How to Build a Multi-Touch FTIR Table

This article will show you how to build and assemble a multi-touch FTIR display, like the one Jeff Han of NYU built.
At any point you can check out my blog http://polymathengineering.blogspot.com and click on the images for notes on the image!
FTIR stands for Frustrated Total Internal Reflection.

Instructions

1

Assemble all the components. That means find someone that will sell you acrylic, get the drills, sandpaper, LED's, resistors, etc.
You can find out where to order electrical components here:
http://www.ehow.com/how_4496412_buy-electrical-componets****.html

Note: Keep the protective covering on both pieces of acrylic until the end of the project!

2




For the frame you will use Canvas stretchers, which are sold at local art stores. In my case, using a piece of acrylic 24 inches by 32 inches by 3/8 inches thick I needed to use canvas stretchers 26" by 34" to compensate for the interlocking components. Also to deal with the thickness of the wood.

3




Assemble two frames out of canvas stretchers. In my case they are 26 in by 34 in.
Go here for notes on the picture:
http://www.flickr.com/photos/28819131@N02/2917233566/
or http://polymathengineering.blogspot.com

4




Begin Sandwiching components. Lay the first frame down, then lay the thin piece (1/8 in) acrylic, then the drafting paper ( diffuser), then the thicker (3/8 in or more) acrylic, and finally the upper frame.

5

Take the 36 in long 1/2 in thick wooden dowels and cut them down so as to lie flush against the acrylic in the sandwich. In my case I cut them down to 34 inches.

6




Using a compass, measure out a 1 inch interval and swing at arcs across the 2 of the dowels at 1 inch intervals. Then make a center line down the center of the dowel.
Go here to see a better picture with notes.
http://www.flickr.com/photos/28819131@N02/2961163713/

7

These dowels will function as baffles for the LED's. That is, they will hold the LED's and direct the light into the acrylic and minimize the loss of IR light.
Drill holes at 1 inch intervals (33 of them) on both dowels.

8




To save time on drilling I used the first fully drilled dowel as a template to make the second baffle.

9




Sand the baffles now and make sure the inside of the holes are smooth as well. Otherwise you can damage the LED and ruin the transfer of light into the acrylic.

10




In this image you can see the Infra-red LED covered by a make shift baffle painted black. If you enlarge the image or go here: http://www.flickr.com/photos/28819131@N02/2916402447/
you will be able to see a tiny red dot inside the acrylic. This is the effect we are going for - transmitting the IR light into the acrylic.

11

Rub the baffles down with mineral spirit in preparation for paint. This helps clean off the sanding left behind on the wood. Wait a few minutes for the mineral spirit to evaporate.

12

Paint the side of the baffles that will lay against the acrylic, and the first half of the holes with white acrylic paint. This will help reflect the light into the screen better.

13

Paint the rest of the baffles, include the back half of the LED's black. This will help absorb any stray IR light in the environment and help keep IR light from escaping the baffle.

14

Place both baffles in position and both remaining dowels on either side of the acrylic screen.

15

Go to http://led.linear1.org/led.wiz for help on connecting your LED's.

16

Once you have power running into your LED's and they are set and soldered into the baffles it's time to remove the protective covering from the plexiglass sandwich. Don't forget to put the diffuser ( the drafting paper back). This will help you see the image the projector is projecting.

17

Now, drill holes for the bolts at either vertex of the frame. When putting the bolt through remember to place a washer between the bolt and the nut and the bolt and the frame. This will help spread the force more evenly across the frame.

18

Now, setup touchlib on your laptop. You can find it here: www.nuigroup.com/touchlib/.
Plug the projector into your computer as well as your IR webcam. Aim them from behind the FTIR display.

19

Calibrate the touchlib points. If everything went smoothly you now have a working multi-touch table. The fun part is programming software you can play with now.