Principles and Techniques of Schlieren Imaging Systems

This paper presents a review of modern-day schlieren optics system and its application. Schlieren imaging systems provide a powerful technique to visualize changes or nonuniformities in refractive index of air or other transparent media. For over two centuries, schlieren systems were typically implemented for a wide variety of non-intrusive real-time fluid dynamics studies. With the popularization of computational imaging techniques and widespread availability of digital imaging systems, schlieren systems provide novel methods of viewing transparent flow. Innovations such as background-oriented schlieren, rainbow schlieren interferometry, and synthetic schlieren imaging have built upon the fundamentals of schlieren imaging to provide less ambiguous, quantifiable studies of schlieren systems. This paper presents a historical background of the technique, describes the methodology behind the system, presents a mathematical proof of schlieren fundamentals, and lists various recent applications and advancements in schlieren studies.