I work it out (ok not me but the website I use) to be 220 ohm.

LED Resistor Calculator

Going slightly bigger wont hurt, so if for instance it was 82 ohm you could use a 100 ohm. But double check that, and make sure I didn't screw up anything on the above link.

Edit: Didn't realise there were 3 of them, so 60mA... 68ohm 1/2watt - but isn't the digital port only capable of 25mA?

Thanks for the input, Rich!

I popped the data in to the series/parallel wizard on that site.. It came out with 82

Solution 0: 3 x 1 array uses 3 LEDs exactly +----|>|----|>|----|>|---///----+ R = 82 ohms

The wizard says: In solution 0: each 82 ohm resistor dissipates 32.8 mW the wizard thinks ¼W resistors are fine for your application together, all resistors dissipate 32.8 mW together, the diodes dissipate 72 mW
total power dissipated by the array is 104.8 mW
the array draws current of 20 mA from the source.

So you'd say using a 100 in place of that would be fine?

Additionally, I'm planning on running six of these, from d0 --> d5. As I understand, 25ma per port, so that should be fine, too?

Ah I see... Yes, forgot how to do it (it's slowly all coming back after 15-20 years of not using the knowledge). Yes I'd say so. If not then whack in a tip transistor switching circuit and run them from the 5v.

Thanks for your feedback Rich, it's been a huge help!

The 6-port 18LED array is now connected to an EZB, and my modified cameras are saying all looks good. I'm going to use PWM and - when I get one - a photoresistor to monitor light above the camera, and then ramp up the power on the array as it gets darker. I think.

Maybe I can help, I tried the online calculator and it returns erroneuos data. What I think you need is 56 Ohms. If you have three leds in series then there is a 3.6 Volts drop accross them all,leaving 1.4 Volts left to be dropped accross the resistor. So 1.4 Volts divided by .025 Amperes equals 56 Ohms. Since this is an emitter you want to have the leds as bright as you can for the receiver to see them, although they will be invisible to humans (infrared). I would use 56 or you can put two 100 Ohms in parallel and that should get you there due to tolerances of the resistors, so you can so you could put two together and then measure them to see if you are close to 56 ohms. I hope this helps. PS Some cell phone cameras can see infrared light.

Thanks for the extra input, Purple!

I have bunch of cameras that I modified years ago, removing the IR filters from them and replacing them with over-exposed film negative (To remove visible light, leaving only IR), so they can all see IR. On a previous project, I created a 12LED array, running from USB, and that's enough to illuminate an entire room, so 18 on this one should be bright enough, even with a reduced brightness.

Thank you again for the input! Knowledge is power!