Hi,

Quick question. Ive started building a small protoype FTIR screen. Im using 12mm Acrylic, the sheet is about 50cm x 30cm. As a test I have wired 8 LEDS into one corner of a frame and thought (well hoped!) that this would be enough to test it i.e. should be able to see a blob. Cant see anything at all.

To clarify:-

1) My camera (Creative Webcam 5 with photo film filter) definitely sees IR light (have tested it with IR pen).
2) My 8 SFH485 LEDs are happily blazing away
3) My webcam can see the LEDs if I tilt it and point it directly towards them (i.e. take it from perpendicular to pointing at the leds blobs galore).
4) My Touchlib seems to working fine and when I wave an IR pen in front of it it picks up blobs no problem.
5) I have polished the edge (of one side only) with wet and dry paper using different grades up to 1000. The edge still seems kinda cloudy, is this a big deal just for a small test?

So when I press on the surface no blobs at all appear?

Note, at present I have NO compliant surface or Roscoe. Im just testing the touch aspect not projecting at this stage and therefore i dont think I need a compliant surface. Is this correct? I tried a bit of paper and a Kleenex just to test but this made no difference.

Im think the problem must be either the polished edge (I read somewhere maybe I should use Brasso?!?) or maybe lack of LEDS i.e. they’re just in a corner but I figured this should at least allow me to test the corner?

Heres a poor quality pic just to show the dimensions.

Any help much appreciated, and apologies if the answers obvious

Cheers,

Al

BTW, if I point my webcam at the opposite side of the acrylic to the LEDS even though it unpolished it floods the webcam with IR light so im figuring it must be getting through to the other side of the acrylic in some form or another?!?!?

i think that your problem is that your edge is cloudy, take a bit of brasso and pollish the edge. hopefully that’ll help

Taha

Did you take off the IR filter from the webcam? Keep in mind that all webcams can see IR, but you still have to remove the filter. I have unpolished edges on my table and things are fine (so I can’t say if that’s the problem). Put your fingers on the side there LEDs are coming from and see if that makes any difference. If you have some light coming out the other side, I would think you’re fine with unpolished edges.

Alex I had the exact same problem on my first test rig!
It turned out that two things helped.

First, I polished ALL sides, It helps trap the light. When it hits an uneven surface, it’s the same as a blob. The light scatters and escapes the acrylic.
Try polishing all four sides. Trust me…

Second, LED angle. I noticed in messing around with my test rig that if I tilted the LEDs at an angle of 40 - 45 degrees to the edge instead of 90 degrees, my finger would show a blob.
Try this while watching the monitor and pressing on the acrylic. Use your free hand to tilt the LED rail at an angle. I’ll just bet you see a blob appear.

Hope that helps you get blobs!

Dino

Many thanks for the info guys. I shall get polishing today and also look at the LED angles etc....

Can i just check, im a bit confused about this IR filter business. My webcam unaltered can see IR LEDs. Ive put some processed film in front as a filter so now all it sees is IR and it sees the LEDs fine. Do I need to dismantle it and remove some internal IR filter? Presumably if there was a functioning filter that blocked IR I wouldnt be able to see the LEDs?!?

Cheers,

Al

Hi Alex,

All cameras can see IR light, but this is not enough to register blobs. You need to remove the IR filter that is inside of your camera, so that when you point the camera at an IR LED the image goes completely white; that’s when you know you got it right.

Good luck!

Thanks for the info everyone!!

I hacksawed through my Creative webcam lens and removed the IR filter, wasnt pretty but it worked!

I also Polished all sides and have one complete side of LEDs working. I get faint IR blobs so im definately on my way Building the other rail of LEDs now so that should help.

A few quick questions:-

1) As per the info Ive noticed that angling the LEDs makes a BIG difference. I have my LEDs mounted in rails (C shape facing the acrylic) though so the only way I can get an angle is if I take the rails off slightly and tilt them. Is there a way around this?

2) Next step is Roscoe and compliant surface I guess. Whats the current take on the best way to do this? UK suppliers ive seen seem to charge about 12-15 quid for 1m Roscoe but then about 15 quid delivery so £30. Anyone found anything better?

3) Im using Touchlib v2 beta. It seems to have far less sliders and stuff than v1 or the Mtmini package. Which version do people find is most useful?

4) In terms of scaling up once my prototype works is 1 LED every 2.5cm (about 1iinch) going to do the trick for a 60 inch job?

Cheers for all the help,

Al

Hi Alex,

about 1):
I know that there are lots of discussions on having different angles, etc.
My personal experience was, that it’s no “must” to do so, depending on the construction of the frame.
I used aluminium c panels with 6 mm space and used a acrylic plate with 6 mm thickness. leds (Osram SFH 485, 5mm diameter) are every 2 cm (2.5 is good as well, might make no difference). I had good ftir effect right from the beginning using this simple construction. i think the edges of the panel, which hold the acrylic already work as baffles on the top and bottom and also do reflect some light from the leds into the acrylic.

I got really good results without changing the angle, but there are surely ways to enhance.

Personally, i just go for the 0 degree alignment (so in one line with the plate), with the heads of the leds touching the border of the acrylic.

I wonder if this (different angle) might have something to do with the thickness of the plate, cause i read on the forums that some people do not get any effect with thin plates.

I get very good results with a 6 mm thick plate (60 x 70 cm). For larger screens, of course, a thicker one should be used due to bending of the plate
when it’s to thin.

The more i think about it:

i work with a 6 mm plate, the led heads have a diameter of 5 mm. So they are almost the same diameter.
So for thicker plates that use 5mm leds, the angle should change due to bigger difference (thickness of plate is almost twice than diameter of the leds).

For polishing i used different grains:

from start to finish: 400 - 600 - 1000 - polishing paste (very fine grain for highlight polishing)