How it works: The technology of touch screens

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

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Pro-cap technology is not without its challenges. The system of conductors is susceptible to electrical noise from electromagnetic interference (EMI). This can be a problem for display devices such as LCD and OLED panels that rely on an active matrix backplane of transistors to rapidly switch the individual subpixels on and off. The touch screen controller must be able to filter out this background noise and figure out which signals are from actual touch points.

The controller is often asked to make other decisions as well. Comparing results from adjacent coordinates can help determine if the touch is hard or soft, or if it is the result of the palm of the hand resting on the screen and thus should be ignored. Some smartphones rely on the touch screen to signal when the phone is being held next to the user's face, so that the screen can be turned off to save power.

All these tasks require significant processing power, which makes the controller more expensive. In addition, the touch screen only works when you apply a conductor; the ball of your finger works, but not your fingernail. Some pro-cap screens will work even if you're wearing thin surgical gloves, but they won't work if you have thick winter gloves on. (The exception is if the gloves themselves are conductive; you can buy gloves with conductors woven into the fingertips so that they can conduct the charge from the screen to your finger.)

In spite of these shortcomings, pro-cap technology has become the dominant choice for mobile devices. And there are improvements on the way that could make them even better.

Can't be too thin or too light

Consumers have made it clear that they want smartphones and other mobile devices to be as thin and lightweight as possible. As a result, design engineers are always looking for technology improvements that let them remove layers and materials from their products. And touch screens are not immune to such scrutiny.

The traditional structure for adding pro-cap touch to a display is to purchase a separate module. You would start with an LCD panel that is made up of two glass layers that contain the liquid crystal material; the top glass sheet is covered with a polarizing layer.

Above that goes the pro-cap touch module, which is made by coating both sides of a glass sheet with a conductor (typically ITO), which is then patterned to create the electrodes. This glass sheet is then laminated to the polarizer layer of the LCD panel described in the previous paragraph.

Finally, a protective cover glass is placed on top of the touch panel so that the top electrodes are not exposed. This cover can also have decorations (such as logos or icons for fixed controls) and be designed to protect the display from damage.

If you've been counting, you'll realize that it all adds up to four different sheets of glass in the stack -- which means that even today's thin smartphones aren't as thin as some might prefer. If manufacturers could eliminate one of these sheets, they'd reduce the space required for glass and the weight of the glass in the display by 25%. Those are savings worth pursuing.

A method that is gaining momentum is called the "one-glass solution" (OGS); it eliminates one of the layers of glass from the traditional pro-cap stack. The basic idea is to replace the touch module glass by a thin layer of insulating material. In general, there are two ways to achieve this.

One approach to OGS is called "sensor on lens." (In this case, the "lens" refers to the cover glass layer.) You deposit an ITO layer on the back of the cover glass and pattern it to create the electrodes. You add a thin insulator layer to the bottom of that, and then deposit a second ITO layer on the back of that, patterning it to create electrodes running at right angles to the first layer. This module then gets laminated onto a standard LCD panel.

The other approach is called "on-cell" pro-cap. (Here the "cell" refers to the LCD display.) A conductive layer of ITO is deposited directly onto the top layer of glass in the LCD panel, and then patterned into electrodes. A thin insulating layer is applied, and then the second ITO layer is patterned with the second layer of electrodes. Finally, the top polarizing layer is applied on top, and the display is completed by adding the cover glass.

This may not make much difference to the end user, but it can make a huge difference for the companies in the supply chain -- including which companies are actually included.

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