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.
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.
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.
We should probably use some acrylic or plexiglass since it is easily available and reasonnably cheap. Acrylic would be better than plexiglass. We can buy this at our local homehardware since it will be cheaper this way
Online store
For acrylic : 3/8" thick
For plexiglass : 1/2" or 3/8" thick
LEDs in the 850nm region. Online shop or digikey. Pre-assembled belts (expansive)
Buy as needed from local radio shack can't know in advance how many or what type will be needed.
Shouldn't buy until the screen is working! By using a compliant surface over the plexiglass, we will reduce the amount of pressure needed to get a clear infrared reading. We can build a working system without one and at it at a later time. A compliant surface is usually a silicon rubber sheet about the same size as the plexiglass.
We can get some rosco grey here. Cheaper if it's not name brand. Needs to be the same size as the plexiglass.
Playstation Eye[wikipedia]. The older version (SLEH-00201) can be modified to filter out visible light becoming a cheap infrared camera. The biggest advantage of this model is that it can go up to 120 fps at a lower resolution. We can get one at our local computer store. Supported in Windows, Mac OSX and Linux.
Online already moded
Some black tape to absorb the infrared light which isn't at a critical angle.