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Choosing the FTIR or DI set up

FTIR and DI are the two main methods used in multi-touch technology. Although it is possible to incorporate both into the one table we opted to choose one method for this project.

FTIR = Frustrated Total Internal Reflection

IR light is shined into the side of an acrylic panel (most often by shinning IR LEDs on the sides of the acrylic). This light is trapped inside the acrylic by internal reflection. When a finger touches the acrylic surface this light is “frustrated” causing the light to scatter downwards where it is picked up by a IR camera.

DI = Diffused Illumination
IR light is shined at the screen from either below or above the surface. IR light is mounted underneath a hard clear surface (ie. glass, acrylic) with a diffuser (ie. tracing paper, mylar, semi-transparent material). When an object touches the surface it reflects more light than the diffuser or objects in the background; the extra light is sensed by a camera. Depending on the diffuser this method can also detect hover, and objects placed on the surface.

Although blobs are not as clear and sharp with DI when compared to FTIR we still choose DI for this project for these reasons:

* The IR-Illuminators are ready to work so no soldering is involved
* It is possible to use other transparent materials like glass in place of acrylic
* Fingers and fiducial marker tracking possible
* The setup is relatively simple although it can be very difficult to get the light working right
* There is no need for a compliant surface, a diffuser/projection surface works well.
* It is possible to use frosted glass in place of a diffuser
* Dragging can be very smooth if done properly

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