With the initial work on the Multi-Touch table I had issues with coupling between the acrylic and touch points. You had to press quite hard to frustrate the internal reflection of the infra-red light. The issue here is that we need to change the interface that the light is encountering from acrylic-to-air, to acrylic-to-finger. However, when you press on the acrylic directly, micro air gaps exist between your finger and the acrylic, and at those points the interface hasn’t changed. To remove those micro air gaps, you can press really hard or moisten your fingertips a bit. Neither of these solutions is particularly great, and an alternative exists — to form a compliant surface between acrylic and your touch points. I’d attempted this several months ago with a silicone rubber product called Ecoflex. Unfortunately that material wasn’t usable when placing the projection material (Rosco Gray) on top of the rubber. I suspect that the EcoFlex didn’t cure properly, or was just the wrong type of silicone rubber. At the time I was under a time crunch, and just resorted to no compliant surface, and tracing paper taped to the bottom of the acrylic as a projection diffuser.
Over the past few weeks I’ve been addressing this issue (with the help of a new mechanical engineer we hired at the lab) and I’m very pleased with the results. This time around, we used SORTA-Clear 40, another silicone rubber, and after a few attempts have much better response out of the table. Here’s a video of the latest effort, with a bit of commentary.