U.S. Army Rangers go through night vision testing in a hypobaric altitude chamber at Langley Air Force Base, Va., Nov. 14, 2014. During the tests, students looked at color charts and maps while experiencing hypoxia, which can alter vision and cause confusion. All Department of Defense aviators and parachutists must go through a chamber refresher course every five years. (U.S. Air Force photo by Staff Sgt. Natasha Stannard/Released)