Hi there,
I got a 6mm thick piece of perspex and want to use vellum
as the diffuser. I must ask will silicone help as part of the
blob detection even though I plan to use a Di set up?
Cheers

I don’t think silicone will help with blobs for DI. As far as I’m aware it’s only used in FTIR to improve the feel when dragging across the surface.

If anything it may make blobs less visible as you are increasing the amount of ‘stuff’ between the IR illuminator and your fingers…

Thanks for your reply,
Thats what I thought I am a little confused. I am currently testing
with sheets of tracing paper as a diffuser and having a few problems
with detecting the blobs evenly. I saw that vellum is the most widely
used diffuser and I was wondering if there is a certain thickness that
would be the most beneficial? Any Idea on how to stick it on to the surface
correctly (as I wish to make the screen vertical).?
Cheers

So you are using tracing paper in front of the illuminators and vellum for the projection material?

Sometimes I think it’s confusing as with DI you have a diffuser for the IR and a projection material which also sometimes gets called a diffuser.  Getting even coverage of IR light can be a bit of a nightmare with DI. In my set up I ended up lining the walls with tin foil and bouncing the IR around the inside of the box! The idea was that the more the IR was reflected the more diffuse the light would be, resulting in even coverage. We still found it difficult to avoid hotspots though.

I have since done some experiments with frosted acrylic and white paint. Both seemed to work well, perhaps the tin foil was too reflective in a way, our illuminators had quite a narrow beam and so even after bouncing off foil were still quite focused. White coloured things seem to diffuse the light more. This kind of makes sense if you think of white light being made up of all visible frequencies of light, then it’s reasonable to assume that IR frequencies would be uniformly distributed by white materials also...but I’m not an expert so just making a guess here. Also with the frosted acrylic I found that it seemed to diffuse the light more if it was further away from the IR source. Again this makes sense to me as the more diffused the light is at the point of diffusion, the more diffused the diffuser will make it! Know what I mean

Which also highlights the problem with buying prebuilt illuminators. What are yours like? Or did you wire some yourself? If so you might try more widely distributed arrays…

Thanks for that great reply.
Well currently I was using the tracing paper as the projection surface and the diffuser.

Is it wrong to do this?  I was told that architects tracing paper would be the right job for both.

The illuminators I am using are prebuilt with an array of 48 leds in a square formation.
I can only use two of them because the college cant give me any more. I get your point about
the tin foil which makes perfect since.
I wanted to make quite a big screen, like a metre by metre, and it is meant to be vertical. I reckon I am mad!

fearbeag - 04 August 2008 03:14 PM

I wanted to make quite a big screen, like a metre by metre, and it is meant to be vertical. I reckon I am mad!

You and me both then!

It sounds like you’ve got the right idea with diffusing the IR, you might just need to play around with different angles/materials etc like we did. I didn’t try architects paper as we couldn’t get hold of a piece big enough. I also heard that people have had good results with it though, but not sure if it was DI or FTIR. We did find that purlfrost white window film worked well, have a look for my recent post that shows some tests I did. It’s not too expensive so maybe you’re uni will pay..

My screen was pretty big (I can’t remember exactly but not much less than a metre wide) and it was certainly a challenge. I don’t want to burst your bubble but we had 4 x 148 led illuminators and still had problems lighting the whole screen so you may have difficulty with 2 x 48....

It seems to me that the bigger the screen (at least with DI) the more problems you encounter. First of all, since DI requires an enclosed box if you have a screen 1m x 75cm (if using a 4:3 display) then you have got to have a pretty big lump of a set up. You’re going to have problems getting it through doorways or even just moving it around at all. Also, all that space behind the screen has got to be filled with IR and that requires a lot of illumination/power/money. In addition to problems behind the screen, if you try to use your box in anything approaching daylight the amount of ambient IR from the sun could swamp your blobs. I’m not saying don’t do it, I’m just trying to explain some of the problems you might encounter. Necessity being th emother of invention perhaps you can find some creative solutions!

Hope this helps!

Thanks alot for the advice, it is really helpful, and much appreciated.

Panic stations I got alot of testing to do in the next 5 days, (prototype due on the 15th)
I saw on http://orionmultitouch.blogspot.com/ that he used 1*140 led illuminators
for his project which had quite a big screen and it worked pretty well.
Might reduce the screen size and get some more tests done, I will have no daylight
in the room I am displaying but I am wondering that maybe I could place a frame around
the back and use black out curtains to cover it with maybe lined tinfoil inside the curtains to spread
the light out . Hmmm.. sure its all part of the fun of it
Thanks
I will keep you posted.

just saw your post on purlfrost,
that looks like the bees knees,
I reckon I am going to go for that stuff

Yeah I wish I knew how he acheived that with just one illuminator! We saw that too when doing research and thought we might be able to do the same. No such luck though. FYI if you want to get really good blobs with low IR then a thorlabs band pass filter will really help you. They don’t come cheap unfortunately though :-(

Glad to hear the guide on purlfrost helped. Please do keep us posted.

Have fun and good luck!

Hi there,
We are working on our setup at the moment but we managed to do a bit
of testing with the idea for our project. Thanks for your help. We couldnt get
any Purlfrost but we are working on vellum and we still need more Inra red but
that is ordered. The screen wasnt calibrated properly either. However we think
that the results so far are promising.
The bubble wrap app . simple but fun

Our drag and drop game

I will put more up when things look a bit better. Cheers

One thought for a vertical display is to use a compact fluorescent or halogen bulb with a 6000K color temperature. Use a 100mm wraten 87 filter to block the visible light. You will get way more lumen’s then LEDs. I did this for my first table, there was not quite enough light for the entire surface (the Problem is the standard size of the filter cutting down the dispersion of the light) and not enough room for another light.  But with a vertical display you won’t have as much of a space problem and you can use more lights. Also if you can find a bigger IR passive filter you can illuminate the whole display with one light I imagine. It was very good light output where it hit the surface of the acrylic. I also just used trace paper and 12.7mm acrylic for the surface.

hi,
well I have been too busy to post
but we have nearly built all
of the touchscreen and it is working
fairly well. We had to use 3*140 ir led
illuminators but the results are really promising.
I will post up a new video tommorrow for you

How did it go with your multitouch project. I would like to know more about your setup if its possibly… I am also going to use some of the same things ... 3*140 ir and vellum and so on…

Ferrari_man