Sensitivity limits of background oriented schlieren (BOS) with a projected background*

Background-oriented schlieren (BOS) requires a background pattern against which a schlieren object is imaged.  Generally these patterns are painted, printed, or projected behind the schlieren object.  Here a  projected dot pattern, which is projected through the schlieren object, was explored.  This arrangement results in double refraction of the pattern, both on the initial projection and on the return light to the camera, similar to a single mirror schlieren system.  The goal of the present work is to quantify the sensitivity of the system.  A high resolution camera was used to image helium jets in air against both projected and traditional BOS backgrounds. Distances between the helium plume and and both background types were varied to determine the range at which sensitivity was lost in each arrangement. The resultant pixel shifts caused in the BOS backgrounds were calculated using optical flow methods, and the results were compared against each other. While traditional BOS backgrounds were found to have a higher sensitivity, the projected BOS backgrounds have the potential to be applied in a wider range of experimental setups.