Advanced Knife-Edge free Self-Aligned Colour Schlieren Imaging with Extended Measuring Range

Abstract

Schlieren imaging is a powerful, non-intrusive method widely used to visualize refractive index gradients in fluid dynamics and heat transfer studies, essential in fields like aerospace engineering, combustion analysis, and supersonic flow visualization. Traditional Schlieren imaging setups employ a knife-edge cut-off to differentiate between deviated and non-deviated light rays, creating contrast. However, this approach requires highly precise alignment and limits both usability and measurement range, restricting accurate observation of high-gradient phenomena such as combustion processes at elevated temperatures. Here, we proposed a simplified Schlieren imaging method that eliminates the knife edge by introducing a novel integration of an extended RGB segmented colour source with a circular aperture. Our approach significantly simplifies alignment, enables colour visualization, and we achieve expanded measuring range by approximately threefold. Experimental validation, involving imaging hot air plumes generated from butane combustion and transparent adhesive tape tests, demonstrate significant improved performance, including enhanced visibility near regions of high temperature gradients, improved edge detection, and precise visualization of large refractive gradients. This simplified yet robust Schlieren imaging approach substantially broadens the scope and ease of application, facilitating detailed investigations of previously challenging fluid phenomena.