Showing papers on "Contrast transfer function published in 2000"
TL;DR: The evaluation of the required accuracy for the compensation of the various residual aberrations in order to achieve a resolution in the sub-Angstrom regime with a probe-forming system.
173 citations
TL;DR: It is shown that in the limit of a large objective (probe-forming) aperture, relevant to a spherical aberration corrected microscope, the Z-contrast image of a zone-axis crystal becomes an image of the 1s Bloch states.
Abstract: We show that in the limit of a large objective (probe-forming) aperture, relevant to a spherical aberration corrected microscope, the Z-contrast image of a zone-axis crystal becomes an image of the 1s Bloch states. The limiting resolution is therefore the width of the Bloch states, which may be greater than that of the free probe. Nevertheless, enormous gains in image quality are expected from the improved contrast and signal-to-noise ratio. We present an analytical channeling model for the thickness dependence of the Z-contrast image in a zone-axis crystal, and show that, at large thicknesses, columnar intensities become proportional to the mean square atomic number, Z(2).
116 citations
TL;DR: A simple modification to the well-known back- projection reconstruction algorithm is described, called the defocus-gradient corrected back-projection, which computationally corrects for the contrast transfer function along a defocus gradient.
55 citations
TL;DR: In this article, an algorithm for determination and correction of the coherent wave aberration has been developed which is based on a combination of genetic algorithm and downhill-simplex algorithm, the basic criterion is the minimization of the amplitude contrast which is a well established correction guideline for weak phase objects.
37 citations
TL;DR: Comparison of different CTF models used to reconstruct 3D volumes from zero-loss images confirmed that best agreement with the atomic model is attained with the new, modified CTF theory.
36 citations
TL;DR: The experimental procedure using amorphous carbon is described and errors in measuring beam-tilt angles, magnifications and image displacements can be kept sufficiently small to achieve the required accuracy.
21 citations
TL;DR: A method is proposed to improve the image deconvolution technique based on the weak-phase object approximation for determining the crystal defects at atomic level by constraining the integral amplitudes to be equal to the corresponding structure factor amplitudes of perfect crystals.
Abstract: A method is proposed to improve the image deconvolution technique based on the weak-phase object approximation for determining the crystal defects at atomic level. Originally, both the amplitude and phase modulation caused by the contrast transfer function (CTF) was removed for all reflections in the diffractogram of the image and then the deconvoluted image was obtained by carrying out the inverse FT. In this paper after removing the CTF modulation the amplitudes of reflections are further corrected by constraining the integral amplitudes to be equal to the corresponding structure factor amplitudes of perfect crystals. [110] high-resolution electron microscope images were simulated for a structure model of Si crystal containing the 60 degrees dislocation with different thickness. A comparison between deconvoluted images with the amplitudes corrected by the present method and those uncorrected is given to show the effectiveness of the method. The amplitude-crystal thickness dependence for images simulated close to the Scherzer defocus condition has been analysed and the result serves as an empirical basis of the method. The validity and limitation of the method are discussed.
20 citations
TL;DR: It is shown both theoretically and experimentally that two-photon fluorescence imaging experiences less spherical aberration and can thus propagate to a deeper depth within a thick medium.
Abstract: We compare the effects of spherical aberration on the penetration depth of single-photon and two-photon excitation for instances in which the aberration is caused by the refractive-index mismatch when a beam is focused through an interface. It is shown both theoretically and experimentally that two-photon fluorescence imaging experiences less spherical aberration and can thus propagate to a deeper depth within a thick medium.
16 citations
TL;DR: There is a relationship between the phase shift and defocus that allows us to find a series of 'alignment-free' focal conditions and a triangular relation among C(s), defocus and lattice spacing is established for proper image contrast and 'aligned' imaging with the C(S) corrected microscope.
Abstract: For spherical aberration corrected transmission electron microscopes recently developed, the Scherzer resolution in proportion to C(s) 1/4 lambda 3/4 is still inevitably limited by the influence of some other electron optical factors, such as chromatic aberration coefficient of objective lens (C(c)), beam divergence and alignment, incident electron energy spread (deltaE), and the instability of accelerating voltage (deltaV) and of lens current (deltaI). Depending on the image resolution guaranteed by C(s) correction, the defocus spread caused by C(c), deltaV, deltaI and deltaE would need to be reduced. The effect of beam alignment as a phase shift in contrast transfer function is also studied for the C(s) corrected microscopes with different values of C(s), defocus and spatial frequency. There is a relationship between the phase shift and defocus that allows us to find a series of 'alignment-free' focal conditions. A triangular relation among C(s), defocus and lattice spacing is established for proper image contrast and 'alignment-free' imaging with the C(s) corrected microscope.
5 citations
TL;DR: In this paper, the authors show that instead of tuning out spherical aberration completely, there is an optimum value for the residual Cs that maximizes information transfer to the best possible resolution and depends only on the information limit of the microscope.
Abstract: With the advent of electron-optical systems able to generate negative spherical aberration (usually called ''Cs correctors''), it has now become feasible to zero-out objective lens Cs in the high-resolution transmission electron microscope. However, I show that - instead of tuning out spherical aberration completely - there is an optimum value for the residual Cs that maximizes information transfer to the best possible resolution and depends only on the information limit of the microscope.
4 citations
TL;DR: In this article, the transverse spherical aberration of a thin lens is calculated and a normalized aberration Y equal to TA divided by the theoretical resolution limit is defined, where Y 1.6 is the angular norm of the Gaussian beam.
Abstract: In this paper, we calculate the transverse spherical aberration TA of a thin lens and defines a normalized aberration Y equal to TA divided by the theoretical resolution limit. As a rule of thumb, (a) a thin lens that suffers only from spherical aberration may be considered effectively diffraction-limited as long as Y 1.6, and, specifically, (c) the lens need be diffraction-limited only over a radius approximately equal to the radius (to the 1/e-point) of the Gaussian beam.
TL;DR: In this paper, an optical pickup with a spherical aberration compensator using nematic liquid crystal (LC) cells was proposed, which is composed of two LC cells each of which has X and Y stripe electrodes, respectively.
Abstract: To increase recording density in an optical disk system, a two-lens system which enlarge its numerical aperture (NA) has been developed. When NA becomes larger, however, the spherical aberration due to disk thickness error increases. We propose an optical pickup with a spherical aberration compensator using nematic liquid crystal (LC). Our LC device is composed of two LC cells each of which has X and Y stripe electrodes, respectively. We discussed how the spherical aberration due to the disk thickness error can be compensated by combination of two crossed cylindrical phase distributions induced by the LC cells, and confirmed the compensation effect in a design example by simulation. We have also fabricated the crossed stripe LC device actually, and have obtained the experimental result which shows that amount sufficient to compensate the aberration is achieved.
01 Aug 2000
TL;DR: A new method is presented that allows a fully automated multaneous determination of the structure factor and the parameters of the Contrast Transfer Function CTF and noise function based on the new precise mathematical formulations of the prob lem as constrained nonlinear least squares.
16 Jun 2000
TL;DR: In this article, a general description of the partially coherent illumination case is given for extended objects like the line and periodical structures for different values of the coherence parameter, and the apparent transfer function is introduced as a quality criterium.
Abstract: A discussion on the interest of the study of diffractional image formation theory in a course of Fourier Optics is carried out. We develop a general description of the partially coherent illumination case. Images of extended objects like the line and periodical structures are given for different values of the coherence parameter. The apparent transfer function is introduced as a quality criterium. For the incoherent illumination this parameter is compared with the optical transfer function.