Human Computer Interaction

Humans communicate with computers traditionally by using input devices such as mice and keyboards.  They are communicated with by the speakers and the screens.  This is a single user experience that is not efficient for groups of people doing work at the same time in the same location.  If you have a crew of people who are getting awesome with a computer, well, then you need a Multitouch Display Surface!  These things can be giant, so they are going to sit in your shared line kit and very rarely be moved.

Multitouch Display Surface?  That sounds pretty expensive!

Well, if you go buy one sure, but of course empowered friends don't let empowered friends pay retail!  It is suprisingly easy to put one together that preforms on the same scale as a commercial device, and the size on most of them is only limited by how big you can actually build it!  Many places (in Europe) have started using such surfaces for information centers and public internet kiosks.  It's a cost effective way to connect people by the dozens to technology!  IT"S ALSO SUPER COOL!

Ok, I'll bite.  How does it work?

The device is actually two seperate devices.  There is a display technology and there is a touch technology.  There are a few schools of thought on how touch detection things should work, and here they are a few!  These methods try and identify 'blobs' which are merely identifiable points on the surface where contact has been made.  The clearer and more responsive the blobs, the better.  You really just want the blobs and not anything that is not in contact with the material, but we will get into that sooner rather than later... but not now!

FTIR Method
FTIR stands for Frustrated Total Internal Reflection.
IR light is shined into the side of a flat surface.  This light is trapped inside by internal reflection.  When a finger touches the surface this light is frustrated causing the light to scatter downwards where it is picked up by a IR camera.

Diffused Illunimation
IR light is shined at the screen from either below or above the surface.
IR light is mounted underneath a surface with a diffuser.  When an object touches the surface it reflects more light than the diffuser or objects in the background, the extra light is sensed by a camera.  A diffuser is merely a layer that smudges an image, such as a peice of paper or some frosting on a surface.

Don't worry about it if you don't understand what that means yet, because you will! 

That's great, but what about a screen?

There are also a few options for this as well!  

Projection

There are lots of ways to project a digital image onto a surface.  You can use a digital projector, obviously, that will project onto a surface.  You can build a projector out of a light source of some kind and an LCD with the back removed.  LCD screens are actually clear with a white sheet behind them and some thin lights on the edges. You can also use any kind of screen placed in a box with a magnifying lens on the end.  You just have to have visible light going through a surface that digitally displays a screen and projects it out.

LCD Screen

Nothing is as crisp as a strait LCD image.  Projectors slightly 'blur' an image in comparison.  LCD screens often have backlights that emit IR light and that can cause trouble.  

All you have to do is put em together!  It's easy as pie!

Infrared Light, how do I love thee?

TFIR and DI touch systems both use infrared light to detect contacts.  Infrared light works exactly like regular visible light, only it's invisible to the human eye.  It goes strait and it goes through some things and not others.  Easy.  There are infared light bulbs and LED's just like visible light, and there are also infared cameras just like there are visible ones.  In fact, most cameras are already infrared but have a filter to block it out.  This is to make sure some nerdlinger's  bulging face veins aren't glowing during his VideoChat with somone in a rabbit suit.

TFIR

Many surfaces, like acrylic, retain infrared light.  If you have a thick peice of it and shine light through one edge of it, it will bounce around inside the material.  When something comes in contact with the material the light will actually bounce off that something and make it 'glow'.  If you put your finger on such a surface the only thing that is reflecting that bouncing light is your finger, and it is glowing like the sun on the infrared spectrum.  Below the surface of the material there is usually a thin sheet of another material called a diffuser.  A diffuser can be many different things, but almost all blur the image of whatever is on the other side except where there is a point of contact.  Place a finger on a peice of paper and notice how the shadow of the hand you see is blurry but the dark circle where you are touching it is well defined.  

Here is an image of how TFIR works.

As you can see, the light is coming from an LED and into the surface of the material.  The internal reflection is causing the light to zam around inside not going anywhere really.  When the finger came in contact with the surface though, it forced light strait down instead of on into the glass to continue bumping around forever.  This causes a blob to form on the diffuser, which can be seen by the camera that tracks contact points.

One of the most prominent types of TFIR surfaces are acrylic or plexiglass or some other compatable surface with LED's all around the edges.  This doesn't have to mean all 4 edges of course, but there you have it.  LED's are placed every few inches along the edge and sealed in with some kind of stripping.  In some cases the LED's are mounted into the screen by drilling very small round holes in the material and mouting them inside.  In some cases they are merely mounted against the surface.  A lot of care is taken to make sure that IR light doesn't escape out the edges by using IR blocking tape or some other material.  Underneath or behind the surface is an IR camera.  Whenever a finger comes into contact with the surface it glows in the infrared spectrum brightly enough to identify it against all background light.  From there, it's all software.

DI

The DI method of detecting blobs is almost identical to the previous, only the IR light that is being reflected is not coming from inside the surface.  This light can be coming from behind the surface in the form of an evenly distributed IR light source.  The light passes through the layers of the surface without being interupted and dissipate after passing through the final layer, which is a diffuser in most cases.  When an object of any kind comes in contact with the diffuser surface though the light reflects off and creates a blob in IR.  The surface is watched by an IR camera and it detects these blobs.  From there, again, it's all software.