[quote author=pullbangdead link=1226362652/0#3 date=1226364779]mW is a units of power, or energy per time. The power coming out of the laser doesn't change due to fog, the power stays the same regardless of what you're shooting the laser into. 50mW is 50mW. Power also doesn't change with color. 50mW is 50mW, whether, is be red, blue, green, or purple.
The visibility of a beam changes due to particles in the air. Smoke, fog, whatever puts more particles in the air, the light scatters/reflects, and more of it is diverted from the beam path to your eye. This makes it more visible only because some of the light that is there no matter what was diverted by the smoke, but otherwise wouldn't have been diverted.
To find the mathematical increase in perceived brightness from an amount of smoke, you're gonna need some heavy math there. It's one of those problems where I say to myself "yeah, I see how I might approach that maybe, but it'll take days to do no matter how I do it". And really, I don't think you need anything specific, more smoke = more beam visibility is good enough for me at least.