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 a MultiTouch FTIR setup?

Tutorial Video:

The layers from top down are:

• Diffuser/Projection Layer: The diffuser stops the image produced by the projector.
• Compliant Layer: The compliant layer increases the brightness of touch points by momentarily coupling with the acrylic.
• Acrylic: The acrylic needs to be thick in order for the FTIR effect to work the best. LEDs surround the acrylic.

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 camera is placed on the opposite site of the touch surface so that is can see the blobs.

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:

Seth (cerupcat) AudioTouch
FTIR Wiki Link