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.
Do you have a question for Touch Guy? Send him an e-mail at firstname.lastname@example.org.