The Science Behind Touchscreens
Originally Written for Princeton Innovation Science Magazine in Spring 2015
It’s a cold winter day; you’re on your way to class when all of a sudden you get a text. You take out your phone, but with your gloves on, your phone’s touchscreen won’t react. You begrudgingly take off your glove to reply to your friend, wishing that you had bought one of those fancy “touchscreen-friendly” gloves instead. The question then arises: What makes “touchscreen-friendly” gloves different from your “normal” ones? Why do some screens need special gloves when others don’t?
To answer these questions, we must look at a fundamental question: How do touchscreens even work? With the rise of touchscreens in nearly every aspect of life, sometimes one comes to wonder how such a magical screen came to exist in the first place. While hard to believe, touchscreens have existed since the 1970s. In general, all touchscreens follow the same principle — they figure out where your fingers are on the screen. However, how they do that can differ greatly. Here, we look at two of the most popular types of touchscreens: resistive and capacitive.
Resistive Screens
Devices with resistive screens have two layers. The screen is a flexible sheet of plastic coated with a conductive material (such as Indium Tin Oxide) in thin horizontal lines lies on the bottom. Beneath the screen, another surface is coated with a similar material in vertical lines. When you touch the screen, you make a dent in the screen so that the top and bottom layers touch. Once the two layers touch, a set of horizontal lines on the top will touch the vertical lines on the bottom. This will cause a disturbance in how the electricity is being channeled through the lines. The touchscreen device senses this disturbance and can then calculate where the dent was made. With these types of screens, you don’t need any special gloves to use them. You only need to apply some pressure, and voila!
Capacitive Screens
The types of screens that require special gloves are capacitive screens. These screens, like resistive screens, have two layers coated with a conductive material in a vertical/horizontal formation. However, unlike resistive screens, these surfaces are hard and are usually made out of glass. Electricity is sent alternatively through the top and bottom layers, and the voltages (the amount of electrical energy) in each location are measured correspondingly. Believe it or not, people also hold small amounts of electrical charge (when was the last time you were “shocked” by someone?), so when you touch a capacitive screen, you change the voltage of a specific location. The touchscreen device senses this change in voltage and uses it to figure out where you touched. It is for this reason that certain touchscreens require special gloves. With a normal glove, the charge in your finger won’t go through the glove. However, special gloves use conductive threads, usually silver-plated nylon, in the fingertips, and these special threads help transfer the electricity from your finger to the screen.
Why Use Capacitive Screens?
With all of this, another question pops up. If I don’t need anything special to use resistive screens in the cold, why bother with capacitive screens? There are many answers to this question, but one of the key reasons is precision. If you touch a capacitive screen with multiple fingers, since it directly measures voltage at specific locations, the device will know exactly where each finger touched, no matter how many fingers you use. With resistive screens, however, the situation is a bit different. Imagine your phone as a graph with x/y-coordinates. If you place your fingers at two diagonal corners, the device can only sense that there has been a disturbance at the left-most and right-most vertical lines and at the top and bottom horizontal lines. However, it cannot tell which corners your fingers actually touched. Therefore, for the magic of multi-touch, many device developers have decided to forgo resistive touchscreens. Although resistive and capacitive touchscreens are the main types of touchscreens out there, there are also many other different types, such as infrared grids and optical imaging. Most modern smartphones, however, use capacitive screens. If you have a smartphone but no special gloves, hate to break it to you, but you’re going to have to either reply later or take your glove off and brave the cold.
