Our low cost multi touch screen was built using the frustrated total internal reflection principle.
Total reflection occurs when light travels from a medium with a high refractive index to a medium with a lower one. The light is then trapped within the first medium.
In our case the Infra-Red beam of light, transmitted by IR LEDS, remains trapped inside an acryl surface, since the refractive index of the acryl is higher than that of the air surrounding it. When touching the acryl surface, light bounces off the finger out of the surface.
The touch-screen is composed of an acryl surface, a standard projector connected to a PC, a mirror, an IR-sensitive webcam and a foldable, portable construction:
The acryl surface is used both to display the PC output and to sense user's input.
The acryl is coated with diluted silicon which enhances the amount of reflected IR light due to user's applied pressure on the screen.
Then, a sketch paper sheet is placed on the silicon layer to protect and isolate it. A diffusing material is layered on top of the sketch paper. The diffuser is semi transparent, allowing quality projection, on the one hand, and blocking access visible light from the other side of the screen.
The entire surface is framed with an aluminum frame which has 112 IR LEDs drilled into. The LEDs' tips, sticking out of the frame, are coated with black silicon. This is done to reduce IR light spilling in the wrong direction.
To keep everything together and protected from short circuits, a wooden frame is applied around the screen.
The 112 IR LEDs in the aluminum frame are divided into 14 sub-circuits, each containing 8 LEDs and one resistor. The 14 sub-circuits are connected in parallel, producing a circuit that consumes 1.4Amps and 12Volts. It is powered by a standard PC power supplier which is short circuited to work without a motherboard.
The reflected IR light is detected by an IR-sensitive webcam. We used the Microsoft VX-1000 webcam. To make the camera sensitive to IR light we removed its internal IR Filter. To make it insensitive to visible light we applied several layers of burnt camera film.
This project was done as a part of the Out Of the Box Week #IV at Microsoft Innovation Labs, Israel, by Hen Fitoussi, Kfir Karmon, Omer Ramot & me.
Helpful SitesNUI group
Instructables - Interactive Multitouch Display
Instructables - Back projection 56 inch multitouch television