How it works: The technology of touch screens

From single-touch to multitouch and why all displays are not equal.

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One of the big advantages of resistive touch panels is that they are relatively inexpensive to make. Another is that you can use almost anything to create an input signal: finger tip, fingernail, stylus -- just about anything with a smooth tip. (Sharp tips would damage the film layer.)

This technology has a lot of disadvantages, however. First, the analog system is susceptible to drift, so the user may have to recalibrate the touch panel from time to time. (If you owned a PalmPilot or other PDA, you may remember having to occasionally go through the recalibration process on their PalmPilot.) Next, the ITO material used for the conductors is brittle and not well suited for bending. Over time, repeated use can cause the ITO to crack, which disrupts the flow of electricity and can result in a dead spot on the touch screen.

In addition, there needs to be a gap between the two sensor planes that must be bridged in order to make contact between the two. Just about the only material suitable for this gap is air, but this presents some problems of its own.

First, the gap adds to the combined thickness of the display and touch module. As the consumers demand thinner and thinner devices, a single millimeter can be a big deal.

Another problem has to do with the optical properties of the different layers. If you look at a drinking straw in a glass of water, it will look as though it is slightly bent where it enters the water, even though it is straight. This is because light can bend, or "refract," when it makes the transition from one material to another. If the materials have the same index of refraction, the light won't change its path, but if the index of refraction is different, the light will bend.

The space between the plastic and glass layers of a resistive touch panel is filled with air, and the air has a different index of refraction than the other layers, which makes the light bend as it passes from one layer to another. This can create visible artifacts that can impact the display quality.

The air gap is especially a problem when you view the display under high ambient light conditions, such as outdoors in bright sunlight. The outside light passes through the top layer, then bends when it hits the air gap, and can then reflect between the glass and plastic layers before exiting out the front of the display again. This bouncing light can reduce the image's contrast, making the display look washed out and impossible to see.

But probably the biggest problem with resistive panels in consumers' eyes is that they can sense only one touch at a time. If you touch the panel in two places at once, the combined effect will produce one coordinate for the touch point, and that will be different from either of the two actual points. There are ways to create resistive panels that can sense multiple touches at one time, but these can be expensive and complex, such as creating a matrix of separate contact pads on one of the layers.

Projected capacitance

Fortunately, there's a better way. Many mobile devices now rely on a technology known as "projected capacitance," often referred to in the industry as "p-cap" or "pro-cap." According to various sources, resistive touch has rapidly lost market share to pro-cap and is forecast to continue to decline.

Pro-cap is a solid-state technology, which means that it has no moving parts (unlike the resistive touch technology). Instead of being based on electrical resistance, it relies on electrical capacitance.

When you apply an electrical charge to an object, the charge can build up if there is no place for the electrons to flow. This "holding" of electrons is known as "capacitance." You have probably experienced this effect first-hand. When you walk across a carpet in rubber-soled shoes in the winter time, electrons can build up in your body. If you should then reach for a light switch or some other conductive object that does not have a similar built-up charge, those electrons can flow from your body to the object, producing a spark of electricity.

If you apply a charge to a conductor, and then bring another conductor near it, the second conductor will "steal" some of the charge from the first one, just as the light switch did when your finger approached it. If you know what the charge was to start with, you can tell when the amount of the charge has changed. This is the principle behind pro-cap touch screens.

Early capacitance touch technologies required that you actually touch a conductive layer. This approach left the conductor exposed to wear and damage. Today's projective capacitance technology relies on the fact that an electromagnetic field "projects" above the plane of the conductive sensor layer. You can cover the touch module with a sheet of thin glass, for example, and it will still sense when a conductor comes near.

Pro-cap touch screens use two layers of conductors, separated by an insulator (such as a thin sheet of glass, though other insulating layers can be used). The conductors typically are made of transparent ITO, just as with the resistive designs. The conductor layers never have to bend, however, so its brittle nature is not a problem with pro-cap screens.

The conductors in each layer are separate, so that the capacitance of each one can be measured separately. As with a resistive panel, the conductors run at right angles to each other, so that the device can sense an X and a Y position when touched. The difference is that the separate conductors are scanned in rapid sequence, so that all the possible intersections are measured many times per second.

When you touch the screen with your finger, it steals a little of the charge from each layer of conductors at that point. The electrical charge involved is tiny, which is why you don't feel any shock when you touch the screen, but this little change is enough to be measured. Because each conductor is checked separately, it is possible to identify multiple simultaneous touch points.

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