Optimizing Fourier filtering for digital holographic particle image velocimetry

In digital holographic particle image velocimetry, the particle image depth-of-focus and the inaccuracy of the measured particle position along the optical axis are relatively large in comparison to the characteristic transverse dimension of the reconstructed particle images. This is the result of a low optical numerical aperture (NA), which is limited by the relatively large pixel size of the CCD camera. Additionally, the anisotropic light scattering behaviour of the seeding particles further reduces the effective numerical aperture of the optical system and substantially increases the particle image depth-of-focus. Introducing an appropriate Fourier filter can significantly suppress this additional reduction of the NA. Experimental results illustrate that an improved Fourier filter reduces the particle image depth-of-focus. For the system described in this paper, this improvement is nearly a factor of 5. Using the improved Fourier filter comes with an acceptable reduction of the hologram intensity, so an extended exposure time is needed to maintain the exposure level.