The FDK algorithm has played an important role in both biomedicine and industry because of its celerity and simplicity for the reconstruction of CT images. However, artifacts and attenuation may occur in FDK reconstruction under large cone angle with circular trajectory owing to the insufficient data. In this paper, a factor k is introduced to change the projected surface, based on the idea of rebinning algorithm. In that case, the path of filtering was arbitrarily changed and a unified pre-weighting formula was presented, which consequently enable the filtering path variable FDK (v-FDK) to be smoothly converted to P- FDK and T-FDK by tuning the k factor. Numerical simulations demonstrated our proposed algorithm an effective method for improving the reconstruction accuracy especially in the case of large cone-angles. On top of that, an optimal k value was found which can be used to obtain a reconstructed image with better quality than the image obtained by P-FDK and T-FDK.
Monochromatic x-ray backlighting is an essential and basic diagnostic in the research area, such as laser or z-pinch driven inertial confinement fusion, high energy density physics and laboratory astrophysics. A monochromatic hard X-ray backlighting system based on transmission logarithmic spiral crystals has been imposed, where the crystal is employed as a monochromator as well as an optical path deflector by taking advantage of the defining characteristic that all X-ray radiated from the pole of the spiral meet the crystal surface at the same angle. According to the model of Laue logarithmic spiral crystal imaging system and ray tracing method, the imaging principle and characteristics are analyzed theoretically, particularly the distance that the monochromatic beam split from the transmitted beam. We have designed and fabricated a logarithmic spiral quartz 2023 (2d=0.2749nm) crystal. Accordingly, the X-ray imaging system has been setup at 17.479 KeV (Mo Kα line), where the monochromatic image and the polychromatic image can be obtained at the same time. The test data and experimental results are presented and discussed. Compared with the most broadly applied monochromatic x-ray backlighting based on the spherically bent crystal, new developed imaging system can achieve higher photo energy and broader field of view.
The laser-induced plasma sources give instantaneous 4π divergent x-ray beams. The x-ray source size and pulse duration depend on the properties of the high-energy laser. Copper is the typical target material giving characteristic photon energies around 8.4keV. Different shapes of bent crystals are widely used as imaging and monochromatizing optics. Focusing and collimating are normally functioned by polycapillary x-ray lenses. Especially, the laterally-graded multilayers are applied as x-ray mirrors, which can reflect hard x-rays with big grazing angles, moderate energy resolution and high reflectivities. To get larger acceptance angel, a higher gradient of bilayer thicknesses from 2nm to 3.6nm within 80mm length scale is designed. And the alignment precision of 10μm is required to make it good performance. The reflected monochromatic x-rays can enhance the traverse coherence for the phase contrast imaging. And multi-frames of the same object can be obtained instantaneously by multi-reflections of the mirrors. The laterally-graded multilayer x-ray mirrors are also used for the pinhole imaging of a Z-pinch target, which benefits from the flat reflection surface and monochromatic imaging.
Transient x-ray diffraction, also called time-resolved x-ray diffraction and dynamic x-ray diffraction, is one novel diagnostic technique for probing shocked solids. It can provide direct information about microscopic mechanisms governing shock-induced deformation and structural changes at atomistic scales with nanosecond and picoseconds resolution, and lately, it has become possible to measure the structure of transients with sub-picoseconds and sub–Angstrom resolution with the development of ultrafast lasers which can produce femtosecond electron and x-ray pulses in the form of characteristics emission lines as well as x-ray continua in the keV range. In this paper, we detect and measure directly the dynamic response of lithium fluoride single crystal shocked compressed by laser irradiation in SHENGUANG II. In our experiments, high-intensity lasers irradiated a thin Cu foil to generate helium-like rays as x-ray source. Film (IP--image plate) recorded x rays diffracted from multiple lattice planes both perpendicular and oblique angles to the shock loading direction [100]. We gained the diffraction signals of the lattice planes (200) shocked and unshocked, what’s more, other lattices (113), (1-13). The positions of the diffraction lines associated with the (200) lattice plane indicated compression of the lattice along [100] direction by 13%. In the experiment, a large-angle detector consists of two films-one rectangular in shape,one triangular in shape that are positioned to record x rays diffracted from a shocked single crystal nearly within a full π steradian. The experiment shows that transient x-ray diffraction can diagnose the dynamical response of solid with higher resolution.
Application of focusing x-ray spectrograph with spatial resolution and uniform dispersion in
measurement of the imploding Al wire array z-pinch plasma is reported. Uniform dispersion (i.e., the
linear dispersion is a constant, or in other words, the x-rays are dispersed on the detector with
uniform spacing for every wavelength) is realized by bending the crystal of a spectrograph into a
special shape. Since the spatial coordinate of the spectrum obtained by this spectrograph varies
linearly with x-ray wavelength, it is very convenient for identification and processing of the
experimental spectrum. The experimental results show that this spectrograph has high luminosity,
high spectral and spatial resolution and is very suitable for the routine spectrum measurement on the
Z-pinch facility or other high-energy-density-physics (HEDP) facilities.
We present here a technique for passively sensing the three-dimensional structure of a scene using a single compact
camera. The iris of a conventional camera is replaced by a mask with a prism array, forming multiple images, in which
the vision disparities between the sub-images are extracted to compute depth. The arrangement of the prisms array can
be regular or aperture coded. For a regular arrangement, each prism forms an independent sub-image that is viewed
from a portion of the aperture, like many mini-cameras encapsulated in one aperture. If the angle of each prism is
designed properly, the sub-images can be separated from each other on the plane of image detector, so that conventional
methods of depth determination in stereo vision can be applied. On the other hand, the macro-prism array also can be
arranged in aperture coded fashion. The coded aperture imaging method then can be employed here for depth sensing.
Unlike the previous arrangement, the macro-prisms are positioned according to certain coding array, such as random
array or non-redundant array, so that the images viewed from each prism are superimposed. In order to reconstruct the
final depth image, a related decoding step for imaging processing is done. The passive range technique we introduced
above should be considered as a multiple imaging problem. Since only a single compact camera is used, we avoid the
need for extrinsic camera calibration, greatly reduced the computational demanding of correspondence problem. The use
of refractive element, prism array, instead of pinhole array, can greatly increase the light transmission and the resolution
of images.
A novel method to construct a quality map, called modulation–phase-gradient variance (MPGV), is proposed, based on modulation and the phase gradient. The MPGV map is successfully applied to two phase-unwrapping algorithms—the improved weighted least square and the quality-guided unwrapping algorithm. Both simulated and experimental data testify to the validity of our proposed quality map. Moreover, the unwrapped-phase results show that the new quality map can have higher reliability than the conventional phase-derivative variance quality map in helping to unwrap noisy, low-modulation, and/or discontinuous phase maps.
KEYWORDS: Optimization (mathematics), Simulation of CCA and DLA aggregates, Coded apertures, Coded aperture imaging, Algorithm development, Signal to noise ratio, Mathematical modeling, Binary data, MATLAB, Cameras
Coded aperture imaging (CAI) has evolved as a standard technique for imaging high energy photon sources and has
found numerous applications. Coded aperture arrays (CAAs) are the most important devices in the applications of CAI.
In recent years, many approaches were presented to design optimum or near-optimum CAAs. Uniformly redundant
arrays (URAs) are the most successful CAAs for their cyclic autocorrelation consisting of a sequence of delta functions
on a flat sidelobe which can easily be subtracted when the object has been reconstructed. Unfortunately, the existing
methods can only be used to design URAs with limited number of array sizes and fixed autocorrelative sidelobe-to-peak
ratio. In this paper, we presented a method to design more flexible URAs by means of a global optimization algorithm
named DIRECT. By our approaches, we obtain various types of URAs including the filled URAs which can be
constructed by existing methods and the sparse URAs which never be constructed and mentioned by existing papers as
far as we know.
Finding the distance of object in a scene from vision information is an important problem in machine vision. A large number of techniques for passive ranging of unknown objects have been developed over the years (i.e. range from stereo, motion, focus and defocus). Nearly all such techniques may be framed in terms of a differential formalism. In the case of binocular stereo, two different images are taken from cameras at different discrete viewpoints, similarly, difference between consecutive images are often used to determine viewpoint derivatives for structure from motion and two or more different images taken from cameras with different aperture size are used to compute the derivative respect to aperture size for range from focus and defocus method. All this methods may be fallen into a discrete differentiation category. Farid proposed a consecutive differentiation method for range estimation which employs the intensity variation of the images along with the aperture changes to measure the range information. In this paper, we first consider the plenoptic function which is a powerful mathematical tool for understanding the primary vision problem. We then show an algorithm within a differential framework for range estimation based on the assumption of brightness constancy. Finally we show several implementations of passive ranging using this differential algorithm.
Three-dimensional imaging techniques are very attractive for many applications. We develop the basic principle of coded aperture imaging used in invisible imaging realm to visible imaging realm, propose a three-dimensional imaging method. The object is captured by a cameras array. Then captured photographs of the object are integrated into an image named coded image. Finally coded image is computationally decoded to obtain a series of longitudinal layered surface images of the object. For good reconstructed images fidelity, we make use of correlation decoding method. With the use of correlation decoding, the distribution of cameras in array is crucial for the quality of reconstructed images. We investigate some typical two-dimensional arrays, choose non-redundant array for its proper imaging property. Experiments have been done to test and verify the performance of the proposed method. We choose a simple discontinuous object. The object is composed of two digit models, digit "1" and "2". Two digit models are displaced from each other. The distance between them is 10cm. Cameras array includes 9 cameras arranged as non-redundant array. The object is placed at the center axis of the cameras array, face to face with the array. After capturing, photographs integrating, computational decoding etc. procedures, we obtain high-quality reconstructed images of digit "1" and "2". The results of experiments show that the proposed method is feasible.
A method using the phase-space representations, i.e. the ambiguity function or Wigner distribution function to compute the optical transfer function (OTF) for an optical system with circularly symmetrical pupils under polychromatic illumination is presented. The phase-space representations is a very convenient tool for display the optical transfer function with varying aberrations such as the longitudinal chromatic aberration and defocus in a single picture, and the monochromatic OTFs can be easily determined from these joint representations. The polychromatic OTFs are computed by synthesizing a suitable number of monochromatic OTFs weighted by the spectral distribution of source and the color sensitivity of the receiver at fixed wavelength. Since the ambiguity function or the Wigner distribution function can be previously obtained by optical method or digital computation, the computational efficiency is greatly improved compared with traditional method, in which every monochromatic OTF need to be determined along. We computed the polychromatic OTFs for an optical system with a clear circular pupil and an annular ring pupil in detail and show some primary applications of the computations in spatial filter designing for color-blur reduction.
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