Duke Vision is a permanent inventory item in Duke Nukem Forever.
Duke Vision amplifies all ambient light levels, allowing you to explore dark areas. The resulting tint is blue with a blurry tracking line sensor crawling repeatedly. It essentially serves the same purpose as the Night Vision Goggles from Duke Nukem 3D.
Unlike the other inventory items, Duke Vision can be toggled at any given time when you need it, as it is a built-in feature to Duke's sunglasses, judging by the Duke Vision inventory icon, which is Duke's shades. Due to the prevalence of many dark areas in the game, you'll find yourself often needing it.