Figure 5.1:  A schematic drawing of the schlieren set-up.

A schematic drawing of the schlieren layout is shown in Figure 5.1. In the arrangement shown, lens produces a parallel beam that passes through the test cell TC. Density gradients arising from temperature gradients in the test cell lead to beam deflection shown by dashed lines 1 and 2 in the figure above. A discussion on this subject is available in the context of refraction errors in interferometry.

A key element in the schlieren arrangement is the knife edge. It is an opaque sheet with a sharp edge. The deflected light beam emerging from the test cell is decollimated using a lens or a concave mirror. If the light spot moves downwards, it is blocked by the knife edge and the screen is darkened. If the light spot moves up, a greater quantity of light falls on the screen and is suitably illuminated. Thus, the knife edge serves as a cut-off filter for intensity. An appropriate term that characterizes this process is called contrast, measured as the ratio of change in intensity at a point and the initial intensity prevailing at that location. The knife edge can be seen as an element that controls contrast in light intensity. The change in contrast depends on the initial blockage and hence the initial intensity distribution on the screen. If the initial (undeflected) light beam is completely cut-off by the knife edge, the screen would be dark. Any subsequent beam deflection would illuminate the screen, thus producing a significant increase in contrast.

In Figure 5.1, the knife edge is kept at the focus of the lens and the screen at the conjugate focus of the test cell. In other words, the distances satisfy the relation

In Figure 5.1, ray 1 increases the illumination at a point on the screen while ray 2 is blocked by the knife edge and this results in a reduction in the illumination. Hence the image of the scaler field is seen as a distribution of intensities on the screen.