Schlieren imaging reveals shock waves due to air density gradient and the accompanying change in refractive index. This typically requires the use of fairly complex optics and a bright light source, and until recently most of the available schlieren imagery of airplanes was obtained from scale model testing in wind tunnels. Acquiring schlieren images of an aircraft in flight is much more challenging. Ground-based systems, using the sun as a light source, have produced good results but because of the distances involved did not have the desired spatial resolution to resolve small-scale shock structures near the aircraft.

More recently, synthetic schlieren techniques have been developed based on image processing methods. One, called background oriented schlieren (BOS), has been particularly successful in wind tunnel tests. First, researchers obtain an image of a speckled background pattern. Next, they collect a series of images of an object in supersonic flow in front of the same pattern. Shock waves are deduced from distortions of the background pattern resulting from the change in refractive index due to density gradients. This method requires very simple optics and a variety of background patterns, including natural ones, may be used. The complexity with this method is in the image processing and not the hardware or positioning, thus making BOS an attractive candidate for obtaining high-spatial-resolution imaging of shock waves in flight.

In April 2011 the first phase of air-to-air flight-testing at Armstrong, dubbed AirBOS 1, showed positive results and proved the feasibility of using the BOS technique for imaging supersonic shock waves created by a NASA F-18. A high-speed camera on the underside of a NASA Beechcraft B200 King Air captured 109 frames per second while the supersonic target aircraft passed several thousand feet underneath in straight-and-level flight at speeds up to Mach 1.09 (Mach 1 is the speed of sound, which varies with altitude, but is about 768 mph at sea level). Researchers acquired imagery with a relatively simple system consisting of a laptop with a frame grabber and using natural desert vegetation as the speckled background pattern, a method the team dubbed “Tumbleweed Tech.”

“Air-to-air schlieren is an important flight-test technique for locating and characterizing, with high spatial resolution, shock waves emanating from supersonic vehicles,” said Dan Banks, Armstrong's principal investigator on the project. “It allows us to see the shock wave geometry in the real atmosphere as the target aircraft flies through temperature and humidity gradients that cannot be duplicated in wind tunnels.”

 “After much planning and a little luck we were able to acquire in-flight images and process the data, achieving results the first time out,” said J.T. Heineck, the NASA Ames principal investigator who originally proposed the idea of using the background oriented air-to-air technique. Ed Schairer, Heineck’s colleague at Ames, where a provisional copyright for AirBOS technology and related flow visualization applications has been filed, wrote the code with which these images were processed. This technique shows not only shock waves but all density changes including vortices and engine plume effects. Future work may include imaging subsonic aircraft flow fields.

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