Hello everyone !

I would like to build a multitouch screen (no way ^^), using a LCD screen. Why ? Because I don’t want a big multitouch screen and I don’t wanna spend more than 300 $/€
I made a projector one year ago, but I don’t want to use it here.

Check this out : here (in french… however the idea is perfectly clear)

Maybe someone tried this, but I didn’t find a topic on it. I think it’s possible to adapt this in order to replace the backlight of a LCD :

(perhaps the webcam position isn’t good, but it doesn’t matter, it can be moved somewhere else where it’s not hiding the light).
I wrote “special” layer because I need to find something to hide the LED panel, like this for instance :

With a flash

Without

The problem is that glass stops IR (I think so, maybe not ?), but maybe something else can do the same job.

What do you think of this ? Do you think this can work ?

i’m starting a LCD screen too.

I’m looking at this product from GE
http://www.modernplastics.com/may05/wdgesolarsheet19.html

it passes visible light, while blocking IR.  I’m thinking of using this to cover the the lightsource so unwanted IR light doesnt get through

By the way, is there a way to make FTIR working with ambient light or the room must be pretty dark ?

FTIR using projection-system can be used in pretty much any condition because the diffuser/projection layer (mylar, rosco, etc...) diffuses incoming light really well

I haven’t seen anyone be able to use FTIR or DI in high amounts of ambient light (like in the sun) though. Mild ambient light would work, but simply using mylar or tracing paper or I don’t believe will block enough IR. Rosco and/or IR blocking foil will block higher amounts of ambient light. If there is only normal room lights around, you should be fine.

true; if you place the sun in the same category as ambient light then IR blocking foil is a good idea.  Luckily, our tables were always set-up in darkened areas because of the projection.

going back to the original topic for a second, my new brainstormed idea is to remove the fluorescent bulbs that originally come with LCD screens, and replace them with a custom made rail of high-powered white LED’s.  Hopefully, the light will bounce inside the acrylic and reflect off the dots just like the fluorescent bulbs, but without the IR pollution

I can’t see how to set a position for a IR blocking foil, I mean is the “FTIR effect” possible, even if you block external IR, because you have to do it like this :

-------Fingers-------
-------IR blocking foil-------
-------Acrylic&co---------

Concerning the backlight, I didn’t understand your idea, because the acrylic is transparent so how the light could be illuminating the screen ?
Having something under the screen seems to be the only solution, no ?

IR blocking foil does not block all IR.  (Actually, I *think* it does a better job at absorbing IR than NIR)

I tried placing the foil directly over the floursecent lights; hoping to cancel out IR light from the source, and it does an *okay* job.... I still want to find out better/more solutions first.

re: light in acrylic

If you look at the acrylic block in LCD screens, it has several tiny white dots.  These reflect light back up to the lcd screen.  It works almost like FTIR, the light enters the acrlyic from the side and exists the acrylic when it reflects off a dot.

If you notice that the dots in the center of the acrylic are bigger than the ones on the side.  This is to prevent a significant portion of the light from existing right at the sides and having a duller center.

And when you turn on the existing lights, you have an even light ? Can you take a photo ?

paillis: in your diagram, the key missing ingredient is obviously the “special layer” that transmits IR light but reflects visible.  Typically, this called a “cold mirror”. But its hard and expensive to find large sizes of this mirror, Ive come across only small filters that go on lenses and stuff.

There are companies that can coat a surface with a cold-mirror surface.  One such company is DSI who have a coating called HeatBuster that they claim they can spray onto transparent surfaces like glass.  Again, I dont know what the costs associated with this process is.

I dont know how even the light will be without the diffuser.

I dont have working LCD screen yet (i was experimenting with a busted one) but I will get one when I get time to go to the store.

I want to get some answers this weekend for sure, so I can decide if its worth transferring from projection based systems to displays.... (I really want to get away from projetors)

Ok, here i go:

I’m working on LCD FTIR technology from the day I came here. Just to say it works well and you have some advantages…

+
High Resolution
Fast Response times

-
Need to solve the backlight problem
Not as big as a projector/beamer

But actually it’s pretty good. Look at my blog http://sandstorm-mm.blogspot.com for more information about my project. But I want to give you some advices:

1. Buy a fast response screen
2. Hope that it has only connectors and a chip on one side, or you have a BIG problem! That is losing a big space where touching the screen will not work
3. Like projectors you have the problem of normal light comming in. Use 2 sheets of tracing paper at the back of your TFT/LCD, and nothing else, just these 2. Also use a layer of a floppy disk for your camera
4. To solve the backlight problem use TL-light bars. Place them inside your box and make sure that EVERY reflective part of the inside of your box is taped with black non reflective tape. Why? TL-light bars are the only one that do not emmit infrared light, except when the light reflects on an object. Then that object will emmit some IR.

awesome! i actually bookmarked your blog a few days ago but didnt realize the amount of progress you made.

As for selecting a good LCD screen; there is a searchable database of monitors that people successfully used for DIY projectors.

http://www.baseportal.com/cgi-bin/baseportal.pl?htx=/Lumenlab/main

Search for LCD’s that have NO FCC issues.....

Thank you for your answers, I bought some components and I will try to start ASAP.

I’ll try to write here all the steps like you do.

nima - 15 November 2007 05:48 PM

awesome! i actually bookmarked your blog a few days ago but didnt realize the amount of progress you made.

As for selecting a good LCD screen; there is a searchable database of monitors that people successfully used for DIY projectors.

http://www.baseportal.com/cgi-bin/baseportal.pl?htx=/Lumenlab/main

Search for LCD’s that have NO FCC issues.....

Great database. :D