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Frustrated Total Internal Reflection

In FTIR, infrared light is placed and directed into the edges (sides) of an acrylic panel. The light is trapped within the acrylic by a phenomena called “total internal reflection”. When a finger touches the acrylic surface, the infrared light is “frustrated” causing the light to escape internal reflection and scatter downwards where it is seen by an infrared camera (modified webcam).


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 frustrate the light and the projection material must be under the acrylic (which is not ideal). With a complaint surface (like silicone rubber), the sensitivity is greatly improved. and the projection material may be placed on top.


Parts List:

  • Acrylic/Polycarbonate/Plexiglass
  • Infrared Light Source (LEDs or similar)
  • Projection Material (Rosco Grey, Vellum, Mylar, Paper, other, etc.) – For displaying an image and scattering light.
  • Infrared Camera/Modified Webcam
  • Optional:
    • Compliant Surface (silicone rubber ie. Sorta Clear 40, Elastosil, Lexel, etc. )

Camera Image Examples




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