How touch screens work

touchscreens 2 - for some reason we don't have an alt tag here

I have mentioned briefly the differences between resistive and capacitive touch screens, but this is a very important subject that can change the user experience of a tablet complete, so I think it merits a deeper explanation.

Resistive touch screens

Resistive touch screens are basically the old ones, the technology that used to be on all touch screen devices but is slowly dying. Touch screen is actually not a very good description of this kind of screen as it is more of a pressure sensitive screen than it is a touch sensitive screen. Here in Norway there are actually two different terms that loosely translate to push screen and touch screen, though they are used synonymously and don’t indicate what technology it refers to- unfortunately.

Resistive touch screens work by having several layers on top of the base LCD screen, and they sense touch input when two of these layers are pressed together. Normally resistive touch screens are single touch only because of the inherent problems that multi touch brings; if you press down on two points on the screen, you will also press down everything in a line between those points. Resistive touch screens also react to only one (small) pressure point that doesn’t take into the account the size of the object pushing down, which means that it doesn’t know the difference between using a stylus or using your entire fist on the screen- it will see either as a single small point of contact. .

This is also why resistive touch screens are often inaccurate when using your finger; the finger is soft, so when you press down on the screen the screen interprets that as something “rolling” down the screen, as the finger creates more and more points of contact as more of it touches the screen in that split second when you press down.

Since there is physical movement involved when pressing down on a resistive screen, there is also some resistance in the screen and you need to apply a bit of pressure to make it work.  Capacitive screens on the other hand will notice you touching them as soon as there is physical contact between the finger and the screen. This might not seem like much of a difference, but you quickly notice the difference when typing on an on-screen keyboard or something similar. Compared to a capacitive touch screen, using a resistive touch screen feels like you have to force the screen to cooperate.

If you do have to deal with a resistive touch screen, a stylus is recommended to get around all these problems with using a finger. If the touch screen is of good quality, using a stylus on it might actually be quick a nice experience especially if you’re drawing or doing any so-called inking, handwriting on the screen. However a stylus is not very comfortable to use for things like on-screen keyboards, games etc- imagine playing a video game console using two pencils on the controller!

Another very big problem with resistive touch screens is that they degrade over time, shift from their default position and become “bent”. This is often noticeable if you use a drawing application and you suddenly notice lines pop up other places on the screen than they should. This is because the touch screen is no longer flat and so when you press down on one side of the screen it makes contact on another part of the screen because the plastic layer is slightly bent. Similar problems might also occur if the device casing is poorly put together or cheaply made, where touching the screen frame actually causes touch input on the screen. Finally, since there are several layers to this type of touch screen the whole touch layer part might shift and cause inconsistencies between where you press down and where the device thinks you press down- in other words, there is a small offset between where you touch and where the screen tells you that you touched. That’s why resistive touch screens need to be calibrated, meaning it shows you some crosses and tells you to touch the middle of those with a stylus so it can tell where the touch layer is in relation to the screen.

Because you need to be able to press down on the screen, resistive touch screens are always less protected than capacitive screens, with softer materials on top which is easier to scratch. Screen protectors will not hinder the operation of the touch screen, but you might actually damage the touch layer by pulling on off if it’s on good.

Capacitive touch screens

Capacitive touch screens really made an entry into the market with the first iPhone and are becoming the default for most touch devices today, at least over a certain price point. Unlike a resistive pressure sensitive screen, a capacitive screen doesn’t require any physical interaction between layers to detect touch input. Instead, it relies on the electrical properties of your body (and certain other materials) to determine when something is touching the screen. It can detect several fingers touching the screen (multi touch) as well as the shape of the thing that’s touching the screen. With the latter and using software a device with a capacitive screen can then calculate the middle of your finger (where you’re most likely to be aiming), simulate pressure sensitivity (by seeing how much of a finger/stylus etc is touching the screen) as well as ignore your palm when you’re writing (by concluding that the object touching the screen at that point is way too large to be a finger).

By being aware of shapes a capacitive touch screen device can also avoid such issues as mentioned above where a finger pressing down is interpreted as drawing a line. It can see your finger and will distinguish between that and a line by looking at the shape and time it took that shape to form. This is all in the software of course but possible because of the way a capacitive screen works.

Since there is no physical movement involved, the screens can be much harder than with resistive screens- often made of glass, even. This also means you get rid of the problem with the finger having to travel that extra distance and press that much harder to trigger the reaction from the device, which combined with the ability to guess where you’re most likely trying to touch makes it a much more accurate technology for finger input. The downside is you need special styli if you do want to use that, and they’re not as accurate.

Which is best?

Normally I don’t like to say that one thing is definitely better than another, but in this case resistive screens have very little to offer compared to capacitive ones. They’re better if you need to use a stylus, but that’s basically it. There is after all a reason why all the respectable brands choose capacitive for their screens these days, resistive screens simply don’t work as well for most uses.

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Andreas Ødegård

Andreas Ødegård is more interested in aftermarket (and user created) software and hardware than chasing the latest gadgets. His day job as a teacher keeps him interested in education tech and takes up most of his time.

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