A number of clinical, in vitro and computational studies have shown the potential for thromboembolic complications in bileaflet mechanical heart valves (BMHV), primarily due to the complex and unsteady flows in the valve hinges. These studies have focused on quantitative and qualitative parameters such as velocity magnitude, turbulent shear stresses, vortex formation, and platelet activation to identify potential for blood damage. However, experimental characterization of the whole flow fields within the valve hinges has not yet been conducted. This information can be utilized to investigate instantaneous damage to blood elements and also to validate numerical studies focusing on the hinge's complex fluid dynamics. The objective of this study was therefore to develop a high-resolution imaging system to characterize the flow fields and global velocity maps in a BMHV hinge. In this study, the steady leakage hinge flow fields representing the diastolic phase during the cardiac cycle in a 23 mm St. Jude Medical regent BMHV in the aortic position were characterized using a two-dimensional micro particle image velocimetry system. Diastolic flow was simulated by imposing a static pressure head on the aortic side. Under these conditions, a reverse flow jet from the aortic to the ventricular side was observed with velocities in the range of 1.47-3.24 m/s, whereas low flow regions were observed on the ventricular side of the hinge with viscous shear stress magnitude up to 60 N/m². High velocities and viscous shearing may be associated with platelet activation and hemolysis, while low flow zones can cause thrombosis due to increased residence time in the hinge. Overall, this study provides a high spatial resolution experimental technique to map the fluid velocity in the BMHV hinge, which can be extended to investigate micron-scale flow domains in various prosthetic devices under different hemodynamic conditions.