U.S. Navy Petty Officer 2nd Class Sean Purvis looks at a color chart during a night flying test on a reduced oxygen breathing device at Langley Air Force Base, Va., Nov. 14, 2014. During the night test, the mixture of gases changed to show the aviators the effects pressure changes can have on vision. (U.S. Air Force photo by Staff Sgt. Natasha Stannard/Released)