Synthetic limb edge schlieren (SLES) is a digital update of the Schlieren for Aircraft in Flight technique developed by Leonard Weinstein at NASA in the 1990s (see "Visualization and Image Processing of Aircraft Shock Wave Structures"). It provides a method for outdoor schlieren imaging of very large moving objects like aircraft. Most past interest in the technique has been on imaging shock waves from supersonic aircraft, but it could work for other schlieren applications such as visualizing flows of different gases (e.g. gas leaks), thermal convection/exhaust, or high intensity acoustic phenomena. This technology is supported by our proprietary SAFRAN software.
The basic idea is that refractive index gradients in the air, distortions caused by shock waves, thermal gradients, or other phenomena, steer light rays away from their default trajectory. With a highly collimated image like the image of the Sun, these gradients manifest themselves as shifts in the apparent position of the solar limb (the edge of the disk image). These shifts are usually very small, but because the Sun is very bright and the edge is sharp, even very small shifts can be detected. If an object is moving over the Sun, in a predictable way, we can use software to take a sequence of edge shifts moving with the object and convert them into an image that is equivalent to a still schlieren image of the object and associated air distortions.
The original Weinstein approach used a film camera and a carefully aligned mask. Our new Limb Edge Schlieren technique eliminates the need for a special mask with software developed at and licensed from MetroLaser Inc. by Spectabit's cofounders, so that virtually any digital camera with a fast enough frame rate (a few hundred to a few thousand frames per second, typically) can be used with simple small refractor telescope. This is a variation of background-oriented schlieren (BOS), but its advantage over the usual BOS technique is that the limb is an extremely sharp, high-contrast edge, which allows us to perform sub-pixel inference of the edge shift without a noisy and unstable digital correlation process. Our software has been successfully applied to imaging shockwaves, wing-tip vortices, Mach radiation, Mach diamonds, atmospheric turbulence, and aircraft exhaust. Few other aerooptic techiques have this potential to detect and analyze aerodynamic properties of a whole full-scale aircraft in normal operation. The simplicity of the equipment required for this approach, which is completely passive, allows it to be used with targets in the loop or out of the loop. Most of the complexity lies in the analysis software, which comes with an easy-to-use GUI and a suite of specially adapted image processing procedures which can extract and render exquisite details from the raw data.
As with most schlieren techniques, the sensitivity is directional. The direction of sensitivity though is determined by the position of transit of the object over the disk. Across the diameter, the sensitivity is to the parallel gradient, that is refractive index differences which change along the direction of transit are imaged. At the opposite extreme, when the object passes tangent or nearly tangent to the solar disk, the sensitivity is to the perpendicular gradient. Crossing the secant halfway between the two extremes, the sensitivity is at 45 degrees, but we also obtain an interesting situation in which the two sides of the solar disk have orthgonal sensitivity, so that the complete schlieren vector can be computed by fusing the two edge signals, with the extremely high sensitivity available from the high contrast edge. The practical implication is that perpendicular features like low Mach number shock waves are best imaged with a diametric transit, and parallel features like wing-tip vortices are best imaged with a tangent transit, while the half-radius secant gives the most complete picture with the right kind of edge fusion.
New results with this technique have recently been publicized by NASA Armstrong at http://www.nasa.gov/centers/armstrong/features/shock_and_awesome.html, and a joint publication of the details of the technique was presented at the 17th International Symposium on Flow Visualization in Gatlinburg, Tennessee June 19-22, 2016. A PDF of the presentation is available from the NASA Technical Reports Server.
See also a recent article in the Economist.