Hi guys.

I recently started out building my first MT-table, a small 15” widescreen FTIR LCD as a school project (with purpose to show how it works rather than building the fanciest table ever).

Today I arranged my IR-LEDs in an array from LEGO Technics pieces (http://i371.photobucket.com/albums/oo151/andrew61987/bricks/1x14_black_x4.jpg) with one emitter in every hole. The IRs are 940nm, 20mA and have a viewing angle at ±10º. My PS3 Eye camera sees it clearly, when just heading them towards the camera.

Though, I had an insight a few minutes ago. Since the viewing angle is just ±10º, will i achieve total internal reflection? The plan is to press the LEGO-pieces against the acrylic (8mm thick) so they are pretty much bunted against each other along the long sides. Therefore the emitters will be sending their light straigt into the acrylic - until now I’ve been thinking that’s just great since I will have lots of light inside the acrylic, but, 10º < 40º so now I suddenly became very unsure whether this will work out at all.

I’d appreciate your help more than anything right now.

Best regards,

Rasmus

High your problem is the IR LED wavelength.

Most web cams peak at 850nm this is the optomal wavelength for IR tracking.

940nm is a little to far out of range and will appear dim.

Technic lego looks like a great way to mount LEDs!

Okey, so 10º beam won’t cause me any problems? At the moment I feel quite binary, haven’nt sleept for nearly 48 hours. But according to Snell’s law my IR should simply leave the acrylic since it’s angle is too low?

About the wavelength; I use a PS3 eye with floppy as IR pass filter. With only one layer (going to use two, if I can’t find some photo negatives) my array looks like this:

http://img96.imageshack.us/i/ir1.gif/

and that ain’t even from an optimal angle but the cam goes over exposed anyway. Feels like it shouldn’t be too weak, or will the acrylic actually weaken it so much?

And yeah, Lego was very neat and nice to work with at least =)

Maybe if you angled them it might work.  Anyway, because of the wavelength as well I think people are trying to say this is a bad choice.  Get the correct LEDs rather than waste your time and money.

You can search for some good ones at the Mouser Electronics site
http://mouser.com/Optoelectronics/Infrared-Data-Communications/Infrared-Emitters/_/N-6qrgl?P=1z0iyv5Z1z0iyugZ1z0wtzfZ1z0wtyrZ1z0wtytZ1z0wsawZ1z0sf62Z1z0wtlpZ1z0wtj3Z1z0wtk2Z1z0wtjbZ1z0wu6iZ1z0wu67Z1z0wsmyZ1z0j5h7Z1z0wtz2&Keyword=infrared+led&FS=True

I like the Knightbright water clear and inexpensive:
http://mouser.com/ProductDetail/Kingbright/WP34SF4C/?qs=sGAEpiMZZMvAL21a&#x2f;DhxMtO6Xj7JS5Ar5BRmVr1&#x25;2bZkI&#x3d;
specs here
http://www.us.kingbright.com/images/catalog/SPEC/WP34SF4C.pdf

50 degree viewing angle is dandy.  880 nm

here are some beautiful flat lens emitters (50 degree half angle) that would look great in a track
http://www.optekinc.com/datasheets/OP200.pdf

or these OSRAMS that have a huge radiant intensity 48mW!!!!  Infrared Emitters Infrared 880 nm HALF ANGLE +/-60DEG
http://catalog.osram-os.com/catalogue/catalogue.do;jsessionid=69037DF9CAC6B5DECD4E43FD7796C389?act=downloadFile&favOid=0200000200008557000200b6

If you want to use the same 20 degree half angle leds that many people here use there are many many choices - although I think you need 48 degree half angle LEDs....

Actually you can use your 10 degree LEDS, all you have to do is angle them at 50 degrees (or even 45 degreess).

The light that is traveling at a 48 degree angle will internally reflect, there is a range that the light is distributed from the LED so you should be fine at 45 degrees.

The wider angle LEDs (particularly SMD chips which have very wide angles but are difficult to solder) are nice in that they have a giant range (typically 120 degrees so you will hit the 50 degree mark in both directions).  It is not entirely necessary, if you place your LEDs at an angle.

Hi Rasmus,

I have had success with using strip mounted IR leds at 850nm wavelength and 120 degree beam angle. 10 degree is way too low and will leave the plexiglass unless you angle them as mentioned here.

If you run out of ideas or just want to solve your problem quickly I can supply the 850nm IR Led strip within within Europe for €40/metre plus shipping.

blu