Schlieren photography (from the German word for "streaks") allows the visualization of density changes, and therefore shock waves, in fluid flow. Schlieren techniques have been used for decades in laboratory wind tunnels to visualize supersonic flow about model aircraft, but not full scale aircraft until the 1990s. Dr. Leonard Weinstein of NASA Langley Research Center developed the first Schlieren camera, which he calls SAF (Schlieren for Aircraft in Flight), that can photograph the shock waves of a full-sized aircraft in flight. He successfully took a picture which clearly shows the shock waves about a T-38 Talon aircraft on December 13, 1993 at Wallops Island. The camera was then brought to the NASA Dryden Flight Research Center because of the high number of supersonic flights there.