Optimizing Fourier filtering for digital holographic particle image velocimetry
Measurement science and technology
In digital holography, the optical numerical apperature (NA) is limited by the relatively large pixel size of the CCD camera. This results in a relatively large depth of focus of the system. Additionally, the anisotropic light scattering behavior of the seeding particles tends to further reduce the effective NA. This paper shows numerically and experimentally that this additional reduction of the NA can be suppressed by applying appropriate Fourier filtering. This filtering comes with an
... ble reduction of the hologram exposure. The measurement setup and the method of numerically reconstructing the images of seeding particles in a volume are described. The light scattering pattern of the seeding particles is measured and its relevance for the design of an optimal Fourier filters is described. Then, the performance of three Fourier filters is analyzed numerically and experimentally. As shown in figure 1, by optimizing Fourier filtering, the depth of focus is improved by a factor 10 without a significant increase in the diffraction limit. Finally, measurements indicate that the holographic setup is able to successfully reconstruct a particle's 3dimensional position. Figure 1. On the left side are slices through three reconstructions of a seeding particle. During the recording of these particles, three different Fourier filters were used, as shown on the right side of the figure. They consist of a concentric opaque disk and a circular aperture. The diameter of the opaque disc is chosen at 1 mm, 10 mm and 20 mm. The diameter of the aperture is constant, 24 mm.