The schlieren system used in the present work is of the Z-type, as shown in Figure 5.15(b).  The optics includes concave mirrors of 1.30 m focal length and 200 mm diameter. Relatively large focal lengths make the schlieren technique sensitive to the concentration gradients. The knife-edge is placed at the focal length of the second concave mirror. It is positioned to cut off a part of the light focused on it, so that in the absence of any optical disturbance, the illumination on the screen is uniformly reduced. The initial intensity values in the experiment were chosen to be less than 20, on a gray scale of 0-255. The knife-edge is set perpendicular to the direction in which the density gradients are to be recorded. In the present study, the gradients are expected to be predominantly in the vertical direction of gravity, and the knife-edge has been kept horizontal.

Shadowgraph images have been recorded using the same optical components as employed in the interferometry setup by first blocking the reference beam and allowing the test beam alone to fall on the screen. Shadowgraph images have also been recorded from the schlieren apparatus by collecting the refracted light beam emerging from the test cell (Figure 5.15(c)). The position of the screen on which the shadowgraph images are displayed plays an important role in data analysis. The screen position is chosen so as to improve the image contrast, while extracting the dominant features of the flow field.  Beam refraction from regions of high concentration gradients can interfere with those passing through one of nearly constant concentration. This factor is taken into account while fixing the camera position. The initial intensity distribution in shadowgraph experiments corresponds to the Gaussian intensity variation of the laser source itself.