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FTIR and DI. How they work and what they mean for multitouch.

FTIR and DI are the two main technologies used in NUIGroup (I will be posting information on alternative techniques at another time):

FTIR = Frustrated Total Internal Reflection (a phenomenon) – This is currently the most popular method, possibly due to the wide internet distribution of Jeff Han’s videos. 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.

Basically, IR light shines into the sides of acrylic (glass does not work) and when a finger touches the surface, IR light, which can’t be seen by the human eye, lights up and an IR capable camera picks up this light.

A silicone rubber layer is often used as a “compliant surface” to help improve dragging and sensitivity of the device. When touching bare acrylic, one must press hard or have oily fingers in order to set off the FTIR effect. With a complaint surface (like silicone rubber) one can press with “zero/no force” and the sensitivity is greatly improved.

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.

Basically the diffuser lets light through, but not all of it. Therefore, the light that hits the diffuser is around 50% of the total light. When a finger touches the diffuser, the light can’t pass through the finger and therefore 100% of the light hits the finger which then lights it up for the camera to see.

For more information, Tim Roth has a great blog entry about the practical differences between FTIR and DI here.

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