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I’m mounting a projected capacitive touch screen to a 17” monitor and am trying to find out how much of a gap I need to leave between the touch screen and LCD glass. Is the gap dependent upon the type of touch screen used? Also, do you have any suggestions for how to mount it to the LCD or bezel?

Dear Victor:

As they say on the London Underground, “Mind the Gap”.  The separation between the back of the touch sensor and the front of the LCD is important for two reasons—optical and electrical.

In general, one wants the touch sensor to be as close to the display as practical, so that there is no pronounced parallax error.  Parallax occurs when the finger or probe comes in at an angle and is detected “too early”, and thus misses the actual target — such as touching a “z” on the on-screen keyboard, instead of the intended “a”. This is most pronounced in older style infra-red touch technology, because the LED’s are relatively high above the display, but can be a problem with any touch sensor located too far from the display.

But putting the touch panel too close can cause problems if you are able to deflect the touch screen so that it hits the LCD. Your 17” touch panel will probably not have this problem because larger touch screens have a rigid back layer that will not easily bend.  Smaller, thinner, touch panels, however, can be pushed into the LCD and a rainbow (moiré pattern) can occur, which is not good for the life of the LCD.  In a few cases, for LCD’s without a metal frame, you can actually put the touch screen on the LCD, and the moiré pattern will be annoyingly permanent.

The LCD and backlight can also create enough electrical noise to slow or stop the operation of the sensor, so there needs to be a gap between the LCD and the touch sensor.  The required gap size, however, is not a constant.  The needed separation varies with the LCD model and touch technology; those lightning-fast tiny switching transistors do emit some electrical noise, believe it or not.  For projective capacitive technology, the necessary gap also varies with the IC manufacturer.

So, to mind your gap, a separation (from the surface of the LCD) should be the thickness of the metal frame plus 10/1000’s, which is usually the thickness of the adhesive that holds the touch screen to the LCD.  Sometimes, you will be tricked by a particularly noisy LCD and need a larger gap, also handled by better tuning of the electronics, but 40/1000’s (bezel plus adhesive), as  a minimum, will usually work.

Ta ta,

Touch Guy

Do you have a question for Touch Guy? Send him an e-mail at asktouchguy@touchintl.com.

I stepped on my fancy iPhone 4 and whole screen is completely cracked! I can barely see the images, but somehow it still works! How is this possible? – Astonished Jim, Little Rock, Arkansas

Dear Astonished Jim in Arkansas,

Touch guy is about to go on vacation, so thank you for an easy question! As I’m sure you know, the iPhone uses projected capacitive technology and one of the main benefits of this technology is that it will “last forever.” Touch Guy did not really mean that you could smash it, or in your case, step on it, and it would still work. But, wow, maybe you can…

So what gives? First, I want you to know that there are several ways to make projected capacitive touch sensors, but all of these methods involve putting the transparent sensing conductors, called ITO (Indium Tin Oxide), behind a protective front layer (I call this layer a lens). The lens on an iPhone is made of a thin layer of glass – you can look in the on/off hole and see how thin this glass is. Behind it is the touch sensor, which is a thicker (0.5mm) layer of glass, and is optically bonded to the thin lens. I say “optically bonded” because there is a continuous glue layer in-between that eliminates the air gap between the touch panel and the lens and is part of the reason the display looks so good.

There are a lot of ways to bond the layers together; the material used is generally known as “OCA” (you guessed it, Optically Clear Adhesive). All automotive windows are two glass layers optically bonded together, and the internal OCA, along with tempering, is what makes the glass “safety glass” with all of its protective properties. When you get a crack in your car window from “that darn rock”, only the outer layer of glass usually cracks. So Jim, that is your answer – when you klutzed the iPhone, you cracked the outer layer of thin glass, but the touch sensing layer remained intact and worked just fine through the broken lens.

But I know you want to know more! Touch International makes all plastic projected capacitive sensors which will not break (think hand-held games for children). They also use chemically strengthened glass for the lens and touch sensor to make it harder to break. There is also another type of cover glass, alumina silicate, which can increase the strength if the lens is likely to get scratched, by, say, the keys in your pocket.

Astonished-Jim-with-the-cracked screen will probably enjoy this story: the iPhone introduction was delayed for months, because the first models of the product had an unbreakable plastic projected capacitive screen. Of course, the big man himself (if you don’t know who this is then tough luck), got the first prototype. By the end of the third week, the junk in his pockets had scratched the lens, making him very, very unhappy. So the lens maker, having invested quite a tidy sum in a plastic lens making facility, had to start over with glass.

We hear that around 25,000 smart phone lenses break each month, so you are not alone. Replacement sensors are available and there you tube tutorials on how to replace them (not easy).

Whew! That was a long answer! If you still want to know more, download the Touch International white paper “Projected Capacitive Touch Screens” – it has a lot of good stuff in it!

And Touch Guy is off! (And planning to opt-out of being X-rayed at the airport)