Investigation of schlieren cutoff filters: A wave propagation-based model

Abstract

Schlieren technique reveals refractive index variations in perturbed transparent media by blocking some the light rays at a cutoff plane. In a physical schlieren setup, it is necessary to carry out test experiments to obtain high contrast images. This fine-tuning task can be challenging and time-consuming. Numerical schlieren simulations circumvents this limitation through computational means. This study applies wave propagation-based model for schlieren imaging which allows complete visualization of wave field on any immediate plane along the optic axis. While in the spatial domain, this approach also permits easy implementation of knife edge at the cutoff plane. Commonly-used schlieren cutoffs include straight vertical knife edge, straight horizontal knife edge, and circular aperture. The filters of interest explored in this work were variations of radially symmetric patterns. Results showed that when a circular aperture is used as a knife edge, the opaque region must be as small as possible to avoid blurry schlieren image. Also, blocking the DC term in the cutoff plane leads to dark field image. Knife edge is a crucial part of schlieren optical setup. The findings provide a stepping stone to unravel the effects of other knife edge shapes and hence offers creative ways to enhance schlieren images.