Background Oriented Schlieren: BOS Imaging

BOS or Synthetic Schlieren is a technique for the visualization of gas motion based on local refractive index variations. This line-of-sight imaging technique measures locally the density gradient as an integrated value over the line-of-sight. In practice, only a random dot pattern in the background of the flow is imaged with a high resolution camera before and during the test. By comparing the two pictures (or more precisely correlating the two patterns similar to the image correlation in PIV) the local displacement of the background pattern can be used to provide lateral information on path-integrated refractive index variations. Tomographic (Tomo) BOS Imaging systems measure quantitatively density or temperature fields in 3D using multiple camera BOS projections. This 3D imaging technique requires an accurate geometrical calibration of the multi-camera system and an iterative tomographic reconstruction algorithm.