Best place to look is
http://nuigroup.com/forums/
Multi-Touch Sensing through Frustrated Total Internal Reflection (FTIR)
The Basics: An acrylic panel is edge lit with infrared leds. When your finger comes in contact with the acrylic, it scatters infrared light out the back where it is visible via infrared camera. As long as nothing is touching the acrylic, very little of the light escapes, instead just reflecting around inside.
Image processing takes care of detecting tips of fingers and relaying their location to application software. Since the camera "reads" the whole display in parallel, it is easy to detect multiple fingertips at once, even those belonging to multiple users. All this sensing goes on in the infrared spectrum, leaving us free to utilize the visible spectrum to display interactive software.
Well basically what you need is
1: A computer built in the last 5 years or so(duh)
2: IR LEDs (some use IR lasers but I prefer LEDs...better scattering)
3: a Webcam with the IR filter removed (I prefer the PS3 webcam. Easy to get, inexpensive, and very very suited for this project. Drivers have been written so it will work in linux and windows.)
4: 1/4" to 1/3" thick acrylic sheet.
5: LCD monitor or presentation projector.
Before you start, you will need to polish the edge of the acrylic to maximize the light that enters from the diodes. Some folks advocate drilling holes in the edge so the leds seat down in but they seem to work ok just butted up against the polished edge (actually butting up against the edge is the preferred method). Start with 200 grit sandpaper and work up to 600 or realms beyond. Some places that sell acrylic will fire glaze or laser polish the edges for you for a few extra bucks. Up to you if you want the pros to do it and save time, or save money and do it yourself. I always opt for the pros to handle that but you can get good results if you take your time sanding.
Next, mount your IR LEDs along the edge of the acrylic sheet. Some people make LED rails and build elaborate mounting kits/brackets that also serve as the edges of the tops of their surface computers. Some just tape the LEDs onto the acrylic. Really it's up to you.
Next take that PS3 webcam and remove the IR filter. Many suggest to put a clear filter in it's place so the camera's focal point doesn't change. I didn't and mine works beautifully but many swear that replacing the film makes the table even more sensitive and functional.
Now you just mount the camera a couple feet under the glass (or however far away you may need so the webcam only sees the acrylic from underneath and nothing else. The acrylic has to fit the webcams entire field of vision.
Mount the LCD screen (after disassembling it) or projector under the acrylic, shining up at it. Many suggest putting a special lens in front of any projector to absorb any IR and heat given off by the projector' light. Hard to explain here what all is needed in adjusting the projector and cam. Best would be to follow the instructions on the forum link listed above.
Some suggest an optional step of protecting your acrylic and giving you a nice screen to project on by buying Velum tracing paper, the size of your acrylic, placing a special, thin (but easy to make) mix of clear silicon caulk/acetone (1:1 mix) on the velum, allow to set for 3-5 days (to cure) and place the velum, silicon side down, onto the acrylic. The silicon does two things. first it keeps the velum on the acrylic. second it gives your finger touches more IR 'pop' to the webcam's eye, meaning better and more accurate responses. The velum gives you a nice screen for the projector or LCD's light to be reflected on.
Anyway, given that, now you just hook power to the LED's (a 3v rail from your ATX powersupply should be just the ticket. Switch for LED's on/off is optional. A simple set of free, multi-touch software is available for Windows, MacOS, or Linux.
This is the basic idea. Costs about $100-$200 total depending on the parts you have laying around. Minus a computer, if you needed to buy every part, you probably are looking $30-$50 for the Acrylic (depending on your size of screen), $20-$30 for the IR leds, $50-$100 for lumber (if you were going to make your own table/case), $5 for the Velum, $5-$20 for the Silicon caulk (I recommend GE's Silicon 1 brand), and $100-$2500 for whatever you are going to use as your monitor (LCD or projector).
