Advancements in Background-Oriented Schlieren (BOS) for NASA Ground-Test Facilities

The application of UV-cured ink for printing intricate patterns on retroreflective sheeting is explored for enabling higher-resolution deflection-sensing algorithms for application in Background-Oriented Schlieren (BOS). When using collinear illumination to illuminate exposed-lens retroreflective sheeting, UV-cured inks provide a precise and accurate way to achieve a wide-range of patterns that can be tailored to a specific BOS measurement need. The use of these inks on exposed-lens retroreflective sheeting, however, reduces the dynamic range of the pattern when compared to conventional patterns printed on diffuse surfaces (typically used in lower-speed BOS applications) and has a non-linear effect on the retroreflected intensity. A calibration methodology is presented to provide printed patterns that have a linear response in gray scale for the imaged retroreflected intensity when subjected to collinear illumination. Fourier checkerboard demodulation (FCD) is presented as a deflection-sensing technique that is highly effective in high-speed BOS applications when using patterns printed with UV-cured inks. Deflection estimates provided by FCD are shown to be sensitive to the contrast of the printed UV-cured ink patterns on the retroreflected material. However, by optimizing the checkerboard pattern, FCD is shown to qualitatively improve the resolvable details when compared to a commercially-available iterative least squares (ILS) algorithm and an open-source wavelet-based optical flow velocimetry (wOFV) algorithm for deflection-sensing. Both the use of UV-cured ink and FCD act as significant advancements for high-speed BOS applications to NASA ground-test facilities, including the further advancement of high-speed, tomographic BOS.