Gradient backlight schlieren: a low sensitivity schlieren for measurement in liquids

Abstract The measurement of schlieren phenomena in liquids has historically been challenging due to strong light refraction effects. This paper presents a novel schlieren technique inspired by side-lighting photography. Derived from the shadowgraph method, the new technique utilizes a gradient background light field. Its setup is notably simpler and more cost-effective than conventional schlieren systems. It provides high-resolution, real-time imaging with an adjustable measurement range, offering a versatile platform for scientific investigation. Calibration and validation experiments confirmed the method’s inherently low sensitivity, making it particularly suitable for liquid measurements and enabling quantitative analysis. The technique was successfully applied to observe underwater ethanol vortex rings. The resulting images effectively reveal fine structural details and support quantitative evaluation.