Vibration Analysis by Phase Shifting Digital Holography

We propose a parallel two-step phase-shifting digital holography technique capable of instantaneous measurement of three-dimensional objects, with a view toward measurement of dynamically moving objects. The technique is based on phase-shifting interferometry. The proposed technique carries out the two-step phase-shifting method at one time and can be optically implemented by using a phase-shifting array device located in the reference beam. The array device has a periodic two-step phase distribution, and its configuration is simplified compared with that required for three-step and four-step parallel phase-shifting digital holographies. Therefore the optical system of the proposed technique is more suitable for the realization of a parallel phase-shifting digital holography system. We conduct both a numerical simulation and a preliminary experiment in the proposed technique. The results of the simulation and the experiment agree well with those of sequential phase-shifting digital holography, and results are superior to those obtained by conventional digital holography using the Fresnel transform alone. Thus the effectiveness of the proposed technique is verified.

We present a technique for phase-shifting digital in-line holography which compensates for lateral object motion. By collecting two frames of interference between object and reference fields with identical reference phase, one can estimate the lateral motion that occurred between frames using the cross-correlation. We also describe a very general linear framework for phase-shifting holographic reconstruction which minimizes additive white Gaussian noise (AWGN) for an arbitrary set of reference field amplitudes and phases. We analyze the technique's sensitivity to noise (AWGN, quantization, and shot), errors in the reference fields, errors in motion estimation, resolution, and depth of field. We also present experimental motion-compensated images achieving the expected resolution.

State of drying paint is monitored from the dynamic behaviors of images reconstructed by phase-shifting digital holography. For quantitative analysis cross-correlation coefficient of complex amplitude, named here, coherence factor, between successive frames with an interval of 20 s. Its modulus shows variation indicating the random dynamic behavior of the paint surface, while its phase is apparently related with the change of paint thickness. In the dried state they reach stationary values. The modulus has been shown to relate to the degree of speckle correlation in the model of Gaussian random process.

Double Encryption of Image Based on Scramble Operation and Phase-Shifting Digital Holography

Recently, it has become necessary to detect cracks in structures such as buildings and bridges. Phase-shifting digital holography is one valuable method that can be used to measure the strain distribution on the surface of an object. However, to simplify the optical measurement setup, more compact equipment is required. An interferometric trigger method with dual piezo actuators for phase shifting is proposed. This method is useful for designing a head discrete-type measurement system that uses phase-shifting digital holography with multiple imaging sensors. The ability to phase shift using the proposed trigger method was confirmed with the reconstruction of an object and an experiment that was performed to measure the displacement distribution.

We propose an algorithm that can improve the quality of the reconstructed image from the single hologram recorded by the optical system of the parallel four-step phase-shifting digital holography. The proposed algorithm applies the image-reconstruction algorithm of parallel two-step phase-shifting digital holography to the hologram so as to reduce errors in the reconstructed image and eliminate ghosts. We numerically and experimentally confirmed that the proposed algorithm decreased 25% in terms of root mean square error in amplitude, and eliminated the ghosts, respectively.

We propose a phase-shifting interferometry technique using only two in-line phase-shifted self-interference holograms. There is no requirement for additional recording or estimation in the measurement. The proposed technique adopts a mathematical model for self-interference digital holography. The effectiveness of the proposed technique is demonstrated by experiments on incoherent digital holographic microscopy and color-multiplexed fluorescence digital holography with computational coherent superposition. Two-color-multiplexed four-step phase-shifting incoherent digital holography is realized for the first time, to the best of our knowledge, using the proposed technique.

High-resolution, real-time 3-D absolute coordinate measurement is highly important in many fields. This paper presents such a system that measures 3-D absolute geometric shapes and positions at 30 frames per second (fps), with an image resolution of 532 × 500. The system is based on a digital fringe projection and fast three-step phase-shifting method. It utilizes a digital-light-processing (DLP) projector to project color encoded computer generated phase-shifted fringe patterns in grayscale, a high-speed CCD camera synchronized with the projector to capture fringe images at 90 fps. Based on the three-step phase-shifting algorithm, any successive three fringe images can be used to reconstruct one 3-D shape. Therefore, the 3-D shape measurement speed is 30 fps. To obtain absolute coordinates, absolute phase is required. In this research, a tiny marker is encoded in the projected fringe pattern, and detected by software from captured fringe images accurately. Absolute coordinates can be obtained when the marker position is located and the measurement system is calibrated. To demonstrate the performance of the system, we measured human faces. The details of facial geometries and dynamic changes of facial expressions are measured clearly. We also measured a hand moving over a depth distance of approximately 700mm, again both hand shapes and positions changes are clearly captured. By using the fast three-step phase-shifting algorithm and a parallel processing technique, we successfully developed a system that acquires, reconstructs, and displays 3-D shape in real-time. Applications of such a system include manufacturing, inspection, reverse engineering, computer vision and computer graphics.

Minimum optical energy required for parallel four-step phase-shifting digital holography is evaluated numerically by using photon-counting method. One of the attractive features of parallel phase-shifting digital holography is the instantaneous recording of fast 3D events where only the complex amplitude distribution of an object wave is obtained. The reconstruction is executed by numerical wave propagation such as angular spectrum propagation or Fresnel propagation. Numerical results indicate that required optical energy of an input image with 512 × 512 pixels is about 11 pJ. Under the criteria used in the evaluation, the required optical energy is independent of the image size.

To improve the security and efficiency of the image message transmission. This paper propose an optical image encryption scheme based on random phase mask and four-step phase shift digital holography. In the process of image encryption, the low frequency sub-band of the original images are superimposed by wavelet transform to increase the encryption efficiency. In this paper, Henon mapping generates two random phase masks. The initial values and control parameters of Henon mapping can be used as the main keys. The use of random phase mask makes the transmission and storage of the keys of sender and receiver more convenient. Moreover, the wavelength and diffraction distance are also used as the keys, which can enhance the security of encryption. With the help of random phase masks and four-step phase-shifting digital holography, the original image is encrypted into four cipher holograms. The simulation results show that the encryption algorithm proposed in this paper can reconstruct high-quality decrypted images and effectively resist various noise attacks

In digital holography, holograms are recorded by a CCD-array, and the complex amplitude of the object wave is numerically reconstructed via computer. For different recording conditions and different properties of objects, different reconstruction algorithms are required. The conventional reconstruction algorithms were conceived directly by replacing the diffraction integral with summation. Each method has its limitation in the valid range for correctly calculating the diffraction integral. The Single Fourier Transform method is valid for far Fresnel zone hologram, whereas the convolution method is appropriate for near Fresnel holograms. Here, we present a general reconstruction model from the perspective of “Generalized sampling theory”. Given that the function space in which the unknown complex amplitude lies, an approximation of the continuous complex amplitude at the CCD can be synthesized from a set of basis functions with the recorded samples as weights. Back-propagation of the approximated complex amplitude to the original object plane yields an expression relating the continuous complex amplitude of the object with the recorded samples. By adopting different basis functions and different formulas for describing the diffraction process, an optimal reconstruction algorithm can be developed for various recording conditions and different diffraction characteristics of the object. Contrary to the conventional algorithms where values are available only at specific grid, complex amplitude at any position of the object can be obtained using this model. In addition, the effect due to the non-zero fill factor of the CCD can also be incorporated into the reconstruction algorithm to be further compensated by over-weighting the high frequency components. Two basis functions: Dirac delta- and Sinc-, are studied in detail.

Alternative use of reflection spatial light modulator in phase shifting digital Gabor holography

Structured light system using a digital video projector is increasingly used for a 3-D shape measurement because of its digital nature. However, the nonlinear gamma of the projector causes the projected fringe patterns to be non-sinusoidal, which results in phase error therefore shape measurement error. Previous work showed that, by using a small look-up-table (LUT), this type of phase error can be reduced significantly for a three-step phase-shifting algorithm. In this research, we prove that this type of phase error compensation method is not limited to a three-step phase-shifting algorithm. It is generic for any phase-shifting algorithm. The phase error compensation algorithm is able to theoretically eliminate the phase error caused by the gamma of the projector completely. It is based on our finding that in phase domain, the phase error due to the projector's gamma is preserved for arbitrary object's surface reflectivity under arbitrary ambient light condition. The phase error can be pre-calibrated and stored into a LUT for phase error reduction. In this research, we captured a set of fringe images of a uniform flat surface board for such a calibration. The phase error of the flat board is analyzed and stored in a LUT for phase error compensation. Our experimental results show that by using this method, the measurement error can be reduced by at least 13 times for any phase-shifting algorithm.

A novel optical image encryption is proposed based on two-step-only quadrature phase-shifting interferometry.Only two interferograms are needed to reconstruct a zero-order-and twin-image-free hologram in this phase-shifting digital holography interference.Such technique precludes the need from recording either the reference wave or the object wave intensity.The object wavefront propagates with two Fresnel transforms in the light path,combined with the double random phase encoding.The following is that introducing zero and л/2 phase into reference waves respectively and recording two digital holograms as encrypted image.As long as the correct key is given in the decryption,a clear original image can be reconstructed by a simple calculation.The feasibility and its robustness against occlusion and noise attacks are verified by a series of numerical simulations.

본 논문에서는 기존의 위상천이 디지털 홀로그래피에서 기준빔으로 사용되는 평면파 대신 보다 일반적인 구면파를 이용하여 3차원 공간상에 위치한 두 물체점들 사이의 상대적인 거리를 측정하는 새로운 시스템 및 알고리듬을 제안한다. 본 논문에서는 이러한 시스템에 대한 모의실험 및 광학실험을 실시하였다. 이러한 시스템에서는 홀로그램 기록에 사용된 기준빔의 정확한 정보를 알고 있지 않더라도 3차원 공간상의 두 물체점들 사이의 상대적인 거리를 정확하게 측정할 수 있다. 본 논문에서는 광학실험을 통하여 약 300 cm 거리에서 서로 0.5 cm 떨어진 두 물체점들 사이의 상대적인 거리를 10% 미만의 오차로 측정하였다. In this paper we present a new method of measuring the relative distance of two point objects in three-dimensional space by using phase-shifting digital holography. In our system the reference beam of a spherical wave is used instead of a plane wave. The system is computer simulated and built on an optical table for experiments. It is shown from computer simulations and experiments that the relative distance can be measured without the exact information on the reference beam used in the hologram record. It is shown from experiments that the relative distance between two point objects separated by 0.5 cm in the distance of about 300 cm from the CCD can be measured with an error less than 10％.