Capacitive or resistive: what’s the best type of touchscreen?

I’m going to deal with a topic that’s sparked more than a dozen emails recently. A typical one is Steven Barrett, who asks:

“I keep reading about capacitive and resistive touchscreens, but I’m not sure what the real-world differences are. Capacitive screens generally receive more favourable reviews than resistive, but I’ve seen some strong views in the other direction on various blogs and online forums, with people saying that resistive screens are more accurate. I’d appreciate your views on which screen technology to choose.”

Well, Steve, that’s quite a can of worms you’ve just opened, and it’s worth taking a quick refresher on how both technologies work. The resistive touchscreen is the older technology, at least in the smartphone arena.

The front surface is made of scratch-resistant, flexible plastic with a thin film of conductive material (usually Indium Tin Oxide or ITO) printed onto its underside. Beneath it is a second layer – usually made of glass, but sometimes of hard plastic – also with a coating of ITO.

The two layers are kept apart by tiny bumps or spacers placed at regular intervals, and the thin layers of ITO create an appreciable electrical resistance – the sandwich is so constructed that electrical charge runs from top to bottom on one layer but side-to-side on the other layer.

When the screen is touched the plastic deforms so that the two ITO films meet, and by measuring the resistance of both layers at their point of contact it’s possible to get an accurate measurement of the touch position. This, of course, relies on an even coating of ITO on the layers, plus accurate calibration: with some early touchscreen mobiles, the calibration could drift as the battery became depleted, but nowadays, unless you buy a fake phone, you shouldn’t experience this problem.

Most older phones use resistive screens, but that isn’t to say it’s an out-of-date technology, as phones are still being churned out using this type of screen (a good clue is normally, although not always, that the device is supplied with a stylus). Most people probably first encounter resistive screens in Windows Mobile devices (apart from the HTC HD2!).

There are two types of capacitive touchscreen generally available, surface and projected, and it’s the latter that you’ll find in smartphones. These again consist of a sandwich, but this time of two spaced layers of glass, again coated with ITO on the inside.

Depending on the particular screen, the ITO layer may be a uniform coat, a grid, or parallel stripes running at right angles on the two sheets. The latter scheme is used in the iPhone and the iPod Touch Duplo, better known as the iPad.

Think back to O Level physics, and you might remember that a capacitor consists of two plates separated by an insulating material, which may of course be air. Now picture those perpendicular stripes on two glass plates – wherever a stripe crosses one below it forms a capacitor so small it’s measured in femtofarads (10-15F).

This small size is both bad news and good: bad, because such a tiny capacitance is difficult to measure and requires complex filtering to eliminate noise; good, because given such a small capacitance it isn’t just the gap between the “plates” that affects the capacitance but also the space around them.

As your finger comes close to a capacitor it changes the local electrostatic field, and the system constantly monitors each tiny capacitor to discover exactly where the finger touched the screen: because the measurement points are discrete, it’s possible to tell whether several fingers are all touching the screen at once, unlike with a resistive unit.
In the real world

So what’s the difference between these two technologies in practice? First and foremost, resistive screens tend to be stylus-friendly, while capacitive screens favour a swipe with a finger. That’s a generalisation, because some manufacturers have recently made resistive screens that are more finger-friendly, while some clever people have come up with conductive styluses that can work (kind of) on capacitive screens.

For finger-based user interfaces capacitive is still far better, though, while if you need the single-pixel accuracy of a stylus then resistive is the sensible choice.

The fact capacitive screens can sense more than one finger press at a time brings me to their major advantage – they can support multitouch interfaces. For those of you who’ve never used such an interface, I can best describe it as one of those light-bulb-over-head revelations. The classic demonstration is in Google Maps, where if you want to zoom in on an area you simply make a pinching gesture on the screen and the map zooms proportionally: spread fingers outwards to zoom in the other direction.

It’s a brilliant and intuitive control mechanism. (And yes, I do know that around a year ago a couple of companies demonstrated ways to get multitouch on a resistive screen, but so far as I’m aware, it hasn’t been implemented on a smartphone yet.)

One area where resistive screens win out is on price, since capacitive screens (plus their associated controller chips and other trimmings) usually cost around half as much again as their resistive counterparts. This isn’t too significant in a high-end smartphone where the margins tend to be pretty large, but it becomes an issue for entry-level devices.

The question as to which technology is more robust is an interesting one. Since the front element of a resistive screen needs to be flexible, it will be some type of plastic, but as anyone who’s ever been sold a “scratch-resistant” coating on their specs will testify, there’s no such thing.

On the other hand, capacitive screens, being made of glass, are more susceptible to shattering by sharp knocks. I have a friend who keeps her iPhone in her handbag, along with all the other junk that women keep in there – she’s now on her third one, thanks to sharp objects such as keys, scissors, tweezers, pens and so on.

Of course, she should keep the phone in some sort of protective case, but that’s a bit of a cop-out as the whole point of a modern phone is to have it readily to hand.

Protective cases raise an interesting point. For a resistive touchscreen you can buy snugly fitting skins in which the screen area is a thin clear window, and because it’s flexible the touchscreen will work through it. It won’t be as responsive, but it will work. The same trick can’t be employed for capacitive screens, where the tolerances are so tiny that any sort of barrier will prevent operation.

You’ll also find that capacitive screens won’t work if you’re wearing gloves (although some manufacturers now make gloves with electrically conductive finger tips). It’s also pretty well impossible to use a capacitive screen in the rain.

Resistive screens win when it comes to accuracy, since a well-calibrated screen should be able to detect stylus position to within a single pixel. However, because the touch surface is actually a millimetre or so above the LCD, there’ll be a potential parallax error: calibrate it while viewing absolutely square on and it will still be a pixel or two out when you use it at an angle.
There’s no point in worrying about pixel accuracy on a capacitive screen because the sensor simply can’t produce that degree of precision – at best giving a reading accurate to two or three pixels in either direction.

In any case, the fact you’re using a blunt finger rather than a pointed stylus means that capacitive screens simply aren’t suitable for tasks that require precise on-screen pointing. An important consequence is that while handwriting recognition works very well on a resistive screen (even using a fingernail in place of the stylus), all attempts to do such recognition on capacitive screens have so far proved hopeless.

When it comes to visibility, there isn’t much difference between the types of screen when used indoors. Outdoors, however, you’ll usually find that capacitive screens look better in bright sunlight. Both types will always look worse than a non-touchscreen, because all those layers of glass and plastic placed in front of the image will scatter the bright ambient light.

This then is a genuine case of horses-for-courses, with both technologies possessing significant pros and cons. In my opinion, if you’re running an ancient OS such as Windows Mobile 6.x with its tiny controls and screen furniture, you really need a resistive screen; for a modern, touch-friendly OS such as Android you’ll almost certainly find a capacitive screen works best.

However, there’s a gap in the middle occupied by those folk who’d like a finger-friendly device, but occasionally need to RDP into their server at work, for example (an activity that requires precise mouse control).

Or perhaps you’re a property agent who needs to sketch rough floor plans onto your phone. For users such as this the best choice currently isn’t obvious, and they may well do better to stick with a resistive screen.