NOISE-REDUCTION PROPERTIES OF A MULTIPLE-SOURCE SCHLIEREN SYSTEM

Schlieren images often contain noise that is caused by density gradien ts in the air that surrounds the wind-tunnel test section. It has been shown that multiple-source, or sharp-focusing, schlieren systems may reduce the region of space in which the schlieren system is most sensi tive to density gradients. In this paper, the potential for noise redu ction by minor modifications to traditional schlieren systems is exami ned. The modifications result in a multiple-source (sharp-focusing) sc hlieren system using the existing mirrors. Analysis shows that, for a system with an incoherent narrowband source, the imaging system, the s lit spacing, and the transverse length scale of the noise govern the d epth of focus. Also shown is that the maximum reduction of the noise s cales linearly with the number of source slits.