How do Touchscreens work? ...What enabled smartphones to dominate as a technology- to be so prolific and critical to our lives that I would rather lose my car keys or wallet than my smartphone. The answer to this question isn’t a simple one-liner, but rather it's a combination of answers, app development, wireless internet, carrier networks, Steve Jobs’ brilliant marketing, though I would argue the most important contributor was the seamless combination of different functions and technologies into a single package. So, we're going to explore the most distinctive feature of the smartphone’s merging of technologies- the touchscreen display.
There are three technologies in the touchscreen display, these technologies are combined all on top of each other. When you felt and used a smartphone for the first time, you just knew that you were holding something revolutionary – something different from all previous phones. These were not new technologies. Many devices used a tactile interface, and color displays had already been a standard for most phones. Even toughened glass had been discovered in the 1800s. But the innovative element was combining them seamlessly. One layer on top of another like magic. Ok, so let’s get into the layers of a touchscreen display. On the top, we have the protective glass.
A lot of us have had a screen shatter, but think about how many times you’ve dropped it and it hasn’t. That’s because a smartphone’s glass is over 5x stronger than normal glass. And, before the first iPhone showed up in 2007 the standard for cellphone screens was plastic and although plastic doesn’t shatter, it is very easily scratched. If the screen were covered in plastic, it wouldn’t last a week sitting in your pocket next to your keys before having dozens of scratches all over it. So, what makes toughened glass so much stronger? A smartphone’s glass is an aluminosilicate glass that is toughened by soaking it in a bath of molten potassium nitrate. This causes the sodium atoms in the glass to migrate out, and much larger potassium atoms take their place. Because the potassium atoms are much larger, they generate a substantial compressive force on the surface of the glass.
Here’s a quick analogy: imagine filling the backseat of a car with 3 average-sized people. They fit snugly but if you push them, they're still able to move. Now replace those 3 people with 3 football linebackers. Those linebackers are just flat-out stuck- unable to move. It would take much more force to move those linebackers from their seats. This is the fundamental concept behind what makes toughened glass special, the atoms are compressed so it would take more force for the glass to break. Below the toughened glass is a projected capacitive touchscreen that senses the presence and location of conductive materials, such as your fingertip. This touchscreen is composed of two transparent diamond grid patterns printed on polyester with an optically clear insulator in the middle. The diamond grid pattern is printed with a transparent, material called Indium Tin Oxide or ITO which acts as a conductor.
Let’s take a closer look at how it works. Say we build up a bunch of electrons on this blue diamond, however, because there is an insulator in the way, the electrons cannot move. The electrons generate a negative electric field which causes a bunch of positive charges to build upon the yellow diamond this is called a capacitor. Now, when we move a conductive material such as the tip of your finger close to this capacitor it disrupts the electric field which changes the number of positive charges that build upon the yellow diamond. The change in positive charges caused by this disruption on the yellow diamond is measured, and the processor registers this as a touch. The location of the touch is detected by scanning the charges or voltage along the blue diamond rows, while actively measuring each yellow diamond column. Note that each row of blue diamonds is connected together, also each column of yellow diamonds is connected. This setup makes a grid of blue columns and yellow rows.
Just to clarify again, all of these components are made with transparent materials. Measuring each point requires too much circuitry, so we only measure each column. The charge or voltage gets sent to each row in quick succession, so the processor can register multiple touches at once below that is a display that uses LCD or OLED technology. While the LCD and the OLED display both produce high-quality images, in this episode we are going to focus on the OLED technology as it is the standard in most new Smartphones. OLED stands for Organic light-emitting diode. This high-resolution OLED display is what generates the high-quality images that we see whenever we look at our smartphone. This is a crazy intricate grid! Currently, high-end phones can have over 3.3 million pixels. That means there are 10 million microscopic individually controlled dimmable red green and blue lights in the palm of your hand. Take a moment to think about the engineering level recurred to control let alone design and manufacture that many microscopic lights! OLED displays are composed of a massive grid of individual pixels and each pixel is composed of a red-green and blue subpixel.
Each subpixel’s light intensity is controlled by a small thin film transistor that acts as a dimmer switch. There are many layered structures in each sub-pixel, however, explaining the function of each layer will have to be saved for a future episode. Photons are produced in the subpixel by electrons that are driven from the negative to the positive terminal. When they pass through this middle layer here, called the emissive layer, photons are emitted through a release of energy. The compounds used to make up the emissive layer determine the color of the light emitted, and the intensity of this light is dependent on how many electrons pass through. This explanation is greatly simplified but the research, engineering, and science behind OLEDs are extensive.
In fact, the 2014 Nobel Prize in Physics was awarded to 3 researchers for their discovery of efficient blue light-emitting diodes! So, let’s summarize: on the bottom is an OLED display composed of 10 million itty bitty little colored lights. On top of that is a transparent projected capacitive touchscreen that can sense one or multiple fingers touches at a time. And on top of that is strengthened glass that protects your screen from scratches and most falls. Now you too are a touchscreen expert! If you have any questions, post them in the comments below. Subscribe for new updates, Like, and tell your friends or family about something you learned.