Digital focusing schlieren relies on accurate mapping between a camera and a digital modulator or display system. Our early development of the technology was challenged by higher-order distortions in the optical systems. A simple perspective transform was adequate for lower-power lenses and smaller fields of view, but for wide fields and for highly sensitive configurations such as dark-field schlieren, correction terms were necessary. The Brown-Conrady model fitting implemented by OpenCV offered little improvement, so we developed a new distortion correction method that empirically optimizes the schlieren filter performance and constructs a map with phase unwrapping techniques borrowed from interferometry. This approach allows operation even with wider-angle lenses and finally makes dark-field schlieren practical. It also appears to improve the resilience of the grid generation with lower resolution camera systems, which is important for implementation in high speed schlieren imaging.

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Benjamin D. Buckner and Drew L'Esperance, "Schlieren unwrapped: distortion correction in digital focusing schlieren," Proc. SPIE 11102, Applied Optical Metrology III, 111020R (Presented at SPIE Optical Engineering + Applications: August 14, 2019; Published: 3 September 2019); https://doi.org/10.1117/12.2528081.