Digital Holographic Method Research Articles

Nondestructive evaluation of residual stress via digital holographic photoelasticity

The nondestructive evaluation of residual stress forms a very important aspect of structural engineering. In this context, as per the theory underlying digital holographic photoelasticity, the residual stress field in a transparency can be efficiently and nondestructively evaluated with high accuracy by varying the polarization direction of the reference light beam. In this study, we use a setup to experimentally verify the above mentioned hypothesis. We first record digital holograms before and after heating optically transparent specimens at four different corresponding positions of the reference light polarization. We next solve the four resulting light intensity equations after digital image processing of the four digital holograms to obtain the residual stress. A comparison of our experimental results with those of the drilling method (the conventional approach to determine residual stress) indicates that our digital holographic photoelasticity method can be suitably applied to the nondestructive evaluation of residual stress in transparencies.

Progress in the Synthetic Holographic Stereogram Printing Technique

The synthetic holographic stereogram printing technique can achieve a three-dimensional (3D) display of a scene. The development and research status of the synthetic holographic stereogram printing technique is introduced in this paper. We propose a two-step method, an infinite viewpoint camera method, a single-step Lippmann method, a direct-write digital holography (DWDH) method and an effective perspective images’ segmentation and mosaicking (EPISM) method. The synthetic holographic stereogram printing technique is described, including the holographic display with large format, the large field of view with no distortion, the printing efficiency, the color reproduction characteristics, the imaging quality, the diffraction efficiency, the development of a holographic recording medium, the noise reduction, and the frequency response analysis of holographic stereograms.

Partial saturation-aided resolution enhancement for digital holography.

Because of the limited dynamic range of common detectors, high-frequency components weaker than the threshold of the detector used are lost in digital holography (DH) even with the aid of coherent diffraction imaging (CDI)-based iterative computation, and this makes DH less preferable for many interesting applications. To overcome this problem, the partially saturated diffraction pattern was adopted to enhance the resolution of DH iteratively. Since more high-frequency components were retrieved in comparison with our former CDI-based digital holographic method, the resolution of DH was improved further. The feasibility of the proposed method is demonstrated both numerically and experimentally.

Fine Metal Mask 3-Dimensional Measurement by using Scanning Digital Holographic Microscope

For three-dimensional microscopy, fast and high axial resolution are very important. Extending the depth of field for digital holographic is necessary for three-dimensional measurements of thick samples. We propose an optical sectioning method for optical scanning digital holography that is performed in the frequency domain by spatial filtering of a reconstructed amplitude image. We established a scanning dual-wavelength off-axis digital holographic microscope to measure samples that exhibit a large amount of coherent noise and a thickness larger than the depth of focus of the objective lens. As a demonstration, we performed a three-dimensional measurement of a fine metal mask with a reconstructed sectional phase image and filtering with a reconstructed amplitude image.

Measurement of moving objects with phase-shifting digital holography using liquid crystal retarder

We propose a phase-shifting digital holography (DH) method that uses a liquid crystal (LC) retarder for use in the measurement of moving objects. Because LCs are birefringent, they can be used as waveplates for phase-shifting operations. In addition, the phase retardation can be adjusted by varying the LC drive voltage. Mechanical operations are unnecessary in phase shifting when using LCs, but fast interference fringe measurements are possible. The proposed method can be realized by incorporating the LC retarder into a conventional interferometer and is both simple and inexpensive. We performed experiments to verify the effectiveness of the proposed method and confirmed that the method is effective for imaging of the complex amplitude distributions of moving objects.

Computational ghost imaging encryption based on fingerprint phase mask

A computational ghost imaging encryption method based on fingerprint phase mask is proposed. In this work, we introduce one’s fingerprint information into computational ghost imaging for the first time. The phase mask key in computational ghost imaging encryption is generated by a fingerprint image using digital holography method. As the phase key links to one’s fingerprint which is uniqueness, this method offers a significant improvement for the security of computational ghost imaging encryption. Furthermore, because a fingerprint can verify one’s identity, the proposed method can combine identity authentication with image decryption. In addition, the distances of recording and reconstruction during digital holography can be saved as additional keys which make the encryption system more secure. To verify the feasibility, security and ability to resist noise attack, some computer simulations are performed.

Communication—Direct Observation of the Shuttle Phenomenon in Lithium-Sulfur Batteries via the Digital Holographic Method

Lithium-sulfur (Li-S) batteries are regarded as the best potential-energy storage systems because of their ultra-high energy density. However, their electrochemical performance is limited by the well-known shuttle effect. We used an online digital holography method to investigate the interfacial changes between the electrode and the electrolyte. An in-situ observation of the shuttle process was achieved in Li-S batteries. This result confirms the existence of the shuttle mechanism, and aids in the selection of preferable composite cathodes and electrolytes to produce high-performance Li-S batteries.

Research on Digital Holographic 3D Reconstruction Software

Abstract Digital holography has a wide range of applications in displaying the surface morphology of three-dimensional objects. There have been many studies on the physical methods and implementation methods of digital holography. One of the main problems at present is to design a stable and fast reconfiguration software. This paper analyzes the basic principle of convolution based off-axis digital holographic reconstruction, and presents the process of 3D reconstruction of off-axis digital holography. The 3D reconstruction software of digital holography is designed and developed according to the basic principle and the process of reconstruction, and the parallel operation of 3D reconstruction is realized by OpenMP. Results show that the software can help researchers conveniently realize digital holographic three-dimensional reconstruction. The reconstructed parameters can be adjusted according to the requirements, and the spectra, intensity maps, phase maps and other information required in the reconstruction process can be obtained. The software meets the needs of digital three-dimensional reconstruction.

Online Digital Holographic Method for Interface Reaction Monitoring in Lithium-Ion Batteries

Understanding the reaction mechanisms at the interface of electrode and electrolyte is both of fundamental interest and essential to improve lithium-ion battery (LIB) performance. Herein, we report an online digital holographic method to in situ observe the entire interface change between electrode and electrolyte in lithium-ions batteries. The accuracy of this technology is well verified in LiFePO4/graphite full-cell systems, graphite/Li half-cell systems in EC-based and PC-based electrolyte, respectively, and supported by the characterized results of conventional instruments, including scanning electron microscopy and X-ray photoelectron spectroscopy. In particular, the time resolution of the digital holographic method is 0.04 s and fast enough to distinguish detail reduction process of ethylene carbonate (EC), for which EC will be first reduced to generate lithium alkyl carbonates, and then the reduction product is Li2CO3 to form a stable SEI films. To our best of knowledge, this is the first report on...

Comparison of numerical reconstruction of digital holograms using angular spectrum method and Fresnel diffraction method

This work reports an experimental comparative study between the two most frequently used numerical reconstruction methods in digital holography; i.e. angular spectrum method (ASM) and Fresnel diffraction method (FDM). As distance between object plane and recording plane is an important factor for selection of a reconstruction method in digital holography, thus here comparison between the two methods is made on the basis of variable distance between object and recording plane (i.e. CMOS image sensor). The ASM is also known as double Fourier transform method as it involves inverse Fourier transform of Fourier transformed incident field and transfer function of the system. While, FDM involves numerical reconstruction by calculating diffracted field using Fresnel diffraction formula. In present work, Angular spectrum and Fresnel diffraction methods are applied for numerical reconstruction of digital holograms of a transparent object, recorded at different distances from CMOS sensor. Experimental results are presented and distance constraints for both methods are discussed in different experimental situations.

Digital holographic high-speed 3D imaging for the vibrometry of fast-occurring phenomena

Digital holography allows production of high-speed three-dimensional images at rates over 100,000 frames per second; however, simultaneously obtaining suitable performance and levels of accuracy using digital holography is difficult. This problem prevents high-speed three-dimensional imaging from being used for vibrometry. In this paper, we propose and test a digital holography method that can produce vibration measurements. The method is based on single-shot phase-shifting interferometry. Herein, we imaged the surface of a loudspeaker diaphragm and measured its displacement due to the vibrations produced by a frequency sweep signal. We then analyzed the frequency of the experimental data and confirmed that the frequency spectra inferred from the reconstructed images agreed well with the spectra produced by the sound recorded by a microphone. This method can be used for measuring vibrations with three-dimensional imaging for loudspeakers, microelectromechanical systems, surface acoustic wave filters, and biological tissues and organs.

Color-image reconstruction for two-wavelength digital holography using a generalized phase-shifting approach.

We propose a color-image reconstruction method for two-wavelength digital holography using generalized phase-shifting digital holography (GPSDH). In this method, color interference fringes are captured by a digital camera with a Bayer array color filter, and phase shifting is simultaneously performed for all wavelengths. Color interference fringes are separated into three monochromatic interference fringes using a color-separation method that suppresses the color-filter crosstalk. The object wave is extracted from each monochromatic interference fringe using GPSDH, which prevents problems due to the phase shift wavelength dependence. Image reconstruction is performed using a shifted Fresnel transform-based method, in which the color reconstructed image is obtained by directly superposing the reconstructed images for all wavelengths. We verify the proposed method through optical experiments with a two-wavelength digital holography system. The results show that the dual-color image can be successfully reconstructed without chromatic aberration.

Surface shape measurement by multi-illumination lensless Fourier transform digital holographic interferometry

This paper presents a multi-illumination lensless Fourier transform digital holographic interferometry method for surface shape measurement. In this method, the interference phases with different effective synthetic wavelengths are obtained by tilting the illumination angle several times, and all are wrapped. A Fourier-transform demodulation algorithm employing all these wrapped phases simultaneously is used to determine the object surface shape. No phase unwrapping procedure is required, and the shape information of each point is calculated independently, thereby offering great flexibility for measuring objects with discontinuities surface, such as holes, steps and gaps. Experimental results demonstrate the validity of the principle.

Fast autofocusing in digital holography using the magnitude differential

Typical methods of automatic estimation of focusing in digital holography calculate every single reconstructed frame to get a critical function and then ascertain the focal plane by finding the extreme value of that function. Here, we propose a digital holographic autofocusing method that computes the focused distance using the first longitudinal difference of the magnitude of the reconstructed image. We demonstrate the proposed method with both numerical simulations and optical experiments of amplitude-contrast and phase-contrast objects. The results suggest that the proposed method performs better than other existing methods, in terms of applicability and computation efficiency, with potential applications in industrial and biomedical inspections where automatic focus tracking is necessary.

Continuous-wave terahertz multi-plane in-line digital holography

Terahertz digital holography is a non-scanning and real time method for reconstructing the absorption and phase distributions of the wave-front diffracted by a given sample simultaneously in the terahertz region. A continuous-wave terahertz in-line digital holographic multi-plane imaging method is presented here for achieving a three-dimensional shape of a specific portion of a sample with the best possible focus. The three enhancement techniques of synthetic aperture, autofocusing and phase retrieval are applied to the single plane recordings for achieving a high resolution, good quality and optimally focused reconstructed image. Later, multi-plane reconstructed images are processed with the threshold mask and a three dimensional profile of the sample is obtained. Experimental verification confirms that the proposed method is a valid tool for acquiring multi-plane information of a target in the terahertz range.

Automatic focus determination through cosine and modified cosine score in digital holography

An automatic focus determination method in digital holography that utilizes the cosine score (CS) of the inner angle between the vectors that result from adjacent axial reconstructed amplitude images is proposed. This method is based on the fact that the optical field near the focus plane contains more regular features of an object than the defocused region. Further, a modified CS (MCS) autofocusing method is proposed to extend the application range and improve the performance of the proposed method. First, a low-pass filter is designed for the object term holograms in the frequency domain. Second, the standardized Z-scores method is applied along the axis to the amplitude images reconstructed from the filtered hologram. Finally, the MCS of the standardized amplitude images is calculated by an improved cosine-like formula. Next, the focus plane is found at the minimum MCS for different types of objects. This method enables accurate focus determination for all types of objects, as well as for the image region with a slow change or small size, which offers time savings and the precise autofocusing of objects with large longitudinal volumes. The simulations and experimental results on a United States Air Force resolution chart and living cells are presented, illustrating the efficiency and potential of the methods.

Linear programming phase unwrapping for dual-wavelength digital holography.

A linear programming phase unwrapping method in dual-wavelength digital holography is proposed and verified experimentally. The proposed method uses the square of height difference as a convergence standard and theoretically gives the boundary condition in a searching process. A simulation was performed by unwrapping step structures at different levels of Gaussian noise. As a result, our method is capable of recovering the discontinuities accurately. It is robust and straightforward. In the experiment, a microelectromechanical systems sample and a cylindrical lens were measured separately. The testing results were in good agreement with true values. Moreover, the proposed method is applicable not only in digital holography but also in other dual-wavelength interferometric techniques.

Optical implementation of cipher block chaining mode algorithm using phase-shifting digital holography

We propose an optical design of cipher block chaining (CBC) encryption mode using digital holography, which is implemented by the two-step quadrature phase-shifting digital holographic encryption technique using orthogonal polarization. A block of plain text is encrypted with the encryption key by applying the two-step phase-shifting digital holographic method; then, it is changed into cipher text blocks which are digital holograms. Optically, these digital holograms with the encrypted information are Fourier transform holograms and are recorded onto charge-coupled devices with 256 quantization gray levels. This means that the proposed optical CBC encryption is a scheme that has an analog-type of pseudorandom pattern information in the cipher text, while the conventional electronic CBC encryption is a kind of bitwise block message encryption processed by digital bits. Also, the proposed method enables the cryptosystem to have higher security strength and faster processing than the conventional electronic method because of the large two-dimensional (2-D) array key space and parallel processing. The results of computer simulations verify that the proposed optical CBC encryption design is very effective in CBC mode due to fast and secure optical encryption of 2-D data and shows the feasibility for the CBC encryption mode.

Phase retrieval method for digital holography with two cameras in particle measurement.

This paper presents a phase retrieval method for digital holography with two high-speed cameras in particle measurement. The conditions for recording two holograms are derived theoretically. We focus in particular on the distance between the two holograms. The relative misalignment of the two holograms is also evaluated by numerical simulations. Finally, we experimentally compared settling particles reconstructed by the presented method and the Gabor method.

The internal defects detection in crystals by digital holographic methods

The internal defects detection method is suggested for crystals intended for the use in the IR part of spectrum. The method is tested on samples of the ZnGeP2 monocrystals, the experimental results are shown.