Focus enhancement in long schlieren imaging systems using corrector lenses

Schlieren imaging is a widely implemented refractive imaging technique utilized to visualize flow fields around objects. Implementation of schlieren imaging may require an extended test section in which the parallel light test section is longer than the focal length of the schlieren optics. Such a configuration introduces a limited region in the test section which is in focus. Here, corrector lenses varying in focal length from -1000 to -100 mm were introduced before the camera lens altering the focus location. The resulting focus range achieved with the corrector lenses was studied using lens type and z-type schlieren imaging systems having test sections longer than the schlieren optics. Lens type imaging systems having schlieren optics of different focal lengths were also compared. The location of optimal focus for the different imaging configurations was determined through implementation of an automated image processing routine. The experimentally determined focus range was compared to the focusing plane calculated using the thin lens equation. The thin lens equation is in good agreement with the experimental results, however a better fit was achieved for corrector lenses with a larger negative focal length. The agreement between the experiments and the thin lens approximation yielded design equations for implementing the corrector lenses in a given schlieren system.