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.
An optimization algorithm is developed to extend the unambiguous range of DWDH, which is suitable for imaging living onion epidermal cells, and meanwhile to reduce the noise. In the experiments, the holograms with two wavelengths can be acquired in a single shot recording by using off-axis dual-wavelength digital holography, resulting to obtain their two sets of spatial spectrum via Fourier transform for these two holograms. The denoised unwrapping phase image of sample is straightforward reconstructed with the optimization algorithm, instead of increasing the noise due to longer synthetic wavelength. As a result, the unwrapped phase images of onion epidermal cells are achieved.
A noise reduction method based on a shorter synthetic-wavelength in DWDH is proposed in this paper. The unwrapped phase at longer synthetic-wavelength is calculated by using the wrapped phases of two individual wavelengths. By comparing the amplified phase of longer synthetic wavelength, of which magnification is equal to that of wavelengths with the wrapped phase of a single wavelength, the difference of phase noises between the longer synthetic-wavelength phase map and the single-wavelength phase map can be calculated, and then accurate height at single-wavelength can be achieved. The proposed method for calculating phase noise is performed by comparing the phases between the single-wavelength with the shorter synthetic wavelength, and then the height at shorter synthetic-wavelength can be obtained. Compared with the existing method, the proposed method can reduce certain noise and benefit the phase reconstruction of fine structures.
The optimization of image resolution for digital holographic scanning imaging of biological cells is investigated. Digital holographic scanning imaging experiments on the upper epidermal cells of onions are performed to demonstrate the validity of resolution optimization algorithm. In the experiments, the holograms of the upper epidermal cells of onion are recorded at a certain scanning rate, and then are processed by using the resolution optimization algorithm. As a result, the phase images of the onion epidermal cells with higher contrast and resolution are obtained. According to the synthetic holograms, the changes of cell nucleus and actin microfilament inside onion’s epidermal cells are displayed. In addition, the dehydration process and plasmolysis phenomenon inside onion epidermal cells are also exhibited by recording longterm scanning holograms of living epidermal cells. The experimental results demonstrate that image quality of living onion epidermal cells can be improved by optimizing the algorithms.
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