The screens of two iPhones.
Touch screens are everywhere nowadays, from smart phones and tablets to dashboard screens and ATM terminals. But how exactly do they function? In this article, we will take a look at the basic principles and technologies involved in touch screens.
The main principle behind a touch screen is to detect where a person touches the screen. There are many types of technology to detect touch, and each one detects touch differently. The most well-known types of touch screen technology are resistive, capacitive, and infrared.
First, as mentioned above, there are resistive touch screens. They’re made up of two layers of conductive material, and between those two layers, there is a very small gap. When you press your finger down on the screen, the top and bottom surfaces meet, making an "electrical path" at that one spot. The precise location of where you touched is easily found because both of these surfaces have a grid of electrical conductors. When they meet, they create a circuit (at exactly where you applied pressure). This information is then read by the controller, which translates the touch into an action on the device.
Next, we have capacitive touch screens, which is probably what your smartphone or tablet uses. These screens use a layer of material that can store electrical charge. Your finger, which typically has some electrical conductivity, changes the distribution of charge on this layer when you touch it. The controller monitors capacitance change, which is the system's ability to hold an electric charge, at many points on the screen. When you touch the screen, you create a change in capacitance at the contact point, which the controller can read and interpret as touch input. This is why when you touch your phone while wearing a glove, which is usually non-conductive, it doesn’t respond. Capacitive touch screens are more responsive and can handle multi-touch input (pinching, zoom, etc.) because they can detect several capacitance changes simultaneously.
On the other hand, infrared touch screens operate on a completely different principle than the ones we talked about earlier. They employ a grid of infrared LEDs and photodetectors around the edge of the screen. The LEDs emit infrared light beams onto the screen, and the photodetectors on the opposite side detect these beams. When you touch the screen, these beams are broken by your finger. The controller then determines where the touch was by analyzing which beams were interrupted. This is a highly accurate method for registering touches, since it's not sensitive to the materials of the screen itself but rather to the physical interruption of light paths.
But, how does the controller translate these touches into something useful? Well, it uses coordinates. When you press the screen, the controller gets data from the light beams or touch sensors about where you pressed on the screen. That data is usually, simple put, in the form of x and y coordinates. The controller then compares those coordinates to the corresponding actions or objects on the screen. For instance, if you tap on an icon using your finger, the controller knows where the icon is and can execute the corresponding action, which, in this case, could be launching an application or navigating to a new page.
The processing speed of such touch events is crucial to responsiveness. Obviously, when you touch a screen, you want a quick response, but the speed all depends on the quality and power of the parts that make up your touch screen and device.
Touch screen technology also includes sensitivity and durability layers. Most touch screens incorporate a scratch-resistant layer on top to prevent scratches and endure wear from repeated touching. Below this, there can be an anti-glare coating to improve visibility in varying lighting conditions. For capacitive screens, there is also the need for an extremely thin layer that can hold and transfer electrical charge well.
Calibration is another aspect of touch screen functionality. Because of time, or due to manufacturing variations, the touch screen can become unresponsive to touches. Calibration is the act of configuring the system so that the coordinates of where the user touches are synchronized with the visual objects on the screen. Users can go through this process when they first start up their device or if the screen starts to feel out of place.
In terms of how physical contact is made with the display, whether it be through fingers, styluses, or even gloves, it depends on the technology. Resistive screens can sense pressure and can therefore be effectively used with gloves or a stylus. Capacitive screens usually require direct skin contact, although they can be manufactured to sense the touch of gloves or styluses specifically made to conduct electricity. Infrared screens are less sensitive when it comes to the types of objects that can be used for touch, since they operate solely based on the interruption of light. In modern times, touch screens also have features like palm rejection, where the system can distinguish deliberate touches from unintentional ones (eg. resting your palm on the screen while drawing). This is especially handy in devices meant for drawing or note-taking.