In a schlieren measurement, beam displacement normal to the knife edge will translate into an intensity variation on the screen. Displacements that are blocked by the knife edge sheet are not recorded. Similarly, displacements parallel to the knife edge will also not change the intensity distribution. Information about these gradients in the respective directions can be retrieved by suitably orienting the knife edge. Other strategies such as using a gray scale filter are available. A color filter leading to a color schlieren measurement is desrcibed later in this module.

Consider the displacement of ray 1 as in Figure 5.2. At point P the illumination is proportional to a, say, equal to . With the test cell in place this becomes . Hence at the contrast with respect to the undisturbed region is proportional to . The contrast increases greatly when the initial illumination is small, but it can lead to difficulties in recording the schlieren pattern. It can be shown that

and the contrast can be adjusted using the focal length of lens .

Figure 5.2:  Initial and final positions of the light spot with respect the knife edge.