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11.
Ya.I. Nesterets 《Optics Communications》2008,281(4):533-542
A theoretical formalism describing the formation of images in a linear shift invariant X-ray optical system is derived within the wave-optical theory. It is applicable to a non-crystalline object consisting of two types of features, with the characteristic sizes which are respectively not smaller and much smaller than the resolution of the imaging system. This formalism is then applied to two phase-contrast imaging techniques, the propagation-based and analyser-based imaging. The obtained formulae for the intensity distribution in the image well explain the “decoherence effect” which is observed in the former technique and the “extinction contrast” which is a characteristic of the latter technique. This formalism is shown to be in good agreement with the results of the accurate numerical simulations, using rigorous wave-optical theory, of the propagation-based and analyser-based phase-contrast images of the model objects. 相似文献
12.
* fluorescence radiation from the ion–ion recombination process in the gas volume during the preionization phase, therefore
allowing spatial resolution by partial imaging of the volume. Volume-integrated and temporally resolved measurements are carried
out to determine the average of absolute preionization densities by comparison with a theoretical model of the temporal behaviour
of the recombination process. Spatially resolved measurements reveal the distribution of the preionization density. The preionization
densities determined from spark and sliding/corona discharges schemes are considerably higher (ne
0?1012 cm-3) than those obtained from pulsed X-ray preionization (ne
0?107 cm-3).
Received: 3 October 1997/Revised version: 6 January 1998 相似文献
13.
The 3D bioplotter, which is one of the rapid-prototyping systems, enables us to produce the design-based scaffolds which could
control good mechanical properties and pore structures for mimicking human organs. Although the plotting system has several
advantages to fabricate a variety of designed scaffolds, the main disadvantage of scaffolds fabricated by the system is that
the strand surfaces are too smooth and tend to discourage initial cell attachment within the scaffolds. To overcome the problem,
we suggest a new 3D plotting method supplemented by piezoelectric vibration system for fabricating scaffolds that have hierarchical
surface structures, which increase the surface roughness of the scaffold without any additional chemical process. The surface-modified
3D scaffold exhibited various positive qualities including enhanced compressive modulus and improved initial cell attachment
and proliferation. Cell culturing results demonstrated that the interactions between chondrocytes and the scaffold were much
more favorable than those between the cells and conventionally plotted 3D scaffolds. This process provides a feasible new
technique for fabricating high-quality 3D scaffolds for tissue engineering applications. 相似文献
14.
W. Stille 《The European physical journal. E, Soft matter》2009,28(1):57-71
The deformation of cholesteric elastomers by mechanical stress applied parallel to the helix axis is studied by calculation
of the free-energy density. The Frank-elasticity contribution is taken into account. A chiral solvent, present at cross-linking
time, is in general considered to be replaced after cross-linking by a solvent with different chirality. Two special cases
considered are zero and unchanged solvent chirality, the first known as that of imprinted cholesteric elastomers, the latter
equivalent to intrinsic cholesteric elastomers with chemically attached chiral groups. Depending on material parameters and
imposed strain, the director can show a tilt towards the helix axis up to the maximum tilt, corresponding to a nematic state.
In case of intrinsic elastomers with low conformation anisotropy, direct transitions from untilted to nematic states can be
induced by straining. The helix structure of the director field is coarsened with an average wave number different to that
of the information inscribed in the network at cross-linking time, if this lowers the average free-energy density. Switching
between different states can be achieved with electric fields of reasonable values applied parallel to the helix axis. Spectra
of the reflection of polarized light are calculated. 相似文献
15.
Improving the performance of DCT-based fragile watermarking using intelligent optimization algorithms 总被引:1,自引:0,他引:1
The performance of a fragile watermarking method based on discrete cosine transform (DCT) has been improved in this paper by using intelligent optimization algorithms (IOA), namely genetic algorithm, differential evolution algorithm, clonal selection algorithm and particle swarm optimization algorithm. In DCT based fragile watermarking techniques, watermark embedding can usually be achieved by modifying the least significant bits of the transformation coefficients. After the embedding process is completed, transforming the modified coefficients from the frequency domain to the spatial domain produces some rounding errors due to the conversion of real numbers to integers. The rounding errors caused by this transformation process were corrected by the use of intelligent optimization algorithms mentioned above. This paper gives experimental results which show the feasibility of using these optimization algorithms for the fragile watermarking and demonstrate the accuracy of these methods. The performance comparison of the algorithms was also realized. 相似文献
16.
In this work, a numerical study on the pattern correlation using wavelet filters is reported. A comparative study of the correlation using the Mexican hat and Coiflets filters is presented. A Coiflet filter acts not only as a band-pass filter but as a high-pass or low-pass filter. Therefore, unlike the Mexican hat-based filter which acts only as a pass-band filter, the Coiflet-based filters allow selecting horizontal, vertical or diagonals details of the original image. Each one of the original images can be discomposed in an average image and several detail images at different levels of multiresolution. We study the numerical correlation between binary patterns using the Mexican hat filter and the first and second multiresolution level obtained by Coiflet filtering. Additionally, an analysis about the noise immunity for the Mexican hat and Coiflet filters is realized. The results show that Coiflet filters are better to identify special characteristics but perform the worst when they are used with noisy images. On the other side, the Mexican filter presents a better noise immunity but performs the worst when is used to compare special characteristics. 相似文献
17.
In this work an optical-digital correlator for pattern recognition and input scene restoration is described. Main features of the described correlator are portability and ability of multi-element input scenes processing. The correlator consists of a consumer grade digital photo camera with a diffractive optical element (DOE) inserted as a correlation filter. Correlation of an input scene with a reference image recorded on the DOE are provided optically and registered by the digital photo camera for further processing. Using obtained correlation signals and DOE’s point spread function (PSF), one can restore the image of the input scene from the image of correlation signals by digital deconvolution algorithms.The construction of the correlator based on the consumer grade digital photo camera is presented. The software procedure that is necessary for images linearization of correlation signals is described. Experimental results on optical correlation are compared with numerical simulation. The results of images restoration from conventionally and specially processed correlation signals are reported. Quantitative estimations of accuracy of correlation signals as well as restored images of the input scene are presented. 相似文献
18.
A new cryptology in dual fractional Fourier-wavelet domain is proposed in this paper, which is calculated by discrete fractional Fourier transform and wavelet decomposition. Different random phases are used in different wavelet subbands in encryption. A new color image encoding method is also presented with basic color decomposition and encryption respectively. All the keys, including random phases and fractional orders in R, G and B three channels, should be correctly used in decryption, otherwise people cannot obtain the totally correct information. Some numerical simulations are presented to demonstrate the possibility of the method. It would have widely potential applications in digital color image processing and protection. 相似文献
19.
We formulate the fourth order correction to a paraxial Gaussian beam propagated along the axis of symmetry of a parabolic index lens. First we examine the evolution of a complex-source-point spherical wave (equivalent paraxially to a Gaussian beam) through the lens in a two-dimensional xz plane. Taking into account the terms of up to fourth order in aperture variables, we find a ray-optical solution to the exit beam that is represented in terms of aberration function. We also analyze the effect of the lens aberration exerted on the degradation in the quality of a Gaussian beam. The fourth order-corrected wave function derived here may be used to evaluate the quality of a Gaussian beam focused with a parabolic index lens. Further it may be applied to the case of an orthogonal system in which the index variations are different in the xz and yz planes. 相似文献
20.
We report an experimental observation of Poisson's spot with pseudo-thermal light. The experimental results show that the diffraction pattern disappears in the intensity distribution behind the opaque disc but emerges through both auto-correlation and cross-correlation intensity measurements. The auto-correlation scheme can take care of both better visibility and higher resolution of the diffraction pattern under the condition that the thermal light source has a larger spectral bandwidth. 相似文献