Hey Touch Guy, I’m working on a sort of digital signage / kiosk type application that will hopefully be everywhere from hotel lobbies to office buildings. The plan is to start out around 32” and eventually grow to 60”. I’m thinking I want Projected Capacitive for the touch screens for all the usual reasons , but people are telling me optical touch is the way to go due to size. So who’s right and what’s the best? PCAP or optical? -The Sign Guy

Hey Sign Guy,

The big ones, usually defined as over 32 inch LCD’s, have long been the domain of optical systems with an occasional sighting of an acoustic (external and internal noise generator) or surface capacitive sensor, as well as a smattering of the other touch technologies. Optical is defined as IR beam or corner camera type. Recent advances in optical touch systems have added the ability to do multi-touch which had been one of the drawbacks. Frankly sign guy, optical is your best choice today. But do not sign any long term commitments. The market wants p-cap with pen for large format displays. Why, you inquire? The same reasons the small display guys want it….low cost, high reliability/durability, multi-touch, gestures, and good image quality – in addition to the twelve other things people like about the technology. The holy-grail of big formats seems to be white-boards (think every school room and meeting chamber) as opposed to the slow growing digital signage market (for touch displays). But, as Touch Guy has pointed out, p-cap has changed the sleepy old days of resistive touch manufacturing to major advances every six months…..that 82 inch, $35,000 p-cap touch panel you saw at the trade show last year will be in your budget within 12 months. Already 32 inch p-cap is common and cheap, due to advances in the materials, IC’s, and manufacturing techniques. Passive pen is not quite ready for the big ’uns yet, but when it is, Touch Guy advises you buckle your seat belt.

Until next time,
-Touch Guy

I’m kind of new to the touch screen world and am trying to figure out why it is so difficult to get large (over 22”) multi-touch resistive and projected capacitive touch screens. Also, what are the big advantages and performance differences between the competing multi-touch technologies?

Hi Jim:

So you want to know about big ‘uns?

Projected capacitive, our favorite multi-touch technology, has pretty much topped out at 32 inch diagonal sizes.  There are bigger ones available, but they have those annoying little wires that nobody likes to see.

In the realm of large format multi-touch, we find that infrared, camera (often called optical) and DST are the most commonly used touch technologies for large sizes (+32”), however IR and DST only support two touches. The mainstream projected capacitive and multi-touch resistive technologies are capable of unlimited touches, but are not as easily scalable (note that unlimited touch is controller-dependent, not sensor-dependent). So if you fancy yourself as Tom Cruise on a big 60 inch display, you will probably have to wait for projected capacitive to make it big.

So, what are we waiting for?  We are waiting for transparent low ohm conductors.  You will recall that projected capacitive is an X-Y scanned technology, using ITO for that purpose.  But, white paper fans amongst you will recall that ITO has a relatively high resistance when compared to something like a copper wire.  As the touch sensor gets bigger, the ITO resistance gets higher, and eventually, too high for the electronics to work.  So, what is to be done?

TA DA!  Debuting now, for your viewing pleasure, are nano-wires (think a Chinese population of angels on the head of a pin) and super-fine line copper webs.  Both of these sport the required low resistivity, and only the most critical alien-eye can see them…. we can even coax them into rows and columns.  Expect the electronics to grow as well, but, hey, what is an extra multiplexer or two…

So, Mr. Proctor, looks like your wait is over.  Technology is getting bigger, not smaller, this time.

For even more info on multi-touch, check out Touch International’s Putting the ‘Touch’ in Multi-Touch White Paper and refer to the graph below.


– Touch Guy

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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

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