Effects of particle locations on reconstructed particle images in digital holography

Abstract

The intensity and phase reconstructed from digital in-line holograms by the convolution approach are analyzed. Distortions of particle images depending on their position in the plane transverse to the optical axis are identified. For this purpose, the object fields of numerically simulated particle holograms as well as of experimental data are reconstructed. The results of three-dimensional correlations of numerical and experimental data are superior when the numerically generated reference volumes are adapted to the transverse locations of the particle. Thus, proof is given that the characteristics of a particle image change distinctly with the transverse position of the particle and that the numerical model successfully simulates these changes. Hence, this knowledge can be integrated in future particle position detection algorithms.

Adaptation of reference volumes for correlation-based digital holographic particle tracking

“…With a diameter of (8.69 ± 0.12) µm, the particles used to track the water flow in the work presented here are smaller than the camera pixel pitch of d pp = 12 µm. This is challenging due to the reduced signal intensity compared to bigger particles and, as the hologram is not magnified prior to the recording, due to subpixel effects [9]. Still, these particles in combination with this camera are used because of two main advantages.…”

“…The depth-of-focus over multiple particle diameters due to the small Mie scattering angle is increased in a resolution-limited system [1], which is often the case in digital high-speed holography [17]. Second, the subpixel position can affect the intensity distribution in reconstructed holograms [9]. Depending on how the particle is located with respect to the pixel grid of the camera sensor, the reconstructed particle intensity distributes differently over the reconstructed voxels of the particle image.…”

“…Depending on how the particle is located with respect to the pixel grid of the camera sensor, the reconstructed particle intensity distributes differently over the reconstructed voxels of the particle image. This means, for instance, that the global intensity maximum caused by one particle can differ by about 50% by the mere movement of the particle from a position that is projected to the center of a pixel to a position, which is projected to the interface of four pixels of the camera sensor [9]. The use of three-dimensional reference volumes for the cross correlation with the object field reconstructed from particle holograms is an algorithm that takes the information of a whole volume surrounding the particle position into account [8].…”

“…It has been demonstrated that the characteristics of particle images change with the transverse position of a particle [6,9] and that this deviation is not only present in the intensity values but also in the wrapped phase [9], which will simply be referred to as phase from now on. One example of a distorted particle image is shown in figure 1(b) for the same recording as shown in figure 1(a) with the difference that the particle image shown in figure 1(b) is no longer located close to the hologram center.…”

“…It is possible to numerically simulate the distortion of the particle image by the hologram truncation [9]. In this study, a particle hologram is simulated as light diffracted by a pinhole aperture superimposed with a plane wave.…”