NOISE-REDUCTION PROPERTIES OF A MULTIPLE-SOURCE SCHLIEREN SYSTEM

Schlieren images often contain noise that is caused by density gradients 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 sensitive to density gradients. In this paper, the potential for noise reduction by minor modifications to traditional schlieren systems is examined. The modifications result in a multiple-source (sharp-focusing) schlieren system using the existing mirrors. Analysis shows that, for a system with an incoherent narrowband source, the imaging system, the slit spacing, and the transverse length scale of the noise govern the depth of focus. Also shown is that the maximum reduction of the noise scales linearly with the number of source slits.