Frustrated Total Internal Reflection (FTIR)

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Contents

Background

FTIR stands for Frustrated Total Internal Reflection.

It is a name used by the multi-touch community to describe the multi-touch methodology developed by Jeff Han (Han 2005). The phrase actually refers to the well-known underlying physical phenomena underlying Han’s method. Total Internal Reflection describes a condition present in certain materials when light enters one material from another material with a higher refractive index, at an angle of incidence greater than a specific angle (Gettys, Keller and Skove 1989, p.799). The specific angle at which this occurs depends on the refractive indexes of both materials, and is known as the critical angle, which can be calculated mathematically using Snell’s Law.

When this happens, no refraction occurs in the material, and the light beam is totally reflected. Han’s method uses this to great effect, flooding the inside of a piece of acrylic with infrared light by trapping the light rays within the acrylic using the principle of Total Internal Reflection. When the user comes into contact with the surface, the light rays are said to be frustrated, since they can now pass through into the contact material (usually skin), and the reflection is no longer total at that point.

Building a Multi-touch Display based on FTIR

This principle is very useful for implementing multi-touch displays, since the light that is frustrated by the user is now able to exit the acrylic in a well defined area under the contact point and becomes clearly visible to the camera below. One point to note is that how well the camera will track your "blob" is directly dependent on the fps speed of the camera.



In order to achieve this effect, all you have to do is to mount some infrared light emitting diodes around the edges of the acrylic at a slight angle, and some of the light should be totally internally reflected in your acrylic. For best results, try slightly modifying the angle at which your leds are pointed into the edges of your acrylic.

A silicone rubber layer is also often used as a compliant surface on top of the acrylic. The layer of silicon helps to help make dragging smoother and increases the touch sensitivity of the device, making it responsive to little or no pressure. If a compliant surface is not used, for example when using an FTIR setup with only 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), this sensitivity is greatly improved.

LED Frame Material

Image:Frame_laser_test.jpg

As you can see from this picture, choosing the right material for the Frame that will holds your LED's is important. So you should use aluminium or some other shiny metal surface to hold your LEDs because plastics and other materials just won’t reflect very much light.

FTIR Parts List:

  • Acrylic/Polycarbonate/Plexiglass 8mm - 15mm in thickness.
  • Aluminium Frame
  • Infrared LEDs (or similar infrared light source)
  • Resistors
  • A Compliant Surface (usually in the form of silicone rubber ie. Sorta Clear 40, Elastosil, Lexel, etc. )
  • Projection Surface (Rosco Grey, Vellum, Mylar, other, etc.)
  • Infrared Camera/Modified Webcam
  • Band Filter


Links:

Wikipedia entry on Total Internal Reflection.

Multitouch Technologies

Compliant surface

From HCI to FTIR diagram

References:

Han, Jerfferson Y. “Low Cost Multi-Touch Sensing through Frustrated Total Internal Reflection.” Symposium on User Interface Software and Technology: Proceedings of the 18th annual ACM symposium on User interface software and technology. Seattle,WA, USA, 2005. 115-118.

Gettys, Edward W, Frederick J Keller, and Malcolm J Skove. Classical and Modern Physics. New York: McGraw-Hill, 1989.


Contributed to by: Cerupcat, Nade