Creating the FTIR Frame

A central piece to my multi-touch table is the FTIR frame. This is simply a piece of acrylic with surrounded by IR LEDs shining into its sides. Finding suitable IR LEDs was a task. For good advice on which LEDs have been used successfully, search around and ask questions on the NUI Group forums.

My initial plan on wiring up the IR LEDs was to use take apart LED Christmas lights from the store, and replace the LEDs with IR LEDs. This way, I wouldn't need to deal with designing the circuit, and the wires would be nice and neat. While the plan sounded good in practice, it did not work out so well. I wasn't able to find any LED Christmas lights, despite the fact that it was only days after Christmas. Finally, I was able to get a hold of net-style lights. With a little modding, they were almost like regular string lights. I found a suitable IR LED with the same specs and ordered them from Mouser Electronics. In my haste though, I hadn't checked all the stats completely. The lights I ordered were not nearly bright enough, only 20 mW/sr instead of the recommended 100 mW/sr. They came anyway, and I paid for them. Perhaps someday I will find a use for them.

After doing more research, I found better LEDs. The LI521 IR LEDs from Mouser Electronics, with a radiant intensity of 80-120 mW/sr, sounded perfect. A little more expensive than my first order though. I went ahead and ordered 30 of them.

My acrylic I simply picked up from a local hardware store. It was 1/4" thick (or so it said, it is actually a bit thinner). I then cut the acrylic to be about an inch bigger than my LCD panel on every side. To maximize the amount of IR light that enters the acrylic, it is important that the sides of the acrylic are highly polished. This actually turned out to be one of the hardest parts of the multi-touch project, as sanding can take a lot of time. I used varying grits of sand paper, up to about 600, polished the sides with Brasso, and then ran the acrylic over a flame from my stove to get it very shiny. The edges needed to be shiny enough you could see colors that are on the other side of the acrylic. Once again, look on NUI Group for tips on how to get acrylic polished.

I wired up my LEDs by cutting thin strips of prototyping board, and then using wire wraps on the opposite side to connect the leads. I then covered the exposed wires with electrical tape. It wasn't a very clean-cut solution, but it worked, and it didn't take too long. If I were to do this again, I would print a circuit board and solder to attach the LEDs. Although I ordered 30 LEDs, I only wired up 20 of them. They would cover three sides of the acrylic, and be space about 1.5" apart.

To hold the strips of LEDs to the sides of the acrylic, I constructed a frame. The frame's sides were made of a three sided u-shaped metal thingy. Obviously, I have no idea what to call it, so from now on we shall call this metal phenomenon a foobar. Here is a picture of the part that I find so difficult to name:

The foobar is deep enough that the LEDs board can with within it. I constructed a frame out of the foobar so that the inside edge of the frame would be the same size as my acrylic. I then put the LED boards inside three sides of the frame, and put in the acrylic. I used plastic insulation strips to hold the acrylic at the right height for the LEDs to be shining directly into the edges. I used electrical tape to seal the gap between the acrylic and the edge of the foobar, so that IR light was not shining above and under the acrylic, otherwise the camera would see my fingers before they made contact with the acrylic.

With all of that done, I could attach it to the table! Here is an infrared picture of the FTIR frame on the MT table.