calculating IR Irradiance

[prarieVole]

Hi, I'm sorry if this question has been answered. I've searched the forums, and while I've found useful information I'm not sure that I understand everything, and so I've decided to ask directly.

I'm in the process of rebuilding an old dark-pupil eye-tracker. Basically, it's a system that tracks the position of the pupil and corneal reflection of a light source using a high speed camera.
The original setup used a halogen bulb with a filter that only passed near-IR light. we had to scrap it because it gets reallllly hot, is very difficult to position correctly, and looks scary.

I'm building an IR illuminator using leds, but I want to make sure that I don't make them too bright.

So, I'm hoping that someone can help me to calculate irradiance at the eye.

the approach is pretty much as follows.
I get a bunch of these leds
http://www.mainelectronics.com/KLInfraredDetLED.htm
kie-7304, which give a radiant intensity of up to 16mw/sr

I wire them up and then point them at the eye.

The question is how do I calculate irradiance at the eye from this? the distance is right now about 10cm to 40cm, and I've got 20 leds.
the viewing angle for these guys is  30 degrees.

the current guidelines suggest that you don't exceed 20 mw/cm2 for extended viewing times.

Thanks very much in advance.
pv

[BVnursery]

25 to 100",  10cm to 40cm means that with most leds you arent going to get alot of light on your target... the eye.
Im sure one of our more knowledgeable folks has a photo transistor circuit that can help.

[prarieVole]

25 to 100",  10cm to 40cm means that with most leds you arent going to get alot of light on your target... the eye.
Im sure one of our more knowledgeable folks has a photo transistor circuit that can help.

Do you mean that there will be 20-100 mw at the eye? How do you calculate that? I might need to move the illuminator or mount it on the head.

I've got a camera setup that's quite sensitive, you can clearly see the eye using one of these leds at 10cm.
However, in order for the system to robustly discriminate pupil and corneal reflection, and not get thrown by slight head movements and shadows from the cheeks, nose, eyelash etc, you want to have a much more light.

thanks again
pv

[BVnursery]

Sorry I missed your response. 

Im not referring to the output level, only saying that at 25" to 100" with an led rated at 16mw/sr you will certainly have a pretty minimal eye input.