In maps that have relative dark places, each United States Personnel (none for IF) are issued with a pair of NVGs, which provide vision in the dark by amplifying ambient light sources. However, while the goggles provide increase night vision, they reduce FOV; flashbang effects are worse and light sources cause extreme glare. Also, in places that have very dense fog, the NVG reduce long range visibility (though do help on closer range).
Luckily, there is currently (2.6) no delay when switching from NVG mode to normal mode. As such, a tip for target acquisition is to toggle between NVG and regular vision.