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A method for quantitative estimation of density in a high-speed flow field ispresented, which uses a structured-light beam probe to interrogate the regionof interest (ROI). Wavefront distortion suffered by the interrogating beam asit passes through the shock-induced flow field is investigated. Dedicate cameraoptics is used to measure the cross-sectional intensities of the exitingwavefront at two different planes, simultaneously. A technique is proposed thatuses the lateral displacement of the structured light pattern on both theplanes to estimate the axial intensity derivative, dI/dz, which is the inputfor the transport-of intensity equation (TIE) for phase estimation. By doingthis, the need for small defocused distance, dz is avoided in experiments andhence the demand for a low f/# imaging system is alleviated, and now larger dzis admissible as the intensity derivative is not estimated through theconventional finite difference (FD) approximation. The recovered phase from TIEis used as an input to phase tomography algorithm to obtain the refractiveindex followed by density distributions in the flow field quantitatively. Theproposed technique is verified through experiments conducted in hypersonicshock tunnel for flow Mach No. of 8.5. Estimated cross-sectional densities ofthe flow field around the aerodynamic test model are presented and comparedwith the numerically estimated values, which shows good agreement.

Structured Light Projection-based single-shot transport of intensity phase imaging for quantitative diagnostics of high-speed flows