the electrical conductivity of skin, not its temperature. That is why a capacitive stylus works and a fingernail does not.
Why does a touch screen not work with gloves?
Most gloves insulate the finger. A capacitive screen needs a conductive touch, so thick gloves block it unless the controller has a glove mode. Touch screen gloves work by weaving conductive thread into the fingertips.
Can you add touch to an existing monitor?
Yes, with an overlay. Infrared frames and some PCAP overlays fit over a standard display and connect by USB, although a touch display built as one assembly is usually clearer and better sealed.
Which touch screen technology is most common?
Projected capacitive. PCAP is on almost every phone and tablet, and it has become the default for new industrial panel PCs and touch monitors too.
We supply every panel PC and touch monitor with the touch controller configured and its driver installed as standard, including glove and water settings where the controller supports them, and we test touch response before the system ships.