In my last column I mentioned a large multi-touch display, like the one seen in Minority Report. Then I looked at the trailer again and remembered that in the movie the UI was driven by gestures, not touches. Well, that notwithstanding, the gadget in this column is still extremely cool, and if you watch the video (fast forward to 2:00 if you like), very Minority Report like.
The instructable describes constructing a large (30×36) display that is multi-touch sensitive. The touch sensing is done via a clever technique called Frustrated Total Internal Reflection. You may recall from your electromagnetics class that total internal reflection occurs when an electromagnetic wave strikes a boundary at an angle larger (relative to the surface normal) than the critical angle. In this case none of the wave propagates through the boundary, it all reflects off the boundary. A fiber optic strand consists of a glass fiber encased in plastic. When a laser is shone along the axis of the fiber, it strikes the boundary between glass and plastic at a very shallow angle and is completely reflected back into the fiber. This is the reason that a fiber optic strand doesn’t leak light despite being, to the eye at least, transparent.
The touch sensor
A similar technique is used in this project. A sheet of clear acrylic is placed into a frame that contains infrared LEDs around the entire perimeter. Everything is aligned such that the infrared LEDs shine directly into the edge of the acrylic. Total internal reflection means that the interior of the acrylic is fully illuminated with infrared light, while little if any of the infrared light leaks out through the top or bottom surfaces. Until something touches the surface, that is. If you touch the top surface the your finger will scatter infrared light out of the bottom surface where it can be detected.
The second component of the touch detection is an IR sensitive camera. The CMOS sensors used in webcams usually have good sensitivity to IR, so one is used in this project to view the bottom of the panel. The camera is able to read the entire sheet in parallel, and software is easily able to keep track of individual touches and gestures. This technique was described in a 2005 paper by Jeff Han of New York University.
The display
That’s the touch sensing portion of the project, what about the display? In addition to the sheet of acrylic described above, there is another, thinner sheet of acrylic placed below the illuminated one. A sheet of tracing paper, vellum, or other translucent medium is sandwiched between the two sheets of acrylic. Finally an LCD projector is mounted above the panel and projects onto the top surface, with the translucent medium making it visible.
The visible portion of the display then all takes place above the panel, while the IR sensing all takes place below. They occur in different spectra (visible versus IR) so don’t interfere with each other.
The final component is the processing software. There is a large DIY and academic community working on multi-touch interfaces; one product of this is touchlib, which is what the author of this instructable used to create the demo video.
Improvements?
I think it would be interesting to try a sheet of frosted glass in place of the acrylic sandwich. The frosted side would be placed up, in order to provide a display surface. The frost would likely defeat some of the internal reflection, causing the entire surface to appear illuminated to the IR camera. However, there may be enough contrast between the reflection provided by a finger and the reflection of the frosted surface that the IR camera could detect it. If frosting the surface of the glass doesn’t work, then applying a frosted adhesive film to the glass might.
This panel would have the advantage of simpler construction, and the glass would be scratch resistant. I would imagine that a scratch in the acrylic appears as a permanent bright feature to the IR camera, and enough of these could “frustrate” the touch sensing software.
When reading this instructable (which is going on 3 years old) you’ll be presented links to other multi-touch projects, including whiteboards, rear projection TVs, and Wii Remotes. Give them a read also and see how those authors may have improved on this design.
Steve Ravet
EDN Gadgeteer
Tweet This
7 
