Multitouch Surface Computer
It all started while we were researching an article on future user interfaces. Touch interfaces are hardly futuristic at this point, but multi-touch hardware like the Microsoft Surface or the iPhone is just starting to become a big deal, and we decided to see what big things are going on in that field. What we found that surprised us the most wasn’t anything about the future of multitouch; it was about something that people are doing right now.
The Theory
Before we can get into the actual, physical construction of the table, it’s important to understand just how it works.
There are several different ways to make a multi-touch surface, but we’ll focus on the one that we employed: the FTIR screen. An FTIR (short for Frustrated Total Internal Reflection) setup involves three vital components: a sheet of transparent acrylic, a chain of infrared LEDs, and a camera with an IR filter. The LEDs are arranged around the outside of the sheet of acrylic so that they shine directly into the thin side surfaces.
Once the IR light is inside the acrylic, it strikes the top and bottom surfaces of the acrylic at a near-parallel angle, and is subject to the effect known as total internal reflection. This causes it to be wholly maintained in the acrylic. This is a little tough to describe in words, so we’ve made a simple diagram:
The net effect of the setup described above is a sheet of acrylic full of internally reflecting infrared light. When a finger is pressed against the acrylic, it causes some of the light to be reflected down, through the acrylic and into the cabinet, where it is detected by the webcam. This effect, called frustrated total internal reflection is a little complicated, and involves something called an evanescent wave, but you don’t really need to understand why it happens, just that it doeshappen, as illustrated in this diagram:
The Screen
An FTIR multi-touch table’s screen is comprised of three basic components: The acrylic sheet, the LED lighting, and the projection surface. Each one requires a bit of work, so we’ll discuss them one by one.
The Acrylic
The foundation of the screen is the sheet of acrylic which serves as the medium for the infrared light. Why acrylic? Acrylic has several properties that make it a good fit for our project. First, it has the right optical properties, allowing for an excellent FTIR effect. Additionally, it’s lightweight, strong, and very clear (more so than glass).
We constructed our screen from a 24” X 30” X 3/8” acrylic sheet, which we bought at local plastics dealer TAP plastics. Acrylic can also be purchased on the web, although high shipping costs mean that it’s best to try and find a local plastics dealer. For a 24” X 30” sheet, 3/8” is thick enough to prevent any noticeable sagging in the sheet, even when firm pressure is applied to the middle of the screen. A larger screen would, of course, require thicker acrylic for stability.
The Surface
Now we’ve got our acrylic, and the LEDs are set up to shine into it, but our setup has still has two problems.
For one, acrylic is very clear. More so than glass, even. This is nice if you’re building a window out of Plexiglas, but it also means that if we tried to project onto the acrylic the light would pass right through. To solve this problem we’re going to use a sheet of drafting vellum, which is essentially a high-quality, durable tracing paper. This will act as a reasonable projection surface, and is fairly cheap. We got a 36” by 24” sheet for about 5 bucks at San Francisco art supply superstore FLAX. If you don’t live near a huge art store, you might have to do some calling around to find a sheet, or you can order them online, usually in somewhat larger quantities.
In our experience, the vellum worked very well as a projection surface, but gave the surface a distinctly "papery" look and sound, and it was sometimes difficult to make it lay flat. In a future revision of the build, we would like to experiment with having the vellum laminated before using it as a construction material.
The other problem with the acrylic surface isn’t really noticeable until you turn on the lights and camera and watch what happens when you actually touch the screen. On the bare acrylic, or the acrylic with the vellum, pressing your fingers down causes the FTIR effect to occur, reflecting light into the camera. However, if you try dragging your fingers on the screen, the effect gets much weaker, or disappears completely. To solve this problem, we need to create a “compliant surface” to enhance the FTIR effect. We made our compliant surface out of silicon sealant.
When it’s spread on with a foam roller, the silicon creates a thin coating of “microblobs,” with a very rough, rubbery texture. This is ideal because it allows the vellum to lie lightly on top of the acrylic, silicon side down. When you press down on the vellum sheet, the silicon squishes onto the acrylic, momentarily bonding with the surface, which alter the way light bounces around inside the screen and allows more to escape down into the camera.
The Lights
The array of infrared LEDs is what floods the acrylic with light and creates the vital FTIR effect. The exact construction of the array can differ greatly from one table to the next: which LEDs to use, how far apart to space them, how many sides of the screen to wire were all variables we had to consider. For our table, we decided to cover all four sides with LEDs spaced just a little more than an inch apart.
Like with the acrylic, there is an easy way and a hard way to connect the LEDS. The easy way is to buy premade infrared LED ribbons. Right now, the only source for IR LED strips that we could find was a company called Environmental Lights. These strips can be installed around the edges of the acrylic using an aluminum channel such as this one. Both the Ribbon and the channels run on the pricey side.
Now, we won’t lie to you, readers; soldering and wiring the LEDs was a pain in the ass. Not only is soldering 96 LEDs together tedious to begin with, but the LEDs’ leads had an unpleasant tendency to break under even slight force, requiring quite a bit of LED repair work. Also, even though our table worked just fine in the end, we would probably try to use even more LEDs in a future build. Because the premade ribbons pack more LEDs per inch and are much easier to use, we would probably go with those the second time around.
Because the voltage drop across each LED is 1.5V, and we’re using a 12V rail from a PC power supply to run them, we soldered the LEDs in chains of 8 (for a total 12V drop), then wired 12 chains up in parallel (leaving us with a handful of spare LEDs, which is absolutely vital). To make it easier to solder, we drilled 8 holes in a line in a piece of scrap wood, just big enough to hold the LEDs in place as we soldered them together.
There are different ways the LEDs can be mounted around the acrylic. Some people choose to drill holes into an aluminum or wood strip in order to make a frame to hold the LEDs in place. Since we were committed to doing things on the cheap, and also on a very tight schedule, we simply used electrical tape to hold all the LED chains in place, three on each side, shining into the acrylic. Sure, it’s not the prettiest solution in the world, but it was effective, and because the outer two inches of the acrylic are covered by wood, the tape doesn’t show up on the finished screen.