Showing papers on "Contrast transfer function published in 1988"
01 Sep 1988-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, the aberrations of a magnetic quadrupole quadruplet lens used as a probe forming lens in a microprobe system have been studied with the grid shadow method.
Abstract: The aberrations of a magnetic quadrupole quadruplet lens used as a probe forming lens in a microprobe system have been studied with the grid shadow method. From a comparison of theoretical and experimental grid shadow patterns the system is shown to suffer from parasitic aberration (possibly from mechanical misalignments) as well as spherical aberration. Three magnetic octupole lenses were designed and built with the aim of correcting the spherical aberration. It was found that the parasitic aberration prevented complete correction of all spherical aberration by the three octupoles but the largest part of the spherical aberration, the cross terms, could be corrected with a single octupole.
27 citations
TL;DR: The optical properties of a general scanning microscope are determined within the framework of Fourier imaging theory and predicts that spatial resolution and depth discrimination vary continuously with detector aperture and that defocus phase contrast is present in transmission images obtained with a symmetric objective, collector lens confocal microscope.
Abstract: SUMMARY
The optical properties of a general scanning microscope are determined within the framework of Fourier imaging theory. For a simple model optical system, with Gaussian lens and detector apertures, the contrast transfer function can be expressed in terms of elementary functions. The theory predicts that spatial resolution and depth discrimination vary continuously with detector aperture and that defocus phase contrast is present in transmission images obtained with a symmetric objective, collector lens confocal microscope.
12 citations
TL;DR: Comparison of laser and monitor contrast sensitivity functions indicates that for these cataract patients interferometric measurements show a normal retinal function, whereas similar measures using the traditional display system (monitor) show considerable loss of visual function.
Abstract: Conventional methods of measuring retinal function are dependent on the optics of the eye. Therefore, an optical opacity such as a cataract that obstructs the normal optical transmission of light can prevent measurement of retinal function. In some cases a laser interferometer designed to bypass the effects of the optics of the eye may be employed to measure retinal function without interference due to the optical components. In this study we compare contrast sensitivity functions determined by laser interferometry and conventional display methods for cataract and normal eyes. Comparison of laser and monitor contrast sensitivity functions indicates that for these cataract patients interferometric measurements show a normal retinal function, whereas similar measures using the traditional display system (monitor) show considerable loss of visual function. The extent to which contrast sensitivity was limited by the lens of the eye is indicated by the ratio of laser and monitor contrast thresholds. In general, the laser interferometry technique demonstrates that relatively minor cataracts decrease the contrast transfer function of the eye' optics over a range of spatial frequencies.
12 citations
Journal Article•
5 citations
TL;DR: In this article, the authors considered the optical spherical aberration caused by a layer of solution, melt or the crystal itself, looking at the surface of the crystal or its surrounding through this condensed phase and derived a formula which gives the maximum aperture which can be used under given conditions (thickness of the layer and its refractive index) without serious distortion of the image by the spherical aberrations.
4 citations
TL;DR: In this article, the authors compared the results of stationary-phase integration with fast Fourier transform calculations as a function of defocus for the imaging of disordered planes of atoms, and showed that for simple scattering functions, it is possible to perform the integral analytically using the method of stationary phase.
Abstract: To calculate high-resolution images it is necessary to convolute the wavefunction generated by scattering from the specimen with the microscope objective-lens wavefront aberration function. This is usually done by a multiplication of the transfer function and the specimen exit-surface wavefunction in reciprocal space followed by a numerical integration over all scattering wave vectors. Examination of the analytic behaviour of the wave-front aberration function in the complex plane shows that, for simple scattering functions, it is possible to perform the integral analytically using the method of stationary phase. Analytic results for the imaging of disordered planes of atoms are compared with fast Fourier transform calculations as a function of defocus. The limitations of stationary-phase integration are also discussed.
3 citations
15 Jan 1988
TL;DR: In this article, a modified Stroke filtering technique is applied to high resolution micrographs of a conventional transmission electron microscope (CTEM) in order to reduce defects of defocus, spherical aberration and axial astigmatism.
Abstract: A modified Stroke filtering technique is applied to high resolution micrographs of a conventional transmission electron microscope (CTEM). In a coherent optical processor synthetic holograms are used. They are produced by an analog electron-optical device. A micro-processor-controlled electron beam writes the amplitude component on a fluorescent screen. This image is photographed through an alternately shifted grating. The defects of defocus, spherical aberration and axial astigmatism can be reduced.
2 citations
TL;DR: In this paper, a Wolter-type x-ray optic was implemented in a high magnification microscope on Nova and the point spread function, contrast transfer function at selected spatial frequencies, xray throughput, alignment accuracy, and field of view were measured.
Abstract: We have implemented a Wolter‐type x‐ray optic in a high‐ (22×) magnification microscope on Nova. We report on the on‐line characterization of this system and show results from the several types of experiments performed. The point spread function, contrast transfer function at selected spatial frequencies, x‐ray throughput, alignment accuracy, and field of view have all been measured as configured for Nova experiments. Such characterization may be used to remove the degradation introduced by the instrument. This work was performed under the auspices of the U.S. DOE by the Lawrence Livermore National Laboratory under Contract No. W‐7405‐ENG‐48.
1 citations
TL;DR: In this paper, a practical method for the Wiener filter is explained by using a HREM image of multiply twinned Ag particle, and the other techniques are also demonstrated with various images.
Abstract: Two kinds of image processing techniques can be applied to the high resolution electron microscope (HREM) images : one in the real space (averaging of periodic images) and the other in the Fourier space. There are several methods in the latter, such as ring masking, window masking and a CTF (contrast transfer function) compensation using the Wiener filter.The Wiener filter is extremely useful for the HREM image processing, because the filter can improve the CTF, and thus provide improve images. The image contrast can partially be reversed and that in the high frequency region can be enhanced by the Wiener filter. A practical method for the Wiener filter is explained by using a HREM image of multiply twinned Ag particle. An application is shown with a high temperature superconducting material, and the other techniques are also demonstrated with various images.
01 Jan 1988
TL;DR: In this paper, the authors discuss the image formation theory for such objects and show that the recorded image is, to a good approximation, the convolution of the projected atomic potential of the specimen with a space invariant point-spread function.
Abstract: Whilst the only practicable technique for determining the structure of biological molecules to atomic resolution is X-ray crystallography, many specimens of considerable interest cannot be formed into three-dimensional crystals. Much useful data can nevertheless be obtained from smaller amounts of material, for example, single viruses, or two-dimensional crystals of proteins, by electron microscopy, because electrons interact much more strongly with atoms than do X-rays. However, because of the stronger interaction, and the smaller amount of material used, the specimen is destroyed by the electron dose necessary to form an image. Traditionally, the specimen has been preserved by negative staining — embedding in a salt of a heavy atom, such as uranyl acetate — and thus the image only records the shape of the specimen, and the resolution achieved is limited by (inter alia) the grain size of the stain. The staining also protects the specimen from dehydration in the vacuum of the microscope. In order to increase the resolution, and also to image the internal structure of the specimen, the specimen has been embedded in other media, such as glucose (Unwin amp Henderson, 1975), and more recently in amorphous ice (Lepault et al., 1983). This necessitates using a lower electron dose, decreasing the signal to noise ratio, and the object also scatters primarily as a ‘phase object’. In section 2 we discuss the image formation theory for such objects, and show that the recorded image is, to a good approximation, the convolution of the projected atomic potential of the specimen with a space invariant point-spread function. To obtain a full 3-dimensional reconstruction of an object therefore requires a complete set of projections to be obtained. This is usually done by means of a tilting specimen holder, but if the object has internal symmetry, there will a proportionate reduction in the number of views required, and, for example, a helical virus may be reconstructed from a single view.