Showing papers on "Contrast transfer function published in 1990"

Development of the modulation transfer function and contrast transfer function for discrete systems, particularly charge-coupled devices [also Comment 35(7), 2105-2106 (July 1996)]

Abstract: The modulation transfer function (MTF) and contrast transfer function (CTF) are considered for discrete systems. The MTF and CTF are derived for the charge-coupled device (CCD). The analysis presented con- siders the full effects of sampling on device response. Experimental tests of the CTF for a line-array CCD are presented. The MTF and CTF are shown to be multivalued functions for discrete systems.

Gain, noise, and contrast sensitivity of linear visual neurons.

Abstract: Contrast sensitivity is a measure of the ability of an observer to detect contrast signals of particular spatial and temporal frequencies. A formal definition of contrast sensitivity that can be applied to individual linear visual neurons is derived. A neuron is modeled by a contrast transfer function and its modulus, contrast gain, and by a noise power spectrum. The distributions of neural responses to signal and blank presentations are derived, and from these, a definition of contrast sensitivity is obtained. This formal definition may be used to relate the sensitivities of various populations of neurons, and to relate the sensitivities of neurons to that of the behaving animal.

Optimization of electron lens shape giving minimum spherical aberration coefficient

Abstract: A novel computer-aided optimizing procedure has been developed to obtain the geometry of the magnetic electron lens that give the minimum spherical aberration coefficient Cs. The minimization of Cs is done by a simplex method in which Cs is regarded as a function of up to 11 parameters characterizing the lens geometry. In this process, Cs and other optical properties of lenses are evaluated using a high-speed FEM (finite-element method) with automatic mesh generation. The optimization procedure was applied to the design of an objective lens for a 400 kV transmission electron microscope. >

Effects of third-order spherical aberration on the 3-D incoherent optical transfer function.

Abstract: We derive the expression for the 3-D incoherent optical transfer function when third-order spherical aberration is present. The normalized verision of the transfer function is numerically calculated for various amounts of spherical aberration. We find the effects of the aberration to be highly dependent on the spatial frequency in the longitudinal direction. We also calculate a structure content parameter, as a quality criterion, from the normalized transfer function. The structure content parameter dependence on spherical aberration is well-fit by a simply Cauchy curve for aberrations out to two waves at the margin.

Electron Microscopy of Biological Macromolecules

Abstract: Two recent advances have substantially improved the information that can be obtained from electron microscopy about the structure of biological macromolecules and the interactions between them. Computer image processing has enabled many of the intrinsic problems of electron microscopy of these materials to be circumvented. For example, many fine details are usually faint in electron micrographs of macromolecules, and they are further obscured by noise deriving from both the image formation mechanism of the microscope and also features of the methods used to contrast and support the materials. Filtering removes much of the background and so enables fine details to be seen more easily. In addition, structural information from different levels in the object is superimposed, and this can sometimes produce confusing moire patterns. These moire patterns can often be decomposed into their constituents by image processing. Low-dose conditions and electron diffraction usually need to be combined with image processing to obtain the highest resolution and, moreover, it is generally possible to combine different views of objects to generate three-dimensionalstructural models.

Streak Tube Modulation Transfer Functions

Abstract: An exponential model is proposed for streak tube MTFs (modulation transfer functions) based on line spread function measurements. This model is used to derive performance characteristics such as temporal resolution, channel width and pixel size. These results are compared with those predicted by a Gaussian model for the MTF.

Correction of axial chromatic aberration improves the contrast transfer in bright-field phase contrast imaging

Abstract: The modulation transfer function of an electron microscope having a corrected axial chromatic aberration of first order and first degree as well as a corrected spherical aberration of third order is determined by the sperical aberration of fifth order, the almost totally controllable aperture aberrations of first to fourth order, and by the axial chromatic aberration of first order and of second degree. We calculated the contrast transfer function for axial parallel illumination for a corrected objective lens system considering a time-dependent change of the electron energy as well as considering a Gaussian energy distribution. These influences render a phase contrast transfer function, which is limited by an envelope function that is mainly determined by the chromatic aberrations. Contrast transfer functions and their envelope functions have been calculated for the case of the chromatically and spherically corrected electron microscope, as it has been investigated earlier by Scherzer, Koops and Rose. Discussion of the modulation transfer functions for the different correction states obtained show, that the correction of the axial chromatic aberration of first order and first degree to a value of

Correction of spherical aberration in an interferometric system with nonzero difference between optical paths and fringeless observation field.

Abstract: The influence of fifth-order spherical aberration in the measurement error in an interferometer system is analyzed. The criterion of the aberration correction and the optimum correction are proposed. The influence of the focusing error and the wave tilt as the adjustment errors are determined. As a practical example, the optimum wave spherical aberration, optimum longitudinal spherical aberration and the error distribution in the observation field are given.

Aberration Correction by Electron Holography Using Numerical Reconstruction Method

Abstract: Electron holography has been performed to correct the aberrations of an objective lens in electron microscope. The aberrations, particularly defocusing aberration, are corrected at the stage of numerical reconstruction according to the wave aberration function. The influence of defocusing aberration on both the amplitude and the phase distributions is eliminated in the reconstruction images. Since spherical aberration is represented as the similar function of defocusing aberration, the results confirm the possibility of correcting the spherical aberration of the objective lens by the high resolution electron holography

Analytical design of holographic optical elements for Fourier transform

Abstract: A new method for designing holographic optical elements for Fourier transform is described. A phase function is assumed as a power series of coordinates of the optical element, and an analytical expression of wave-front aberration is derived. The coefficients in the phase function are explicitly determined such that the wave-front aberration may be kept small during variation of inputs. The location of the circle of least confusion and the balancing of the aberration over an entire spatial-frequency range are taken into consideration. By the simulation of spot diagrams it is confirmed that our designed phase function is optimum for a sufficiently wide spatial-frequency range.