Tech 101 – How Touch Screens Work

Previously, we’ve explained How Liquid Crystal Displays Work as well as How OLED Displays Work. Today, we’re going to explain how Touch Screens Work.

Touch Screens are all the rage today. You’ll find them in many devices like tablets, smart-phones, computer monitors, PDAs, ATMs, table tops, kiosks etc. A touch screen can use multiple inputs from your fingers to styluses and even sausages! But how does this marvel of technology really work? There are many different types of touch screen displays. You will hear many marketing gimmicks regarding capacitive or resistive technology, but most people just don’t know the difference. Read on to learn more about this fascinating technology.

 The first touch screen was made all the way back in 1965 by E.A. Johnson at the Royal Radar Establishment, Malvern, UK. Since that first prototype, many methods of detecting touch on a display have been developed. The most widely used touch screen technologies are explained below:

Resistive Touch Screens

A resistive touch screen is composed of many layers. The two most important layers are made of a flexible polymer which are coated with a resistive material and are separated with an air gap or microdots. The resistive material is applied in lines on each sheet and they are placed perpendicular to each other. When a person touches their finger to a resistive touch screen, the two layers are pressed together, and the points of intersection on the two layers allow the processor to accurately measure the position of the touch.

When pressure is applied to the screen, a uniform voltage is applied to the first sheet, and the second sheet measures the voltage as distance along the first sheet, which gives the X coordinate. Similarly, when the X coordinate has been ascertained, a voltage is applied to the second sheet, and the first sheet is used to measure the distance, which gives the Y coordinate. These measurements take place in only a few milliseconds, which means that a touch is registered as soon as contact is made.

Since these types of touch screens rely on a point of contact between the two resistive layers, any pointing device like a finger or stylus can be used on them. These screens are also quite inexpensive to manufacture as they don’t require any specialized components. Due to the design of these screens, registering multiple points of contact was not possible due to vectoring issues. However, new technology is now available that overcomes these vectoring issues and allows multiple points of contact to be measured.

Capacitive Touch Screens

Capacitive touch screens can be based on two different technologies:

Surface Capacitance

This is the most basic form of capacitive touch screen technology. One side of the insulator is coated with a conductive layer. A voltage is applied to this conductive layer, which results in a uniform electrostatic field. When you bring your finger in contact with the screen, a capacitor is formed dynamically. This changes the electrostatic field across the screen and this change is measured by sensors placed in the four corners of the screen. The point of contact can be accurately measured based on the change in capacitance in the four corners of the screen. The greater the change in any corner, the closer the point of touch is to that corner. As there are no moving parts, these types of capacitive touch screens are very durable and are used in industrial applications as well as kiosks.

Projected Capacitance

Projected capacitive touch screens work on a principle similar to the matrix found in Liquid Crystal Displays. There can be either a single layer on which an X-Y grid is etched to form a grid pattern of electrodes or two separate perpendicular layers on which parallel lines are etched to form the grid. There are two basic types of projected capacitance technologies – Mutual capacitance and self capacitance.

• Mutual Capacitance: These types of capacitive displays have a capacitor at every intersection along the grid. A voltage is applied across the rows or columns which creates a local electric field across the display. When it is touched with a finger or capacitive stylus, the capacitance at each point on the grid changes, which can be measured and gives an accurate location of the point o contact. This technology can measure multiple points of touch.
• Self Capacitance: This type of capacitive touch screen can have the same grid pattern as a Mutual Capacitance screen but the rows and columns operate independently. With this type of screen, the capacitive load of a finger is measured on each row and column by a current meter which gives the point of touch. These types of screens can only measure one point of touch at a time.

Projected Capacitance touch screens can operate without direct contact and can have a layer of insulation between the user and the screen itself. However, they require the use of a capacitive input device in the form of a human finger or a specialized stylus with a capacitive foam tip. These types of touchscreens are more expensive to manufacture than resistive touch screens. They also face issues when in contact with any capacitive material like water.

Infrared Touch Screens

These types of touch screens work on a very simple principle. Infrared LEDs or lasers are used to form an X-Y grid on the surface of the screen. These transmitters are coupled with receivers that monitor the grid pattern. When a touch event occurs, the infrared light from the transmitters is blocked in that region. The receivers that are no longer detecting an infrared signal are used to measure the exact point of contact.

Infrared touch screens face many problems from objects that obstruct the infrared grid such as smudges on the screen and dust particles. Any kind of input can be taken on these screen from fingers to gloves to styluses.

So now that you know how a touch screen works, we hope that you can have a new-found appreciation for all the devices that use this technology. A tremendous amount of research and hard work has been put in to make an input interface for a sense that most of us take for granted – Touch.