FTIR Touchscreen Hardware FAQ

What kind of Acrylic are you using?

I am using a 1/4" piece, measuring 24 x 18. I also polished the edges. It can be purchased at any glass / window store usually.

How many LEDs are you using?

Right now I have 42 LEDs, but I think you can get by with much less. Probably 14 will do.

What kind of LEDs are you using?

I am using some IR leds I bought off ebay. They are 880 nm, 1.5v, 100 ma LEDs with a 12 degree angle. A larger angle might be better. I ended up sanding the heads to get more dispersion.

How are your LEDs mounted?

Originally I had them UV glued to the edges of the acrylic, but now I just have them pointing into the side (straight in, not angled or anything). I also sanded flat the heads of the LEDs. This helps them sit closer to the acrylic and helps disperse the light more.

How did you wire the LEDs?

I wired them in a series in groups of 7 with one resistor on each group. Your wiring will vary depending on what kind of LEDs you get. You definately need to wire them up with a resistor or you will burn them out pretty quick.

How did you polish the acrylic edges?

I sanded it using high grit sand paper. First file down any big bumps. Leave the protective paper ON the whole time. Then sand with 100 grit sandpaper, then 200 grit, then 400, then 600. You can find the high grit sandpaper at your local hardware store. My store had it in the Paint section, not over by the sanding tools which was confusing. After sanding I then flame polished the edges using a small butane torch. Using the torch takes practice and can be dangerous. Make sure you take necessary precautions. I found that holding the torch on a spot just long enough to melt the plastic and singe the protective paper produced the best results.

What kind of camera did you use?

Right now I am using a Phillips SPC 900NC camera which is a CCD camera and can run at 640x480 @ ~50 fps on my computer. I bought it for 79$ at Comp Usa. There are other more expensive options such as the cameras from PtGrey which are firewire and come with their own software SDK. Touchlib does support these cameras using the videowrapper capture filter.

Was the IR filter easy to remove?

On this camera, no. It is impossible to remove. I ended up having to buy a new lense assembly but they are fairly cheap. Check this page for info about removing the lense. Look for a lense that is for a 1/3" CCD, M12 x 0.5 style. I used one that has a 4.3 mm focal length and is specifically for night vision (IR).

What kind of filter are you using to block out the visible light?

I am using an IR cutoff filter (made for IR photography) that I got fairly cheaply off ebay. Ideally you would use an IR Bandpass filter tuned to the wavelength of your LEDs. These run a bit more.

What did you use for a diffuser (projection surface)?

I took a sheet of thin plastic from a poster frame and sanded it with fine grit sandpaper - this makes it kind of a milky white/translucent. I also saw there is a glass spray you can get to achieve the same thing (maybe.. haven't tried it). Then I just put that piece of plastic underneath the acrylic - there must be an air gap (don't glue it to the acrylic or anything like that). Don't try doing that to the acrylic itself or you will destroy the FTIR effect. You could also use tracing paper.

What could I use for a 'front surface' (aka compliant surface)?

I did some experiments coating paper with Plasti-Dip on one side. Paper blocks/scatters IR and acts as the projection surface. The plasti-dip allows the paper to couple with the acrylic screen when you press down. It works well, but is not nice to touch. Jeff Han recommends trying Rosco Grey 2105 film screen. I haven't tried it yet. Concievably there is some magical combination of materials that works well as a compliant surface - something that scatters IR, couples well with the acrylic and is smooth (low friction) to touch. Oh and it also has to be a good projection screen.

What lighting conditions do you use the screen in?

I use the screen in the dark since all the lights in my house output a lot of IR. High efficiency bulbs and flourescent bulbs supposedly do not output much IR.

What are you using for a stand?

I made a stand out of PVC pipe.

Does it work well?

It works well, but like most things, it's not perfect. Dragging operations can be a little difficult - you may have to press harder as you drag. Also you have to get used to the fact that your finger is a lot larger and more unwieldy than a pixel accurate mouse cursor. Either way, you can design an interface that works well with the screen (large buttons, etc.). Otherwise, it is accurate and responsive - I have no problems with lag as long as I don't overload the CPU. A system where one machine analyzes the table input and another cpu runs the simulation may work best. Future experiments may improve dragging and sensitivity. Some things to try are new surface coverings or treatments.

I would approach it more like an experiment that you do for fun or a hobby, rather than expecting some end result.

How long did it take?

It took me 2-3 months to do the hardware and software, putting in many hours every day. Just finding the materials can take a lot of time. However, I had a lot of trial and error and no one to ask questions of.