[Mobhack]

The units vision rating is how far they can see at night. So if scenario visibility is 5 and the unit has a vision rating of 20 it can see out to 20. A unit with no night sights could only see to the scenario limit.

40+ vision devices are the game's version of thermal imagers (TI). These can also see through smoke obscuration (Though lots of smoke and fire will have some effect in reducing the distance seen, a change over the original code). TI (or GSR which is treated as TI) therefore costs more than lesser (<40) night fighting gear.

A dark night is typically less than 5 hexes visibility with a moonless night being 1-2.

On vehicles, IR sights are typically 15-20, Image intensifiers or LLTV 30. Infantry night sights are usually much less. All use 40+ for thermal sights since 40 is the "magic range" for thermals.