Intro: Back Projection 56 Inch Multitouch Television.
Step 1: 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
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
Step 2: Material Needed
This is the component I use now in my system. You can use different gear and get same or better results.
-Computer core2 recommended
-Projector 550$ on ebay
-Webcam 55$ on tigerdirect
-Special lens 25$
-IR Filter 25$
-36 x 48 inch Plexiglas 1/2 inch 150$
-36 x 48 Rosco Gray Rear-Projection Screen 30$
-168 Osram SFH485 880nm IR LEDs 115$
-Wire and resistors 20$
-Protective foil Different from mine--- Seems similar 30$
-Mdf, hdf or plywood sheets
-Front speaker fabric 40$
-Many different tools
-wood finishing product
-Sound system 115$
Step 3: Preparing Acrylic
DO NOT Use the pouring method as this has been found to result is Crazing (Cracking) of the Acrylic due to the Toluene.For best results use the spray method in thin layers. I suggest you to run some tests on spare acrylic before you do your final screen.
The silicone rubber layer helps coupling between the Rosco projection screen and the acrylic. It allows the user to have brighter blob for less pressure onto the screen. Without it users should have to press really hard to get decent blob.
You can watch a video that illustrate the differences at:
'''''Dowload my explicative PDF for rubber'''''
Step 4: Ir Section
In this setup I put one led every inch on four side of the acrylic. 36x2+48x2= 168 led. Big quantity of leds helps to have really bright blobs. For my experience 1 led every inch is enough to get nice result. I got the best performance by drilling 5mm holes on side of the acrylic and then glued them with testors glue. Then I wired the leds and paint the acrylic edges using really dark black paint. I don't use any frame on side of acrylic, black paint block all IR and it's a lot simpler to avoid leaking of IR.
Step 5: Fixing Other Layers
Your acrylic is now ready to be covered with other layers. Before you stretch the Rosco on the acrylic you have to put a 1mm spacer on edges of the screen to avoid the Rosco being directly on the silicone rubber. I personally use arborite which is 1/32 inch thick. This small air gap helps to avoid the Rosco from sticking to the acrylic.
The easiest solution I find out to this problem its tape, I used the best tape I found. For the simplest result you start by stretching the center in both directions horizontally and vertically and then you process to the outside in rotation. Go by small steps to avoid wave in foil. First you have to stretch the Rosco than your protective foil. I use to cut both exactly the same dimensions of my screen so it simple to tape it on edges and at the backside.
As protective foil I personally use Insulating film which are really thin and retract when you blow warm air onto it. I took the thicker of the 3 size. You have to stretch it using the same method as the Rosco and then you blow warm air from an air dryer to eliminate waves. My results with this product were awesome compare to those obtain with my previous vinyl which was too thick and hard to hold with tape. I suggest you to put some layers of tape around the edges of the acrylic to help other tape to hold.
NEVER STICK TAPE DIRECTLY ONTO THE ACRYLIC OR COVER IT WITH BLACK PAINT!
Step 6: The Box Plans
Size of the box
Height: 73 inches
width: 50 inches
deepness: 24 inches
Size of the screen
33.75 x 45 inches
56 inches diagonal (4:3 ratio)
If making this box is a problem you will find a lot information on this website:
Step 7: Position of Components
Position of components
Step 8: Software and Link
I made a website with video reviews of softwares with download link
You can see more pic here: Link