A bit of background to wet your appetite...

The Acrylic sheet
The Acrylic sheet (this is the surface that you actually touch) has a bunch of IR-LED's around it's edges (lots and even more depending on how big the sheet is, roughly about 1.5cm apart appears to be the norm). Some people seem to use just two sides, others have gone right round the whole thing - it all depends I suppose on where you are using it (background "light"), the power of the LED's and the size of the Acrylic sheet. But how does this mean that the computer can see where you are touching?

FTIR
FT.. what!.... stands for Frustrated Total Internal Reflection. What's it mean ? - well in a knutshell, if you shine light inside through the edge of the acrylic sheet the light will bounce around inside the sheet like a rubber ball in a box. If you then touch the sheet the light "scatters" as they call it (or "gathers" as you would see it) where your finger is. This creates a big white "blob" - everything that touches the surface of the acrylic will create this effect to a greater or lesser extent. The multi-touchn setup uses IR (infrared) LED's so that your naked eye can't actually see this process - but your webcam can.....

Hows it all work then?
Well your webcam is pointing at the acrylic and therefore "sees" the white (IR appears like white light to the camera) light where you touch it. The computer thats connected then uses these touch points to work out where on the screen you are pointing and then moves something or does "something" on the screen. The screen is the interesting bit. The current setup as I said, involves you placing some form of sheet (i've seen tracing paper used) on the back of the acrylic (but not touching it) so that you can use a projector to shine the computer image onto the underside of the acrylic sheet. The problem is, hence this little blog, that the projector is damned expensive and the bulbs cost a fortune to buy. So.... is it possible to do this without the projector?

Yes.

What you need is a little lateral thinking. Laptop and TFT displays are perfect - find one that you don't need. An external display has the advantage of having long cables but a laptop also has everything you need, including the computer system to drive the whole lot. What you need to do is take the TFT panel apart, and by apart I mean COMPLETELY apart so that you are left with just the panel, no backlight etc - the whole thing should be a mere 1.5 to 2mm thick. It will still work fine except that without the backlight it is hard to see the screen. (actually its quite weird / cool to see the XP desktop sitting on a sheet of glass that so thin). Anyway, the overriding thing you need to be aware of is - take some care, then more care and then even more care when you take the TFT screen apart - the slightest twist or extra force will crack it = put it in the bin. The moment you stop caring about it, is the moment its dead - hold it and treat it like your life depended on it.

This is the panel - no really - the vertical plastic tube is one of my Harmon Kardon II speakers (its holding it up!) - the right hand edge of the speaker that looks white is actually the TFT matrix (you can see some pcb electronics curved round the speaker that lead from it).

Just to prove it - heres the front view. The white object behind it all is the backlight from the panel - you can see the ribbon cable linking the panel to the laptop between the two. For the curious, the laptop concerned is an old (but working) Toshiba Tecra 8200. You can get these for peanuts on Ebay, not literally - though I suppose you could if the guy selling really did want peanuts as payment ;)



Its kinda strange seeing the XP desktop on something that is only slightly thicker than your average birthday card.