Correlation of fluid refractive index with calibration coefficient for micro-defocusing digital particle image velocimetry

Using the 3D micro-defocusing digital particle image velocimetry (mu-DDPIV) method, Yoon and Kim (2006 Meas. Sci. Technol. 17 2897-905) found that an experimental calibration was necessary to correlate the particle images with corresponding depthwise particle locations for a specified test fluid. Furthermore, the calibration needed to be repeated for different fluids with different refractive index (RI) values. In an effort to alleviate the cumbersome requirement of repeated calibrations, this note presents an experimentally extrapolated correlation between the fluid RI and the calibration coefficient (partial derivative z/partial derivative D) that can now be used to apply the mu-DDPIV technique for a wide range of fluids without requiring individual calibrations.