A method of the zero-order-removal off-axis digital holographic reconstruction by recording three holograms with different beam ratios is presented. The zero-order-removal hologram can be constituted using two fitting coefficients in the combination of three holograms, in which the three off-axis holograms with different intensity ratios of the object and reference beams are recorded via arbitrarily turning a half-wave plate. The validity of the zero-order removal of the resultant hologram is proved by expression. The filtering region for the zero-order-removal hologram can extend to the center of its spatial-frequency spectrum domain, which makes higher spatial frequencies on the positive first-order intercepted. In the experiments, the reconstructed amplitude and phase images demonstrate the effectiveness of this zero-order-removal approach and the improvement on imaging resolution.
In this paper, we present a method to eliminate the zero-order term in Fourier spectrum of off-axis digital holograms based on the linear change of the hologram. In a typical digital holographic setup, the three off-axis holograms are recorded by the object waves and the reference waves with the different intensity ratios. The three holograms were Fourier transformed, and then the spectra were linearly fitted to obtain two fitting coefficients. Then, the second and third holograms are multiplied by the corresponding coefficients, and the first hologram is subtracted from the hologram that is multiplied by the coefficient to obtain the hologram with the zero-order term eliminated. In the procedure of spectrum filtering, more high-frequency information can be intercepted in this spatial-frequency spectrum after removing the zero-order. The experiment results show that this method can obviously eliminate the zero-order term.
In this paper, automatic filtering for amplitude and phase reconstruction in off-axis digital holography is developed. A user-friendly interface for automatic filtering is given via program design with MATLAB. The hologram to be processed is input at the front end, and automatic spectrum filtering in Fourier spectrum domain of digital holograms is realized by using clustering algorithm at the back end. The amplitude and phase images are reconstructed from the intercepted spatial-frequency spectrum by using the reconstruction algorithm. This automatic filtering program has high robustness, which can achieve reconstruction imaging for off-axis holograms correctly and effectively in the case of different off-axis angles or different image sizes. For the user interface, upon inputting an off-axis digital hologram and confirming the operation, the reconstructed amplitude and phase images can be quickly output. This user interface has the advantages of simple operation, adjustable parameters and clear feedback. Since K-means clustering is used, this filtering algorithm increases the efficiency in processing experimental data and the reliability of reconstruction imaging. The digital hologram computer-generated is used to simulate filtering processing. The results show that the quality of reconstructed images by using the presented automatic filtering is not inferior to that by conventional manual filtering.
We present a spectrum filtering approach for reconstruction of off-axis digital holograms. The high frequency aliasing terms in Fourier spectrum domain of hologram are generated by non-fitting interpolation. After interpolation processing, the zero-order terms in the aliasing spectrum region are obviously suppressed. Accordingly, for the spectrum filtering of high-frequency signal terms, the available range of frequency band will increase significantly, which can result in the improvement of resolution of the reconstructed images. In order to avoid the effect of non-uniform distribution on the aliasing spectrum terms, all the same-order signal terms in the aliasing spectrum (positive or negative first-order spectrum) are intercepted, respectively, and then added together to reconstruct the complex amplitude distribution of the sample. We perform the reconstruction imaging with a conventional method and the presented method from the same hologram in simulation. The imaging results show that such interpolation method can obviously improve the resolution of the reconstructed amplitude and phase images. This method for off-axis digital holographic reconstruction only relies on a single frame acquisition to achieve high resolution reconstruction imaging without additional requirements to the optical setup, which will have a promising application in real-time imaging for living biological cells or moving objects.
KEYWORDS: Digital holography, Holograms, Image resolution, 3D image reconstruction, Super resolution, Digital imaging, Holography, Reconstruction algorithms, Spatial resolution, Charge-coupled devices
In this paper, an off-axis digital holography multi-frame image super-resolution reconstruction method is presented. Each low-resolution hologram will have small displacement in different directions. The obtained low-resolution holograms are processed iteratively with the super-resolution algorithm to obtain the super-resolution hologram, and then the amplitude image is reconstructed from the super-resolution hologram. The imaging results show that the resolution of the reconstructed image after super-resolution processing is obviously improved, because of increase of high-frequency information in the high-resolution hologram.
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