I had a very interesting lesson in the use of optical slave triggers on my last outdoor shoot.
I use Elinchrom D-Lite's and although I think I can safely assume that most, or all optical triggers are the same, I won't make that assumption and I'll only speak in terms of the optical trigger found on the D-Lite's.
Like most strobes with a built in optical trigger, you can find it on the back panel and is a half dome, with a sensor that 'sees' 180 degrees from front to back and 360 degrees from top to bottom.
I've always used strobes in the studio and I can only recall one or two times that the trigger would not work because it was being blocked. Generally speaking, as long as there are no gobos or modifiers shielding the optical slave it works flawlessly, but I never really thought about how it would work, or if it would work outside.
Well, it does work but there is one big issue that comes into play. The optical triggers are not actually all that sensitive in an outdoor situation because what I hadn't considered was that a good part of what makes those triggers seem sensitive is the fact that they rely heavily on the light that bounces and reflects off other surfaces and not just on the incident light that falls on it.
Once you remove the walls and ceiling it's as if you've turned the sensitivity way down or disabled the trigger. I discovered this while setting up for an outdoor shoot. What was so amazing was that with the main and fill lights about 10 feet apart, I could only get the slave to fire when the dome was directly facing the main with the main on full power. Even angling the strobe slightly killed the trigger.
So. in an attempt to do some testing I cupped my hand over the far side of the dome so the pop from the main light would be contained and bounce off my hand and back to the sensor. Sure enough it worked!!!
The only thing I could find to use as a shield was the black sleeve from a softbox. I folded it over a few times, created a reflector-like shield and taped it in position. After doing that I could move the strobe around, angle it, and do just about anything I would normally do and still have a functioning trigger.
If you've ever tried to use your strobes outside and had this problem then give the reflector a try and you'll find that your triggers work fine and you don't have to break out the sync cables, or run out and buy another Pocket Wizard or radio receiver.......... yet. 
Just thought I'd pass on this tip. 
