A numerical compensation technique is proposed to enhance the quality of reconstruction of phase-shifting lensless Fourier digital holographic microscopy. Introducing phase-shifting technique into the in-line lensless Fourier digital holographic microscopy is widely used, because this system possesses high resolution and is able to suppress the zero-order of diffraction and the twin image in reconstruction. Practically, it is likely to suffer from the lower resolution due to the blurred image in reconstruction plane in respect that the normal of object plane is not perpendicular to the CCD plane or the distance between the source of reference wave and the CCD plane is not equal to the distance between the object plane and the CCD plane accurately. Mechanical adjustment has difficulty in achieving high accuracy of adjustment and needs a great deal of time. Based on the theories of Fresnel diffraction and holography, it is found that numerical compensation can complete adjustment in high accuracy. It uses chirp function as compensation factor. The values of the three compensating parameters in three directions are obtained by repeatedly applying the numerical reconstruction procedure through gradual approach until the best in-focus image is obtained. The precisions of the three compensating parameters are decided by requirement. The main advantages are: (1) the accuracy of adjustment is high, (2) the quality of reconstruction can be greatly improved. In order to testify the feasibility of the numerical compensation technique, experiment is carried out. Its results show that fine numerical compensation is necessary to advance the quality and resolution of reconstruction.
Synthetic aperture technique can be used to improve the reconstruction image resolution of digital hologram. In this paper, a synthetic aperture digital holographic method is proposed. In the method, the recording of synthetic aperture digital hologram is carried out by means of object movement in one direction, and the reconstruction is achieved by cross-correlation synthesis technique. Compared to the digital holography without synthetic aperture technique, only a one-dimension movable bracket is added in the experimental setup. The recorded object fixed on the movable bracket is placed in Mach-Zehnder interferometer and moved along horizontal direction. For the purpose of aperture synthesis reconstructing, four sub-digital holograms have been recorded by a CCD camera. While recording, superposition area of about half pixels of single digital hologram size should be assured between the adjacent holograms in order to ensure that the sub-digital holograms are synthesized accurately. The experimental result shows that every horizontal line of the reconstruction image is distinct in the synthetic aperture digital hologram, but the transverse is unidentifiable in the images reconstructed from the single digital holograms. It can be seen obviously that compared to sub-digital hologram the reconstruction image resolution of synthetic aperture digital hologram is improved.
In the interference phase measurement of slow-changing process, by use of the phase-shifting technology in the initial state, the background and amplitude of interference fringes can be obtained, and a novel phase unwrapping method based on cosine function is proposed. The holograms of the changing process are recorded, and then the phase cosine functions can be obtained by removing the background and amplitude from the holograms. The arccosine functions of phase cosine functions, which are called phase cosine wrapping function in this paper, can be unwrapped by utilizing the additional normal orientation information. The experimental analyses show that the residual noise and the phase-shifting errors have great influence on the accuracy of unwrapped phase. The tangent wrapping phase can’t be filtered by traditional method due to the π phase jumps, and the existing phase unwrapping algorithms are very complex. The phase-shifting errors can only influence the positions of phase jump points in the tangent wrapping phase. It is difficulty to optimize the tangent wrapping phase further. Compared with tangent wrapping phase, the phase cosine wrapping function is consecutive and can be filtered, and the unwrapping process is easier than that of tangent wrapping phase. The influence of phase-shifting errors on phase cosine function is not only positions but values of the wave crest and wave trough. The more precise the phase-shifting is, the closer the values of cosine function to ±1 at wave crest and wave trough are. The Experiment results show that cosine unwrapping method has the equivalent precision with tangent unwrapping method.
In a series of the experiments of real-time holographic interferometry, some dark shadow areas appear among the interference fringes. The appearing of the dark shadow area plays a very important role in the fracture process. We find that the appearing of these dark shadow regions are the phenomena of caustic in Fracture Mechanics. The shadow region is so called shadow spot in Fracture Mechanics and the factor of stress intensity can be calculated by measuring the size of the correosponding length of the shadow spot and the strain field can be calculated by traditional holographic method. Therefore, holographic method and caustic method can be combined together in holographic interferometry and it will lead to a wide application in Experimental Mechanics.
The principle of holographic CT measurement is introduced briefly, and the possibility of measuring 3D variation of the refractive index from the intensity distribution of a real-time holographic interferogram is proven theoretically. Based on this method, simulated study on calculating the variation of refractive index of an axis symmetry media is made by computer and the example of application is also presented in this paper.
on the theoretic analysis, this paper reports a new two-step color reflection holography and has made some holograms successfully. Comparing with making Denisyuk hologram, this new two-step recording method can adjust the ratio of reference light and object light, and the reconstructing image can be in any location in holographic recording plate.
In order to decrease the phase shift error, this paper presented a new object phase shifting method by using fringe stabilizing device to stabilize the interference fringe. The experimental result shows the fringe drift is less than ?/20 in five minutes , and the phase shifting precision is more than 0.004 ? , furthermore the system is very simple, performance reliable and operating convenience, it will take an important role for improving the measure precision of 3-D phase profilometry, more strong practicality and popularly application prospect.
Six novel methods in real-time holographic interferometry recently worked out by the authors of this paper. They are: the one for recording a real-time hologram with high contrast interference fringes and high brightness of the testing optics field, the one for measuring the phase modulation of a thin-phase hologram, the one for controlling the phase modulation in making a real-time hologram, the one for measuring in the 'reference wave field', the one for fast predicting the direction of displacement, and the one for avoiding the light reflected from the surfaces of collimation lens. All these methods are helpful to holographers in pursuit of high precision and efficiency in real-time holographic interferometry.
In this paper the physical process of real-time holographic interferometry is simulated by the computer calculations based on scalar diffraction theorem and the deformation field of the specimen is calculated by the inverse calculation of the diffraction from the interference fringes. The application the study of micro-crack nucleation process leads to interesting result in good agreement with experiments.
The fracture process of uniformity solid is simulated by a series of transparent specimen in this study in order to discover relationships between the micro-crack nucleation process and the surrounding stress field. A real-time holographic interferometry is used to record the stress fields and its variations in the specimen while the wave-forms of ultrasonic emission stimulated by the micro-cracks are recorded by a transient automatic wave- form recorder to determine the time, location and intensity of the micro-cracks. The development process of fracture can be visualized clearly by this real-time holographic interferometry. The initial experimental study has brought some phenomena worthy of study. This method provides a powerful new approach for experimental mechanics and seismological research.
In this paper, the holographic recording materials is thought as a scattering object, while a laser beam illuminates to the materials, the scattering light must be emerged. For the reconstructing hologram, the scattering light is thought as the scattering noise, it comes from the scattering particles. By using a narrow laser beam illuminates to the recording plate and measures the scattering and diffraction light intensity and the size of diffraction ring, the scattering noise and the size of the particle of the recording materials can be measured respectively. As the size of the particle of the recording materials determines the resolving power of holographic plate, the resolving power determines the quality of hologram, so the measurement of the size of the particle of the recording materials is very important. This paper also presents some new methods to measure the quality of holographic plate, model plate, model plate and PET materials by using the diffraction pattern of a narrow laser beam. The experimental results shows these new methods with wide measuring range, high precision and short measuring time are very practical.
To evaluate a hologram by measuring its optical density D, diffraction efficiency (tau) and signal-noise ratio SNR is a conventional method in holography. In this paper, a novel method is proposed for evaluation of a thin, phase holograms by testing phase modulation together with D, (tau) and SNR. Many properties of the thin, phase holograms are related to phase modulation, so that to control the value of phase modulation in making a thin, phase hologram is important. The value of the phase modulation of a hologram can be obtained by measuring the irradiance of its zero and first- order diffraction and calculating their ratio. This method will help holographers to obtain a perfect understanding of a thin, phase hologram and help them to choose the optimum parameters of exposure so as to obtain an expectation result. The theoretical analysis of this method are given and some applications are also introduced.
The holographic information transferred from photoresist master to replica is the relief groove depth. Many properties of the replica are related to groove depth of master, so to control the groove depth of photoresist master is important. In this paper, a novel method is adopted to obtain the value of the phase modulation by measuring diffraction light intensity of the photoresist master, since there is a certain relationship between groove depth and phase modulation, so the groove depth can be obtained indirectly.