[Rupert]

The only 'visibility' penalty for active defences seems to be "can't see the attacker: -4". 'Flank' or 'Rear with Peripheral Vision' attract a -2 which could also be considered a penalty for poor vision.

Either way, this implies that an Active Defence only attracts a -2 when it's "hard" or "very hard", and a -4 when it's "almost impossible" to "impossible" in the sense these terms are used on B345-346. At most I'd only assign a -2 when the lighting penalty is -6 or worse, and the -4 is clearly for when there's no light at all, and I'd be inclined to go with what the rules imply - no penalty at all as long as you can see at all (or have some other targeting sense to use instead).

As for this resulting in a comedy of people flailing around madly, hitting nothing, that's how brawls in the dark tend to go, and the real damage is done by getting hit by things you never because they blind-sided you or were aimed at someone else.

Firefights are even worse. You're firing at vague shapes and muzzle flashes, unless you have good modern low-light sights you can't see your own sights, and friend vs foe is based mostly on their relative position, so if someone gets turned around in the confusion (easy enough even in daylight!), they could well be shooting at their own side. Night Vision gear helps to an extent, but it cuts into peripheral vision, making it easer to lose orientation - to my mind that's probably the biggest advantage of the gear in UT over today's gear - the goggles and visors don't have this limitation.