Frustrated Total Internal Reflection (FTIR)
HOW IT WORKS:
The acrylic is filled with infrared light rays bouncing around inside it due to internal reflection. When an object comes in contact with the surface of the acrylic, it "frustrates" the infrared light and sends it down to the camera as a bright blob. The addition of a compliant layer helps couple the point of contact with the surface of the acrylic so that the reflection can take place with needing less pressure to be applied.
How do I make an MT FTIR setup?
The diffuser stops the image produced by the projector.
Compliant Layer
The compliant layer is used to increase the brightness of the touch points by momentarily coupling with the acrylic.
Acrylic
The optically clear acrylic needs to be about 8-10mm thick in order for the FTIR effect to work the best.
The source of infrared light for an FTIR setup comes from infrared LEDs that are placed around at least 2 sides of the acrylic. Typically the more sides surrounded, the better the setup will be in IR prevalent lighting conditions.
The webcam is placed on the opposite site of the touch surface so that is can see the blobs.
(The following comparison is from the nuigroup.com/forum thread)
Advantages
An enclosed box is not required
Blobs have strong contrast
Allows for varying blob pressure
With a compliant surface, it can be used with something as small as a pen tip
Disadvantages
Setup calls for some type of LED frame (soldering required)
Requires a compliant surface (silicone rubber) for proper use
Cannot recognize objects or fiducial markers
Cannot use a glass surface
Examples of FTIR setups:
FTIR Wiki Link
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