|Multi Touch Screen, By Mike B and Alex C (Page 1)|
A Multi Touch Screen is just what it sounds like, a screen that allows multiple input points for location and movement tracking simultaneously. These multiple points can be used for interaction with computers, just like a mouse or key board but with a more natural tactile approach with the computer screen reacting to every point of touch with instant visual feed back at the points of contact. Until recently most "touch" screens on computers could only track one contact point at a time. Recently multiple technologies have been developed to track more than one position, the Apple iPhone is just one of those devices that utilize a variety or techniques to track multiple points.
This particular build works off of a technology developed by Jefferson Y. Han and many others called "Frustrated Total Internal Reflection". Basically this technology works because light moving around inside of a solid clear plastic or glass tends to bounce around inside and only escape in a even "leak", however when one places an object against this surface it breaks the clear materials ability to evenly reflect the light. At these contact points where the reflective nature changes more light escapes. this increase of escaping light can be "read" using various techniques. Its a hair more complicated than that, but this is the simplest explanation of how it works.
The particular technique we will be working with uses Infra Red light. IR light is just outside of of the visible spectrum so it can be used without effecting or compromising a rear projected image on the same surface. with this technique you can have two different bands of light one acting as an input device (the IR), and the other acting as a visual output device (a visible spectrum projected image) on the same surface, and since the eye can only see the second, the first does not intrude on the integrity of the visual output. and to "read" the IR escape points, you simply need a camera that is modified to only "see" IR.
We started construction on the screen in June of 2008 In hopes of having it ready by Critical Massive. We started by preparing the screen itself which is a 1/2 inch thick sheet of lexan sandwiched between two thinner sheets of similar plastic. Mike acquired the screen material from tap plastics where they did a rough cut to make the 2 foot by 3 foot screen that we wanted. To assure than as much IR light as possible entered the screen we heat treated the edges to make them more transparent.
To do this we ran multiple passes with a propane torch along the edge, this melts the rough (diffused) edge into a glossier finish. we started first with a bit of scrap to get a hang of the process. As you can see to the right the heating warps the edge a bit, but as long as you don't over do it you can keep it fairly even and fairly shallow.
I believe that we found that three slow passes with the torch parallel to the length (see first photo) and then allowing it to cool and repeating 3-4 times resulted in the most even and clearest finish. As you can see the clear edge on the right (of the photo on the left) is far more transparent than the rough cut side from the saw at tap plastics on the left. We prepared all 4 sides in this manner and moved onto building the back frame.
We routed two groves down the entire length of wood, one grove that ran down the center and created a channel for the LED power leads to run through and a second router pass along the inside edge of the frame for a track the screen would sit in. we made this second grove deep enough so that when all three pieces of screen where in the frame, they would be flush with the top of the wood. we then used a chop saw to cut our 45 deg corners. This is one of those places where we measured a hundred times, and still got it wrong on one of the pieces,. it all has to fit fairly tight,. my only advice is measure a 101 times and plan on having a little extra wood on hand just in case.
Next we drilled out evenly spaced holes from the inside edge to the center channel, 8 holes on the short lengths and 11 holes on each of the long ones. these drill holes where for the IR LEDs to sit in, evenly spaced, and pointed towards the center of the screen. This gave us a total of 38 LED holders, which we figured would be enough if as long as we got the right Super Bright LEDs. We found what we where looking for at mpja.com 940nm "super bright" IR LEDs with "17mW/sr min. intensity" whatever that means,..
Mike chose a fairly unique hard wood called heart wood which stained to a beautiful soft purple for the front of the screen. it was meticulously cut and glued together to nearly match the inside edge of the previously constructed back frame.
Later we painted the back frame and the legs flat Black so that only the beautiful purple finish of the heart wood could stand out without distraction.