here is a screen-shot of the first prototype with an LCD screen and FTIR..... the blobs come out beautiful and easily recognizable. 

also; i managed to achieve these results with minimal modification to the LCD screen.  The only modifications i made are:
1.) removal of last reflective layer (this is necessary obviously)
2.) adding IR-blocking film to the sides of the interior LCD acrylic.  This blocks any stray IR light from the CCFL.

I left the fresnel lense and the diffuser in the LCD and was still able to get crisp and workable blobs.  I dont think it is necessary to remove these filters, although some LCD’s may have two fresnels and two diffusers which may attenuate the IR signal.

In the end, I don’t think it is necessary or wise to try create your own back-light. Lining the acrylic with IR blocking film will considerable diminish unwanted IR light.

Looks really good! What camera are you using? Do you have more information on your project? Do you have compliant surface or do you just use bare acrylic?

Really impressed!

/Mijo

I’m trying to build the same thing but I’m not having as much luck getting any blobs to show up. I think the problem is IR light coming in from the cathodes, but I’m not sure what to do about it. You mentioned IR blocking film, but I haven’t been able to find a place to get that. Any suggestions?

He talked of the sides of backlight-acryl. So you block the IR before it gets to the backlight at the first place.

Thanks for your results Nima. Only problem is that I dont like the feel of bare acryl. I wonder if its possible to create some layering with silicone rubber and some other materils to get smooth surface…

M-Azing

You should post a how-to… A very detailed one at that.
I would really love to do this, seeing how youve done it here (and your vague instructions) I think you should post more detailed ones (with pics) and maybe take a look at what others ahve made (ive checked, to my knowlege, no one else have made a multi-touch screen with an LCD under it)…

AWESOME!!!

-Tyeo

I don’t think im going to end up even using acrylic, just touch the actual LCD screen.

In this picture, i just have the rail skimming across the surface of the screen.... i think it’s like the setup of the Tangent table where they had the LED’s on top of the acrylic instead of the side.

here is a raw image from the camera (no filtering)

Kamelisko - 24 November 2007 04:15 AM

He talked of the sides of backlight-acryl. So you block the IR before it gets to the backlight at the first place.

Thanks for your results Nima. Only problem is that I dont like the feel of bare acryl. I wonder if its possible to create some layering with silicone rubber and some other materils to get smooth surface…

I am going to try not to use any acrylic overlay at all.

Tristrex - 23 November 2007 06:11 PM

I’m trying to build the same thing but I’m not having as much luck getting any blobs to show up. I think the problem is IR light coming in from the cathodes, but I’m not sure what to do about it. You mentioned IR blocking film, but I haven’t been able to find a place to get that. Any suggestions?

If you go to Home Depot (or any other hardware type of store), go to the “Window” section and ask if they have any film that goes on windows to improve heating and such.  It is pretty common, a company called GLIA (that is own by CP Films http://www.cpfilms.com/) makes a film that blocks IR and some NIR (and some visibile)

Also, what LED’s are you using?

I am not using the osram sfh485 because they are too big (5mm) so i ordered 3mm IR leds from Fairchild semi.

any pictures of the box itself yet?

I dont even have any IR leds yet… and I dont think Ill be getting any anytime soon…

Anyone wanna donate some extra LEDs??? =\

Hey, this is really great !

I tried the same thing (you can find the result on my blog), and it works !

The only thing is that I have to apply a little pressure, because of the layer on the TFT (I think it’s the antiglare). Nevertheless, I tried this with 3 leds on 5cm and they were not aligned, maybe a clean setup will avoid the small pressure.

nima - 24 November 2007 03:59 PM

Kamelisko - 24 November 2007 04:15 AM

He talked of the sides of backlight-acryl. So you block the IR before it gets to the backlight at the first place.

Thanks for your results Nima. Only problem is that I dont like the feel of bare acryl. I wonder if its possible to create some layering with silicone rubber and some other materils to get smooth surface…

I am going to try not to use any acrylic overlay at all.

Hmm. How the FTIR effect works then? And where is the leds? Or is it some kind of DI solution? Or are you illuminating the LCD:s glass on sides? I wonder how LCD will last the contiuously touching (especially if you have kids

Sounds good tough that there’s no need for another layer…

I don’t know how Nima makes this work. I’m illuminating the LCD’s glass on sides (right now one side but it’s a prototype). 2 sides for the IR leds and 2 for the backlight.

I’m also concerned about how the screen will react some months later, but because of the little pressure I have to practice. It seems that Nima is not experiencing this problem ?

I must say that I didn’t really turn on my screen, I’m just able to display a black screen, but nothing changes when I’m doing that so I think it’s exactly the same with a working screen.

here is a quick video i made this evening of the LCD set-up.

Very nice nima! It looks like you’re using a modified front DI approach.

Although you’re already getting good results, If you haven’t tried it yet, you may get even better results and less CPU usage with the “simple highpass” filter rather then just the “high pass” filter.

yeah i should update my SVN to get the latest touchlib version (im still using a version from march or april or something!) Then I can test out the highpass and brightness/contrast filters.

as for the DI method;i actually started out designing the prototype using FTIR but removed the acrylic and discovered that it still worked.... actually it worked better because the anti-glare coating on the LCD glass doesnt get mucked up with fingerprints as quickily and easily as acrylic.  The 3mm LED’s im using help out a lot because they are a narrow angle (15 degrees).  Next, I will try to see if i can do DI from underneath the display.  I don’t mind having to touch the actual LCD glass because it just slightly soft and nice to touch.

more importantly, i want to get my hands on a large HD LCD TV (36” - 50") and convert that into MT.  Hopefully i can find a bargain on boxing day at a department store; If i cant make a large MT display from LCD’s then i’ll have to go back to projection-based systems.

I’m going to show a prof (for whom i made 2 DI tabletops already with the casio xj-s35) the prototype to see if there is any interest in upgrading the system to large LCD displays

Looks good nima,

a recent binary build can be found here: http://www.multigesture.net/downloads/touchlib-rev260.zip
It does requires some extra DLL’s from the cmu driver (http://www.cs.cmu.edu/~iwan/1394/) and OpenCV.