  The background-oriented schlieren (BOS) technique with the physics-based
optical flow method (OF-BOS) is developed for measuring the pressure field of a
laser-induced underwater shock wave. Compared to BOS with the conventional
cross-correlation method in PIV (called PIV-BOS), by using the OF-BOS, the
displacement field generated by the small density gradient in water can be
obtained at the spatial resolution of one vector per pixel. The corresponding
density and pressure fields can be further extracted. It is particularly
demonstrated that the sufficiently high spatial resolution of the extracted
displacement vector field is required in the tomographic reconstruction to
correctly infer the pressure field of the spherical underwater shock wave. The
capability of the OF-BOS is critically evaluated based on synchronized
hydrophone measurements. Special emphasis is placed on direct comparison
between the OF-BOS with the PIV-BOS.
