EEVblog #1346 - How An Infrared Optical Touch Screen Works

Dave tears down and investigates a Dell infrared IR optical touch screen monitor found in the dumpster.
Infrared touch screen are not often used in consumer computer monitor applications compared to resistive and capacitive touch screen technologies.

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#Teardown #Infrared #Touchscreen

[GnuReligion]

I want equal access to Dave's dumpsters.

[Andrew_Sparrow]

🐞They are great until a fly lands on the screen and starts walking around 'clicking' things and dragging files around! (from experience) 😣

[oliverthane]

I worked on the firmware for similar optical touchscreens back in the 2008-2012, but I dont think it was this one. Going to higher number of cameras helped in several ways. First we had to meet the new 10 touch requirements for Windows and with only 2 cameras those touches were very easily occluded by the other fingers etc, so having more triangulation's meant the points didn't have to coast in the Kalman filter/particle filter for so long. Also as you found tracking something close or far away is really hard with just 2 cameras as there is lots of reflection or basically no shadow, so getting more cameras around the screen meant you normally got a better touch signal. There were some other benefits to multiple cameras like redundancy and getting away with dead zones from dust etc.

[fonkbadonk]

Industrial automation engineer here: I've never encountered anything but resistive touch in industrial application. Optical would be waaay too easily disrupted by dust and other gunk that there is plenty to go around. Not many production facilities are even close to being clean and neat. They'd have to deep-clean such monitors hourly to keep them working =)

[dg-hughes]

At an old job I recall one time at the bar a fly landed on the POS cash machine screen. The fly ordered himself a sandwich and two drinks. It was an IBM POS model which must have been infrared optical touch.

[alandouglas]

Dave you forgot one other type of touch screen technology. IBM had touch screens back in the 90's that were strain gauge based. The CRT (Yes old school!) was mounted on strain gauges (Like in an electronic scale.) and when you touched the screen the pressure and position of the touch was measured. These only worked for a single touch but you could have software that also reacted to the pressure of the touch. ie touch lightly to select and harder for activation.
You did have to have the screen on a very rigid surface otherwise you could get false touch touches if someone walked by or slammed a door. These IBM screens connected through the PS/2 mouse port and had connection for the mouse if you wanted that as well as the touch capacity.
Ahhh memories!

[ZoeyR]

The 3rd sensor is to prevent occlusion enabling multi-touch. Also helps with dust rejection

[mcconkeyb]

I worked at SMART who holds the patents for all of the optical touch screens. We took over the NZ company who infringed on our patents and rolled them into the SMART corp. Now SMART has been taken over by Foxxconn. I haven't worked at SMART for about 8 years, but I'm sure that design hasn't changed much in that time. As you've said many times, most of my time was spent reducing the cost of goods in the manufacturing of these devices. The technology can acquire and track up to 75 objects, it is software limited to a much smaller number, usually either 2 or 4, but some products can track up to 20.
There are 2 configurations, 1 with a tx/rx in each corner, 1 with a tx/rx in the top corners. This Dell unit appears to be not of SMART design, and thus very likely in violation of SMART's patents.
The waveform has peaks and dips as the retro-reflective tape has different reflection coefficients at different angles, the big dip is the drop off of the signal at the opposite corner. Each corner is multiplexed and there would be no way to determine where the shadow is, if the screen was just flooded. Also we drive the LEDs so hard that we can't drive them continuously.
If you have any other question, I'll do my best to answer them. :-)

[zaprodk]

Thank you for doing this teardown. Really interesting technology. It's amazing that they can get so good touch with three small linear sensors like that!

[JimmyMish]

Really common for POS systems, easy to clean, and you can use anything to activate them, gloved hands, pens, i've used a handheld scanner or even product :)

[vinzzbe]

I have one from HP at work. Once i thought the touch was malfunctioning but it was just a small piece of paper that got stuck between the sensors at the bottom edge of the screen.

[andrehu]

The third sensor is needed for the multitouch. If you put one finger on the screen, for the sensor, there is an invisible "shadow"-Area behind your finger. So if you put a second finger along the sensor line behind the first finger, the sensor will not see it. Therefore there is placed a third sensor.

[tomgeorge]

Great video Dave.
I have had across my workbench some industrial grade optical "touch" screens and they have a scanner type linear array around each edge, a TX and RX pair opposite each other.
A bit more electronics around the frame of the screen.
Luckily not on the bench for any touch fault, usually backlight powersupply failure.

[adriansmith]

These were used on the early Acer all in one PCs. I used to work in the returns department for a high street retailer and they often were returned faulty with erratic touchscreen. Always was dirty screen and sensors / reflectors etc. I think they only had two sensors if I remember correctly.

[ultrababa]

8:07 Looks really futuristic without that back cover and bezel.

[MrJef]

Simple in principle, probably a bit tricky to get working properly (re. signal processing). Cool video!

[Veptis]

I recall being 12 years old and seeing those giant 1.5meter vertical touchscreens in a Sport Check. What I was wondering is how it worked and I asked. But I only learned that corporate in Munich handles it. Back than I wasn't that much into the internet to do my own research. I do recall that it would notice your touch before you actually so touch if.

Seeing this now made me wonder if that's it.

[steveunderwood]

He showed the screen sensing pinch operations. You need the third sensor to properly resolve two fingers.

[randycarter]

There is one touch technology that is really tricky. It uses 3 or 4 pressure sensors (load cells). When the screen is touched the screen moves. The loads cells measure the individual weights and triangulate where the pressure is applied. Never really took off due to resolution and size limitation issues.

[Robonza]

The LED is on momentarily. for many reason.
1 The led cannot stay on as it will overheat. It's a very small package and is driven hard.
2 It is only open during the "shutter" phase of the camera
3 There is an ambient acquisition cycle which is subtracted in software.