Schlieren imaging is based on the deflection of light beams crossing gradients of the index of refraction in a transparent medium. These index of refraction gradients can be introduced by density discontinuities in a fluid or in mixing processes of different optical materials. Schlieren is a line-of-sight imaging technique allowing only a qualitative flow visualisation.
In conventional Schlieren setups a collimated light beam is deflected across a knife-edge causing a change of light intensity in the direction perpendicular to the knife-edge. A direct quantification of the light deflection is not possible. The digital version of Schlieren imaging is called Background Oriented Schlieren (BOS) or Synthetic Schlieren based on Digital Image Correlation (DIC).
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.
What is the difference?
Compared to the conventional knife-edge Schlieren method, BOS can visualize large scale flow phenomena and measures the optical flow distortion in form of a digital vector map. For 2-dimensional or axisymmetric flows, BOS provides the potential to measure absolute 3- dimensional density and temperature fields.