Let’s learn a bit about different touch screens that are found on consumer and commercial devices. There are many types of touch screen technologies but we’ll focus on those most commonly found on mobile devices; resistive, capacitive, projected capacitive and digitizer. Chances are there is a touch device somewhere close at hand; touch it with something like a plastic pen tip and you can deduce if it is resistive or capacitive, the two most popular screen types. If resistive, it will pick up the touch point; if capacitive it will not.
A resistive touch panel is three main layers laminated together; the top layer usually is a plastic film, the middle layer is an adhesive with a border similar to a picture frame, along with spacer dots spread over the active area of the touch screen and the bottom layer is glass. The inner surfaces of the top film and bottom glass are coated with a micro thin layer of electrically conductive indium tin oxide (ITO). Additional layers might be anti-reflective coatings or other optical treatments. The top layer has traditionally been a plastic film that can easily be deflected and pressed past the spacer dots so it touches the bottom glass layer creating an electrical touch point between the layers. Resistive screens transmit about 75% of the LCD’s light.
Single layer capacitive screens have the conductive ITO layer on the outside of the glass. Voltage gradients are applied to the edges of the glass and an electric field is created on the glass. Since fingers are slightly conductive, when a finger (or any moderately large conductive object) touches a particular point on the glass, electrons are absorbed from the ITO layer and the change in the voltage readings is decoded to determine the location of the touch point on the screen. These capacitive screens must have a physical contact between the conductive pointing item and the ITO layer.
Projected capacitive (PCAP) panels are made of two layers of glass (usually) sandwiching a plastic film that has rows and columns of ITO coating. Voltage gradients are applied and create an electric field that projects above the top layer of glass. When a conductive object comes into the field it absorbs electrons and changes the electric field’s characteristics (a disturbance in the force) and touch location is decoded. These screens differ from single layer capacitive in that there is no physical contact between the conductive pointing item and the ITO sandwiched between the glass layers, only the electric field disruption is necessary. Thus, PCAP panels can be “tuned” to work with gloved fingers at the expense of accuracy to an ungloved finger. It is a quickly evolving technology and becoming more popular on all devices. Depending on construction details, capacitive panels transmit 82-90% of the LCD’s light.
A fourth, less common type, occasionally used on rugged commercial tablets, but hardly ever on consumer devices, is an electronic digitizer which is actually layered behind the LCD. Like a capacitive panel, it projects an electric field that actually goes through the LCD. A stylus with a passive coil of wire or a stylus with a battery powered coil inside interacts with the electric field to position the cursor. The advantage of a digitizer is that a clear glass front can be put in front of the LCD thus allowing 100% of the LCD light to be visible. The disadvantage is that if the stylus is lost, the touch function is lost. Both MobileDemand’s T7200 7” rugged tablet and T1200 10.4” ruggedized tablet have capability to support a digitizer, besides the standard resistive touch panel.
Rugged Environments vs. Consumer:
Gorilla glass is an increasingly popular and quickly evolving glass type, developed by Corning. It is specially manufactured and strengthened to resist breakage. The primary marketing point of Gorilla glass is that it can deflect significantly without breaking. It allows for very thin, light weight touch screens on tablets. While extreme deflection seems a great characteristic, once a Gorilla glass touch panel is installed over an LCD and a point impact occurs, the Gorilla glass deflects and the impact is transferred to the LCD glass which probably will break. So you have a tablet with an intact touch screen and a broken LCD. Thus, as noted previously, fully rugged tablets, such as MobileDemand’s T7200 and T1200 use thicker glass to resist impacts and protect the LCD: A balancing act between touch screen thickness and LCD protection.
Fully rugged tablets have been much slower than consumer devices in making the transition from resistive touch with minimal scratch resistance to capacitive for several reasons. The resistive screens need a protective overlay to avoid scratches from repeated touches in one spot (often used icons on commercial programs requiring input at the same touch point many times) and from dust and grit covering the touch screen is dirty environments. However, resistive touch does not have the limitations of cold temperature use, sensitivity to moisture and use with gloved hands that hamper capacitive screens.
Capacitive screens do not work well, or perhaps not at all, when moisture is present. Try using a wet finger to operate your capacitive screen mobile device. It’s not happening. Fully rugged tablets with resistive touch can be used in a downpour, with gloves; essential for a utility worker in the outdoors mapping physical asset locations with GPS and Geo-tracking, standard features on MobileDemand’s T7200 and T1200. Capacitive panels also cease to be reliable at extreme temperatures, current typical operating temperatures being -10C to 60C. Resistive panels remain reliable at operating temperatures below -30C and above 70C.
To solve the scratch and wear issue of plastic top layer resistive panels, highly ruggedized resistive screens replace the top plastic film with a very, very thin layer of glass. When touched, the top layer of glass is deflected to touch the bottom layer but obviously it takes a higher finger pressure than a plastic film does. The advantage is the top glass is much more scratch resistant than the plastic so it does not require the use of a protective overlay in dirty environments.
So we see there are several types of touch screens in use on various types of commercial and consumer mobile devices, each has its advantages. Resistive panels continue to evolve incorporating multi-touch capability, more scratch resistant top coatings, and improved transmissivity. Capacitive panels, especially the projected capacitive technology will evolve further to improve use with ordinary (no woven in special conductive fingertips) gloves, more moisture insensitivity and extended operating temperature ranges.