With the Night Vision Perk, you can see in the dark better. Each level of this Perk will reduces darkness and darkness penalties by 10% (Fallout) or 20% (Fallout 2 and Fallout Tactics).
Also only in Fallout, in dark indoor areas or outdoor areas at night-time, this results in an increase in monitor brightness and ranged weapon accuracy. The perk increases ranged accuracy by 15%, and possessing all three levels increases ranged accuracy by 30% in total.
The ranged accuracy bonus from the Night Vision perk is temporarily lost if a savegame is reloaded. In outdoor areas such as towns, the accuracy modifier is re-applied after approximately a minute of game time. In indoor areas, the area must be re-entered before the accuracy is re-applied. [verified]
The above also applies after the Night Vision perk is selected. In other words, if selected this perk while indoors, re-enter the area so that the accuracy bonus for this perk is applied. [verified]