Calibration of a Background Oriented Schlieren (BOS)

A Background Oriented Schlieren (BOS) experiment was designed to measure density gradients in a transparent medium. The BOS technique presented here consists on a comparison between a reference image of a dot matrix and another image of the same matrix behind the flow of a transparent gas. The presence of the flow induces changes in the refractive index, so the dots in the matrix appear displaced in the second image. The displacement of each dot is determined through a cross-correlation algorithm between both images. From the displacement, the changes in the refractive index can be determined, which in turn are related to the density by the Gladstone-Dale equation. In the experiment described in this paper, two different gases with known properties were used for the calibration.