Back projection 56 inch multitouch television.

Step 1 Basic principle

Basic Principle

The principle is to modify a webcam to allow it to capture only IR light and point it to your screen. Then the software analyzes the images and detects press where there is a blob. A blob is the light projected back of your screen when you press onto it (Scattered Light on the schema).

Multitouch technology grows at a lighting speed during those years. There are many different methods for sensing the touch on the screen. As far as I know there's electronically and optically based setup. Iphone and Ipod touch are multitouch devices and use electronic circuit for sensing touches.

Optical systems are the simplest to build for home users. Those systems require a low light environment and may not be as stable as electronical systems. There's two big principle in multitouch optical system, the first Is FTIR (Frustrated total internal reflection) like I use and DI (Diffuse illumination) like Microsoft surface uses.

Frustrated Total Internal Reflection

When light encounters an interface to a medium with a lower index of refraction (e.g. glass to air), the light becomes refracted.
The extent of refraction depends on the angle of incidence, and beyond a certain critical angle, it undergoes total internal reflection (TIR).
Fiber Optic cable is a common technology that employs TIR.
However, another material at the interface can frustrate this TIR, causing the light to escape the waveguide there instead. Ref : http://www.cs.nyu.edu/~jhan/ftirtouch/

Under "ordinary conditions" it is true that the creation of an evanescent wave does not affect the conservation of energy, i.e. the evanescent wave transmits zero net energy. However, if a third medium with a higher refractive index than the second medium is placed within less than several wavelengths distance from the interface between the first medium and the second medium, the evanescent wave will be different from the one under "ordinary conditions" and it will pass energy across the second into the third medium. (See evanescent wave coupling.)
A transparent, low refractive index material is sandwiched between two prisms of another material. This allows the beam to "tunnel" through from one prism to the next in a process very similar to quantum tunneling while at the same time altering the direction of the incoming ray. Ref: Wikipedia

Diffuse illumination
In DI system, The IR light isn't inside the acrylic but projected onto it by the top or the bottom. The acrylic should be evenly projected onto the screen to be reliable.
+ Simplest construction
+ DI don't require a compliant surface (Rubber)
+DI don't require acrylic, it can be made out of any transparent material
+ Allow tracking of Fiducial
-Blobs are not as clear and sharp compared to ftir blobs

http://youtu.be/Wi8DqXYYehE

Comments

[CaptHowdy1173]

"DI don't require acrylic" ... is there a disadvantage to acrylic? I designed mine using acrylic and was going to coat the bottom side of it with a material that's about 20% opaque (using silicone glue). If anyone's having trouble understanding this there are TONS of materials elsewhere on this project.

[biotecnix]

endlightin plexi glass you will find works allot better if your using 850nm leds

[junior2387]

what is a good thickness for the glass... ?

[09wklaehn]

.5 inches is a good glass thickness. I would use plexiglass, much lighter.

[Tomcat94]

Um.... Can you explain that in english, plz?! LOL j/k. But seriosly. It's a tad hard to understand...

[Tomcat94]

Btw, gr8 ible.

[Christopher Jette]

Thank... I will work onto it to make it easier to understand but I suggest you to log on nuigroup.com/forum . Theres a big community working the same project as me. In fact, I've tooked every information there!