Night Vision Goggles appear as two greenish lens attached to a head harness. When used it will make all heat sources (from fires to living/enemies bodies) to glow bright green. You can only have one of these at a time, and has a game play shortcut by pressing the letter "N", but they can freely deactivated anytime for latter use.
Night Vision Googles are useful when entering dark areas. It's also useful to highlight shadow Evil Ninjas or brown Ripper and Baby Ripper variants, which can sometimes be hard to notice from the terrain.