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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).

FTIR1 thumb1 Frustrated Total Internal Reflection

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.

FTIR2 thumb1 Frustrated Total Internal Reflection

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

ftircamera thumb Frustrated Total Internal Reflection

ftircam2 thumb Frustrated Total Internal Reflection

ftir3 thumb Frustrated Total Internal Reflection

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  • http://www.facebook.com/Taurusong Taurus Ong Kok Aik

    must need to(  Infrared Camera/Modified Webcam ) ?
    or Normal webcam suitable ?

    • http://sethsandler.com/blog Seth Sandler

      For FTIR, you need a infrared/modified webcam. You can’t see the infrared light from the FTIR without it.

    • http://www.facebook.com/Taurusong Taurus Ong Kok Aik

      Dear sir,
      Can i Use White Color LED , if  i using normal Webcam?

    • http://sethsandler.com/blog Seth Sandler

      Yes you can do that as long as you don’t want to project or have an image on the surface.

    • http://www.facebook.com/Taurusong Taurus Ong Kok Aik

      Get it your mean… another Question About the IR LED. 
      today i  buy a IR LED , but the Guy told IR LED Transmit or IR LED receiver .
      so which one needed ? transmit or receiver ?

    • http://www.facebook.com/Taurusong Taurus Ong Kok Aik

      Projection material mean diffuser ?mean just a Normal A4 paper?

  • Pingback: Frustrated Total Internal Reflection | Seth Sandler « bleeding logic

  • http://www.facebook.com/nour.hage.9 Nour Hage

    I have trouble getting the camera to correctly see my fingers when placed on the screen i am using ur software and yet im not sure if its a hardware or software problem the fingers are definitely more distinct on the edges closer to the LEDs, maybe the screen is too thick? i dunno pls help!

    • http://sethsandler.com/blog Seth Sandler

      Have you made sure to remove the IR filter from the camera? There’s a lot of reason there could be issues. I recommend going to http://nuigroup.com/forums where there’s hundreds of others building these and they can help you get things right.

  • Moosa Mahsoom

    We tried a lot to make the table using FTIR. But, specs of the IR we get at my place are quite low. Also, it was very difficult to make the Silicone layer come right. As a result, the project didn’t go very well. We’re hoping to make a 2nd model based on rear DI.

  • Jean

    Can we use laser modules instead of infrared LED with FTIR ?