Optimizing Fourier filtering for digital holographic particle image velocimetry

Abstract

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.

Particle imaging techniques for volumetric three-component (3D3C) velocity measurements in microfluidics

“…This can be achieved by mirrors (Malkiel et al 2003;Yamamoto and Uemura 2008) or different angles of the reference wave (Yang and Chuang 2005). One way to reduce the problem of the stretched particle images is the application of a spatial filter (Ooms et al 2006). However, microscope objectives have large numerical apertures and thus a relatively small depth of field, which is advantageous for holography.…”

“…Thus tracking techniques seem to have a higher potential when dealing with steady or periodic flows and when averaging over a longer time period is Ooms et al (2006) possible. If the instantaneous velocity field is needed and the experimental conditions only allow for a very low SNR, a robust method such as cross correlation, which makes use of larger interrogation window sizes, is the technique of choice.…”

Application of digital holography to filament size analysis

“…This will help in improving the measurements of the position of the filaments [8]. Second is to premagnify the field before recording the hologram [18]. This will increase effective resolution of the system.…”

Digital holographic microscopy applied to measurement of a flow in a T-shaped micromixer

“…In practice, this limits the NA to a value of a few hundredths. This leads to a large particle image depth-offocus (DOF pi ) compared to the transverse size of a particle image and consequently to a relatively large inaccuracy of measured longitudinal velocity (Ooms et al 2006). This problem could be overcome if cameras were available with a pixel spacing of the order of the illumination wavelength.…”