IR-LED panel vs. IR-Illuminator
Posted: 23 March 2012 08:06 AM   [ Ignore ]
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Hi to this awesome and helpful community,

This is my first post, so please don’t be to harsh on me smile

We’re going to build a multi-touch & fiducial table and decided on “rear DI”. That’s why we have to place IR LEDs or some IR Illuminator in our box. Here are some specifications of our table/box: the “screen” is 100cm (40") * 75cm (29.5"), so the diagonal is 125cm (49") and the height of the box is 91cm (36")

Is it easier to get a even illumination with IR illumnators or with LED panels placed around all sides of the box, shinning upwards?

Below i made a little sketch to illustrate the options i mean:

seb9h.jpg

I’m thinking about building the panels with one LED for each inch, which would be 150LEDs in total.

I checked a lot of “LED theory” and it seems that almost everybody recommends the OSRAM SFH485(P), which has a radiant intensity of 48mW/sr and a angle of half intensity of +-40 degree.(http://catalog.osram-os.com/jsp/download.jsp?rootPath=/media/&name=SFH485p_Pb_free_2009_08_21.pdf&docPath=Graphics/00052522_0.pdf&url=/media//_en/Graphics/00052522_0.pdf)

I also found this one TSAL7600, which has a radiant intensity of 200mW/sr and a angle of half intensity of +-30 degree.(http://www.mouser.com/catalog/specsheets/tsal7600.pdf)

The TSAL7600 seems much better to me, even so it has a smaller angle of half intensity, it has four times the radiant intensity
Does anybody has experience with the second kind of LED’s?

The radiant intensities stated above are for I=1A. Do I have to check the radiant intensities for 1A or for 100mA. Sorry, I’m quite new to electronics.

Another thing is that at one page in the wiki (http://wiki.nuigroup.com/IR_LEDs) it says that the OSRAM SFH485P LEDs are the most popular, but on another page in the FAQ “What kind of LEDs are best and where can I buy them? “ (http://wiki.nuigroup.com/Frequently_asked_questions#What_kind_of_LEDs_are_best_and_where_can_I_buy_them.3F) it says that the radiant intensity should have a minimum of 80mw. But as I understand it, the OSRAM SFH485P has a radiant intensity of 48mw, so why is it the most popular LED for this purpose then?

Thank you very much and keep up the great work

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Posted: 23 March 2012 10:30 AM   [ Ignore ]   [ # 1 ]
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I’m new to this too, so take my $0.02 with a grain of salt.  I used the SFH 485 P’s in my touch table, but I think they’re popular for the FTIR / DSI crowd more than the rear DI.  They’re great for making LED frames, but maybe not so much for illuminators.

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Posted: 23 March 2012 01:46 PM   [ Ignore ]   [ # 2 ]
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Pointing the LEDs straight up to the touch surface is a poor choice regardless of what arrangement the LEDs are in.  The reason is that the LEDs will create terrible hot spots and make it difficult to get an even diffusion of IR light across your entire touch surface (the goal in any Rear DI setup).  You are better off placing the LEDs at the top of your enclosure and pointing them down towards the bottom of the box, but keep them out of site from the camera.  This allows the LED light to bounce off the bottom and sides and more evenly diffuse out the top.  The beam angles of the LEDs used in a Rear DI setup should be as wide as possible so as to reduce the hot spots.  This means around the 120 degree beam angle, not the 30-60degree that those OSRAM LEDs you mentioned are (we carry 120degree LEDs).

Watch our video below on how to build a Rear DI setup, it should help explain a lot:

Also, check out the Rear DI section of our store if you are having trouble finding parts for your setup.

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Posted: 24 March 2012 07:43 AM   [ Ignore ]   [ # 3 ]
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Ok, thanks for the great input. I looked for some LEDs with a big (120°+) beam angle and a decent radiant intensity, but I think I’ll just go for some IR Illuminators instead. They seem to be cheaper, I safe all the solder work and they gonna be easier to rearrange.

So i guess I’ll try it with the following arrangment:
ghp4s.jpg

Illuminating the screen indirect by pointing the IR-Illuminators to a diffuser palced on the wall of the box.

I think i’ll go for 4 of this 48 LED Illuminators (http://www.ebay.co.uk/itm/48-IR-LED-Night-Vision-light-Infrared-Night-Vision-Light-CCTV-/251016569419?pt=UK_CCTV&hash=item3a71c0e24b). EfeNDy seems to have quite a good result, even with pointing them directly on the screen. (http://nuigroup.com/forums/viewthread/3176/).

Thanks again

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Posted: 26 March 2012 12:27 PM   [ Ignore ]   [ # 4 ]
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Again, having them at the top of the enclosure pointing down with give you the best, even diffused results.  Those IR lamps that you’re looking at using are pretty much flashlights since they have a super narrow beam angle.  They will create a small circle of light on the opposite wall, making it very difficult to get good illumination of your fingers.

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