Posted: 09 May 2007 07:39 PM   [ Ignore ]
Joined  2007-01-08
Total Posts:  1008

Marek wrote an interesting reply in another thread and I thought it was worthy to put this in a special thread:

By the way, I found some interesting patents (from the eighties) referenced in hans original paper

These are two patents for something similar to a FTIR multi-touch. The first one is using “silicon rubber” (the flexible membrane)! “So the answere was there from the beginning” I thought first. But then I read another patent, building up on the first one, saying the silicon rubber is not good enough, but see for yourself:

A touch sensitive device is arranged with substantially parallel surfaces which are at least partially transparent to light. A flexible membrane is supported above the top surface of the screen and when the overlay is flexed into contact with the screen, light which had passed through the screen, is trapped between the screen surfaces by total internal reflection....

The above-mentioned Kasday patent suggests using silicone rubber for the compliant overlay. While this material is somewhat soft, the user nonetheless must apply an uncomfortable level of force to the silicone material to increase the level of trapped light to a point where the resulting photodiode signals can be processed using inexpensive detection circuits. Although materials that are softer than silicone rubber are easy to deflect and hence trap a higher level of light signals when they are touched, they are, for the most part, nonetheless unacceptable as a compliant overlay, since such materials disadvantageously tear easily when they are stressed and are also tacky. A material that is tacky sticks to the surface of the CRT screen and is easily marked by fingerprints.


We have recognized that a soft compliant overlay is achievable, one which substantially deflects when touched and one which can be used with inexpensive detection circuits, by forming, in accordance with the invention, an overlay pouch from two semirigid, or flexible, plastic sheets that is filled with a gel. In accordance with a feature of our invention, the gel-filled pouch may be attached to a semirigid plastic frame to hold it taut and to provide an easy way of attaching it to the face of a CRT screen.

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