Hi,
since this is gonna be my first multi-touch table iam really worried about doing something wrong, because i also was not planning on spending thousends of Dollars
so i have some questions which i wanted to ask the comunity for helping me out.

1. What size for the acrylic plane is good?
I thought it shouldn’t be too small because iam not sure if iam gonna build a second one so if that will be the only one i should be happy with it, but also it shouldn’t be too big because of the costs. What do you think about a 100 cm x 75 cm?

2. How thick should the acrylic plane be?
I heard something between 8 to 10 mm.

3. How much power does the IR-LEDs need?
I heard in different forums it should be about 50mW/sr or greater? I was planing on setting those LEDs all around the table (4 sides) so i guess they don’t need to be too bright

4. What is better, an IR-LED with an acute shining angle or and wide?
I guess a more acute one is better because it has better internal reflection and it will get more even because i was planning on doing the leds on all four sides. Am i right?

5. What distance should i leave between each LED?
I thought at a distance of 3 cm between each LED, see Attachement

6. Do i have to put something on top of the acrylic? if Yes what?
some sort of special silicon?

7. Do i need to cover the Edges with something?
I mean when you have the acrylic in the frame (see attachments) should i maybe put some black acrylic/silicon on the edge so the IR-light is not coming out of the edges

Is a IR-Filter way better then a self-made-IR-filter?
I made one out of the black things from old picture thingies

I probably have a lot of more questions but i can’t tjhink at any more right now ... it is already quite a bit ^^
well ... i hope i get a lot of posts from helpful people =)

oh and yea i made the pictures in the attachement with SketchUp

Well, let me see if I can give you quick answers for each of your questions:

1. What size for the acrylic plane is good?
I thought it shouldn’t be too small because iam not sure if iam gonna build a second one so if that will be the only one i should be happy with it, but also it shouldn’t be too big because of the costs. What do you think about a 100 cm x 75 cm?

100 x 75 cm sounds like a nice size, but bear in mind that it won’t be easy especially for a first try. I would say an easy size to start with would be about 40 x 30 cm, just to test your approach. It shouldn’t raise costs much, because you would be able to reuse the most expensive components (namely camera and projector).

2. How thick should the acrylic plane be?
I heard something between 8 to 10 mm.

8 mm would be fine for the 40 x 30 cm tryout, but for the bigger 100 x 75 cm setup I would advise a stronger 10 or even 12 mm thick piece.

3. How much power does the IR-LEDs need?
I heard in different forums it should be about 50mW/sr or greater? I was planing on setting those LEDs all around the table (4 sides) so i guess they don’t need to be too bright

4. What is better, an IR-LED with an acute shining angle or and wide?
I guess a more acute one is better because it has better internal reflection and it will get more even because i was planning on doing the leds on all four sides. Am i right?

The two above questions actually depend on one another, because radiant intensity depends on the beam angle. But different manufacturers use different methods and criteria to give their leds specs, so you’re safer by going with the widely tried and approved OSRAM SFH 485.

As for the angle, you have basically two options: the rounded head SFH 485, with a +-20 half beam angle, and the flat head SFH 485P, with a +-40 half angle. They both work fine, but to get the most out of the rounded head one you should place your leds at an angle of about 40 degrees towards the surface, which makes your setup engineering a little harder.

5. What distance should i leave between each LED?
I thought at a distance of 3 cm between each LED, see Attachement

Most people leave 2.5 cm, but since you’re using all four sides, 3 cm should be alright.

6. Do i have to put something on top of the acrylic? if Yes what?
some sort of special silicon?

For FTIR, yes. Have a look at the Compliant Surfaces section of the forums. The info you need is all there.

You’ll also need a diffuser to project the image on. There are many options, so I’d advise you to look around the forums for that one too.

7. Do i need to cover the Edges with something?
I mean when you have the acrylic in the frame (see attachments) should i maybe put some black acrylic/silicon on the edge so the IR-light is not coming out of the edges

Sorry, I’m confused with what you mean by “edges”. The U frame you’re using should be enough to cover that actual edges of your acrylic (and I mean the faces where your leds are ponted to). But you’re gonna need a bigger baffle, which is the area covered by the U frame on the touch face. How bigger would depend on your leds angle, but a minimum of 3 cm is desirable.

Is a IR-Filter way better then a self-made-IR-filter?
I made one out of the black things from old picture thingies

Yes, it is. An industrial bandpass filter tailored to your leds specs is ideal. But that depends on your budget, since good filters can cost a lot. Photo negatives work ok for home made setups if used in low light environments.

I hope I could be of help. Good luck with your MT setup.