at firts sorry for the english!
I have used the QED233 / QED234 Leds:

. IR EMITTER, 5MM 940NM
. Radiant intensity:200mW/Sr
. Angle 40°
. Current, If av:100mA

datasheet--> http://www.robot-italy.net/downloads/QED234.pdf

I have used 20 leds for 30x40cm surface

the camera can see the led in front (can see infrared lights)!
but if I put the leds at the border of plexiglass for FTIR, when I put a finger or hands on the screen I can’t see the blobs.
this leds are right?Are enought?
the problem could in the camera?

@FairyShine…
so you don´t get any blobs correct?
This could have a couple of reasons..
What Camera are you using and how did you treat the edges of the acrylic?
BTW 940nm Leds are not the best choice, because CCD Chips in Cameras are less sensitive in that
part of the spectrum. You should go or something around 850-880 nm. Maybe you should use
more Leds as well…

hey FairyShine ... m not sure if it really is a problem of the cam… i guess you are mis-reading the datasheet cause the Power Dissipation that is rated at 200 mW whereas the Radianty intensity is on the second page in the “ELECTRICAL / OPTICAL CHARACTERISTICS” table and is rated at a minimum of 10 and 17 mW/sr respectively for QED 233 and QED 244 and a max of 50 mW/sr for 233 at 100mA current.

so i am sure these are NOT the best options for an ftir and even if they were supposed to work… u’ll need way more than 10 on a side.... maybe 20-25 on each side… but again i am not very sure if they will work.

i would suggest to look for alternative models and check for the correct specs in the data sheet… dont get confused by the units.... look for mW/sr (radiant intensity) and not mW (power rating). u’ll need something with atleast 100 mW/sr for a good contrast of blobs on a ftir....

i am personally using 130 mW/sr led’s placed one every inch along the 2 long sides of my plexiglass… (32” X 24” , 64 led’s , 32 on each side) and they work great.

and whatever tim said are also important issues that cause problems… so take care bout them as well

hope this helps....

At First thanks at all for the reply!

What kind of IR LEDs do you think I could use?
I have read about:
.SFH485 SFH485-2 http://docs-europe.electrocomponents.com/webdocs/08ae/0900766b808aed9d.pdf
.SFH485P.  http://docs-europe.electrocomponents.com/webdocs/0028/0900766b8002885f.pdf

All of OSRAM.
I’m not sure of what’s the best choice.Could you help me..?

well osram SFH485… tried and tested and easily the best option....  and yaa.. with these.. ten to a side will be good enough.... for the size you mentioned.

Regarding the 485s, what about deciding between the round and the flat head (P)?
The P version has a wider half angle (+-40 vs +-20), and the flat head should allow for a better coupling with the acrylic sides.
I know some people here have discussed placing the leds at an angle with the acrylic, but I find that a little tricky to implement. In addition, the wider angle of the P should compensate for that.
Even though, I’ve seen many more people using the regular 485s. Is there a reason?
Thanks.

Okay so now the mW/sr matters aswell? I got the datasheet, but it kinda gives three mW/sr values. Here it is:
[img=http://img88.imageshack.us/img88/8794/irledsdatasheetul5.th.png]

So do you think these LEDs are right? I’m going to put 14 on each side btw.. 72x54 cm screen. 28 leds in total.

@kneli.... the image you posted states the RI (radiant intensity) at 35 mW/sr at 100mA (not 3mW/sr)… but its still too less for FTIR. the value under it (350mW/sr) is achieveable only when you pass 1A current thru the led, but to do so you need a LED driver circuit, cause you need to constantly keep switchin the led’s on and off at a high frequency to prevent the led’s from burnin. usually its not a desirable option to go for LED driver circuits cause they require good amt of electronics knowlwdge (unless someone else can do it for u). i would suggest to look for led’s that can deliver a RI of 100+mW/sr in the current range of 50 - 150 mA… the ones you found might be a little too weak and would not work except for completely dark rooms.

Okay I’m a bit confused here. This link here (http://docs-europe.electrocomponents.com/webdocs/08ae/0900766b808aed9d.pdf) which was posted earlier, states that the SFH 485 LEDs have a radiant intensity of 160 mW/sr at Ie max. Though another link I found (http://pdf1.alldatasheet.com/datasheet-pdf/view/45674/SIEMENS/SFH485.html) gives 160 mW/sr rates with the SFH 484 and only about 80 mW/sr with the 485.

So I don’t know why these datasheets show different numbers, but can it be said that where ever I am and where ever I buy Osram SFH 485 or 485P, they will work with an FTIR setup? They are more expensive than your average IR LED btw, but that’s the least of my problems ^^

Thanks,
kneli

the only reason i can figure out for the difference in mW/sr vaules between the two datasheets is that one is siemens and the other is OSRAM… even tho i think OSRAM was owned by seimens but i might be wrong. i guess you can safely go for OSRAM SFH 485.. (just make sure they are osram and not seimens or any other equivalent) ... i dont think it shud be a problem

all the best!!!