Among these advancements, perhaps one of the most ingenious is “touch-screen” technology. Though the touch-screen is utilized on everything from laptop computers, GPS units, MP3 players, digital cameras, and even printers, most anyone would agree that Apple’s iPhone was the device most instrumental in bringing touch-screen technology to the masses. However, it’s a safe bet that most of us have no idea how the technology in an iPhone’s touch-screen actually works.
In all reality, the basic idea is pretty simple: when you place your finger or a stylus on the touch screen, it changes the state that the device is monitoring. In screens that rely on sound or light waves, your finger physically blocks or reflects some of the waves. Capacitive touch-screens use a layer of capacitive material to hold an electrical charge; touching the screen changes the amount of charge at a specific point of contact. In resistive screens, the pressure from your finger causes conductive and resistive layers of circuitry to touch each other, changing the circuits' resistance.
Whereas most touch-screen systems are good at detecting the location of exactly one touch, which can result in some pretty erratic behavior if another finger happens to touch on the screen accidentally, the touch screen on an Apple iPhone is different. Many of the elements of the iPhone’s “multi-touch” user interface actually require you to touch multiple points on the screen simultaneously. For example, you can zoom in to web pages or pictures by placing your thumb and finger on the screen and spreading them apart. To zoom back out, you can pinch your thumb and finger together. The iPhone's touch screen is able to respond to both touch points and their movements simultaneously.
To allow people to use touch commands that require multiple fingers, the iPhone uses an innovative arrangement of preexisting technology. Its touch-sensitive screen includes a layer of capacitive material, just like many other touch-screens. However, the iPhone's capacitors are arranged according to a coordinate system. Its circuitry can sense changes at each point along the grid. In other words, every point on the grid generates its own signal when touched and relays that signal to the iPhone's processor. This allows the phone to determine the location and movement of simultaneous touches in multiple locations. Because of its reliance on this capacitive material, the iPhone works only if you touch it with your fingertip. Surprisingly enough, the iPhone’s multi-touch screen won't work if you use a stylus or wear non-conductive gloves.
At its base, the iPhone's screen detects touch through one of two methods: Mutual capacitance or self capacitance. In mutual capacitance, the capacitive circuitry requires two distinct layers of material. One houses driving lines, which carry current, and the other houses sensing lines, which detect the current at nodes. Self capacitance uses one layer of individual electrodes connected with capacitance-sensing circuitry.
Both of these possible setups send touch data as electrical impulses.
However, the iPhone's processor and software are essential to accurately interpreting input sent from the touch-screen. The capacitive material sends raw touch-location data to the iPhone's processor. The processor uses software located in the iPhone's memory to interpret the raw data as commands and gestures.
What follows is a step-by-step explanation of this process:
1.) Signals travel from the touch screen to the processor as electrical impulses.
2.) The processor uses software to analyze the data and determine the features of each touch. This includes size, shape and location of the affected area on the screen. If necessary, the processor arranges touches with similar features into groups. If you move your finger, the processor calculates the difference between the starting point and ending point of your touch.
3.) The processor uses its gesture-interpretation software to determine which gesture you made. It combines your physical movement with information about which application you were using and what the application was doing when you touched the screen.
4.) The processor relays your instructions to the program in use. If necessary, it also sends commands to the iPhone's screen and other hardware. If the raw data doesn't match any applicable gestures or commands, the iPhone disregards it as an extraneous touch.
All these steps happen in an instant, much like the interpretation of electrical impulses in the human brain. You see changes in the screen based on your input almost instantaneously. This process allows you to access and use all of the iPhone's applications with your fingers and all but eliminates the need for raised button operations of the past.
Information courtesy of Discovery’s HowStuffWorks.com