Moiré tomography has been considered as an effective tool in studying flow fields because of its advantages such as non-contact measurement, strong anti-disturbing capability, and wide measurement range. The spatial phase-shifting method, which can simultaneously obtain several phase-shifted interferograms, can be applied in the flow field measurements by the moiré tomography when the flow field varies rapidly. In this paper, we present a new spatial phase-shifting shearing interferometry. The optical structure of the interferometry, which only consists of a crossed grating and a linear grating, is very simple. With it six phase-shifted interferograms can be acquired simultaneously. Based on the scalar diffraction theory, the explicit forms of intensity distribution of the interferograms containing the phase information can be derived and a corresponding four-step phase-shifting algorithm is proposed to extract the first-order partial derivative of phase projection from the interferograms. Finally, the spatial phase shifting optical system is used to retrieve the first-order partial derivative of propane flame phase projection produced by plane incident wave. This work is crucial to accuracy reconstruction the physical parameter of the varied flow fields in moiré tomography.
Phase distribution detection of cells and tissues is concerned since it is an important auxiliary method for observing biological samples. Here, in this paper, we have proposed phase
retrieval algorithms dealing with microscopic interferograms in order to solve two-dimensional phase
distribution. Based on phase distributions solved by phase retrieval algorithms, three-dimensional
refractive index distribution of biological sample is reconstructed which could reflect inner structure of
the cell. We believe these methods could be powerful tools in biological and medical fields.
Volume optical computerized tomography (VOCT), which can realize real 3D measurement rather than traditional 2D
OCT, has great superiority in quantitatively measuring the thermo physical parameters of transient flow field. Among the
refractive index reconstruction techniques, filtered back-projection (FBP) method performs better than algebraic
reconstruction techniques (ARTs) with higher accuracy and computationally efficient. In order to apply FBP to VOCT,
the radial second-order derivative of projection wave front passes through the tested phase object should be obtained
firstly. In this paper, a projection device with two circular gratings is established. In particular, owing to an inherent
phase shift exists between moiré fringes of +1 and -1 diffraction orders, a two-step phase-shifting algorithm is utilized to
extract the wave front’s radial first-order derivative which is contained in the moiré fringes. The reliability of the
two-step phase-shifting algorithm is proved by a computer simulation. Finally, the radial first-order derivative of wave
front passing through a propane flame is measured and retrieved by these methods.
KEYWORDS: 3D image processing, Image processing, Radon transform, Integral imaging, 3D image reconstruction, Charge-coupled devices, 3D acquisition, Microlens array, Image quality, 3D displays
Integral imaging is one of the most promising techniques for capturing and displaying the three-dimensional information of the object. Most integral image analysis and processing tasks require each elemental image can be identified with high precision, which is difficult to implement in a real pick-up process. For the acquisition of the three-dimensional information in integral imaging process, the lens array should be aligned precisely with respect to the CCD. In this paper, we present a method to accurately correct geometric distortions triggered by the misalignment between lens array and CCD. The method for calculating the skew angle of deviation and the accurate gridline structure in the three-dimensional integral images is based on the Radon Transform algorithm. Then using projective image transformation, the geometrical distortion in the elemental image set can be rectified. The size and position information of the rectified element images also can be calculated by the gridline structure, which will prevent the image splitting, shifting along the lateral or longitudinal direction, or blurring according to reconstruction distance in the optical or computational reconstruction process.
Moiré tomography is an important technique to diagnose the flow field. However, the traditional moiré deflectometry cannot meet the requirements of Volume Moiré Tomography (VMT). In this Letter, an improved moiré deflected system based on double orthogonal gratings is introduced for real 3-D reconstruction. The proposed method could obtain the first-order partial derivatives in two vertical directions of the projection in one time. Comparing with the traditional moiré deflectometry, the proposed system is more effective and easier to realize the multi-direction data acquisition.
Optical Computerized Tomography is a technique which is famous for real-time, stable and non-contact characteristics in
various flow fields' diagnosis. As a result, it shows superiorities in many domains, including the aerospace survey and
the measurement of the thermo physical parameters. Due to most of the traditional reconstruction methods of OCT are
based on 2-D Radon Transform, they are pseudo three-dimensional in essence. That is to say, the flow field is divided
into several parallel slices firstly, and then, the stack of tomogram slices is subsequently used to compute the 3-D
representation. However, all the flow fields own real three-dimensional character. Therefore, in this paper, based on the
3-D Radon Transform, the optical interferometry is studied on the model of the volume CT. Meanwhile, the sufficiency
condition of accurate reconstruction is studied. Besides, the transform reconstruction algorithm for volume OCT is also
presented and verified by simulated experiments. In a word, this study will be better to visualize and display flow fields.
In this research, the feasibility of moiré deflectometry for diagnosing the temperature and the electron
number density of rocket exhaust plume is studied. The dependence of the refractive index on the
temperature, the pressure, the composition and the probe wavelength is established firstly. Based on
which, the theoretical analysis and discussion indicate that it is impossible to achieve the measurement
of the electron number density by both one- and two-wavelength moiré deflectometry in the given
temperature, pressure and probe wavelength regions. Yet, it may be feasible to measure the temperature
distribution of rocket exhaust plume by moiré deflectometry, if the imprecision of the temperature
reconstruction is permitted by the practical measuring requirement.
A novel spatial phase-shifting method is presented from the moire phenomena. With the analysis to moire patterns from
the scalar diffraction theory, there is stable phase shift between different order diffraction interferograms of moire
deflectometry. This phase shift just depends on the distance between two gratings and the grating period. In this spatial
phase-shifting method, no beam splitter, polarizer or wave plate is used and the disadvantage of uncertain phase shifts
due to the methods using polarizer or wave plate is removed.
A new iterative reconstruction algorithm is developed and applied to moiré deflection tomography for flow field measurements. The algorithm is derived from the basic deflection formula and based on a modified algebraic reconstruction technique. The precision and convergence of the algorithm are analyzed through a numerical simulation. To capture multidirectional projection data, a rotatable deflectometric system is developed. The efficacy of the new algorithm is assessed by reconstructing an asymmetric temperature field. Furthermore, the algorithm is employed to investigate the image of a cross section of a free jet containing steep gradients and to reconstruct the density field of the rocket exhausted jet.
Moire tomography is applied in fully quantitative measurement of temperature distribution. Rotary moire deflectometer is designed to capture multidirectional optical projections, and moire patterns of temperature field can be obtained in 180 degrees. Then the temperature distribution of axisymmetric or asymmetric flow field is calculated by a new iterative reconstruction algorithm. The algorithm is derived from an existing moire deflection formula and based on an improved algebraic reconstruction technique. The method is demonstrated by reconstructing a temperature field generated above the top of two electric solding irons.
Reconstructions with complex flow fields and limited-projections are two main serious problems that are often encountered in practical OCT cases. While moire tomography has its inherent advantages, such as low mechanical stability demand, to be applied in these complex flow fields. But due to the complex style in its formulation, it is not used as widely as general OCT methods. A proof is given to get an equation which explicit expresses the relationship between moire tomography and general CT methods. A Bayesian maximum a posteriori (MAP) approach is used to reconstruct fields with limited-projections. Based on these works, Numerical experiments are proposed to reconstruct simulated fields with ART and MAP approaches separately.
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