Showing papers on "Contrast transfer function published in 1973"

Image Formation in the Electron Microscope with Particular Reference to the Defects in Electron-Optical Images

Abstract: Publisher Summary This chapter describes image formation in the electron microscope with particular reference to the defects in electron-optical images. In the transmission electron microscope, information on the structure of a specimen, through its electron scattering properties, is transmitted to an image plane by a set of electron lenses. The intensity distribution, with respect to the angular deviation and the energy change of the incident electron beam, may be determined experimentally, but the information required on the amplitude and phase of the scattered electron wave may be inferred only by a detailed comparison of theory and experiment. The formation of an image from the scattered wave may be expressed mathematically in terms of a Fourier transformation. The chapter states that the angular and energy characteristics of the transmitted electron beam are important in the determination of the effects of phase shifts introduced by the spherical aberration and the chromatic aberration of the objective lens.

Mechanisms of contrast and image formation of biological specimens in the transmission electron microscope

Abstract: SUMMARY A systematic account is given, in relation to amorphous and paracrystalline (especially biological) material, of electron scattering and image formation for the range of electron energy from 50 to 100 keV. Contrast mechanisms in both elastic and inelastic scattering are discussed and image formation is considered under both scattering contrast and phase contrast conditions.

Enhanced contrast in electron microscopy of unstained biological material

Abstract: SUMMARY Electron-optical methods of enhancing contrast are being investigated in order to eliminate the artifacts associated with heavy-metal contrasting procedures. This report gives a quantitative discussion of the application of the strioscopic dark-field method to model biological objects. Beam-stopping apertures were constructed by microwelding fine platinum wires on to platinum objective apertures. These were precisely located and centred in the back focal plane of the objective lens. Contrast was evaluated by a sensitive micro-Faraday cage system. Bright-field and strioscopic contrast of organic layers up to 557 nm thickness were investigated and related to the transition from single to plural scattering. The spherical-aberration phase-contrast contribution could be separated from the amplitude contrast in bright-field microscopy by extrapolation to zero object thickness. Strioscopic contrast of thin layers was found to be much greater than bright field aperture contrast and shown to be non-linear with respect to thickness in the region 0·7–10 nm. For diffraction patterns made up of a few discrete beams the resolution is high, but is considerably less for diffusely scattering and thick objects. Strioscopy may be the method of choice for obtaining contrast of unstained or weakly-stained objects at high acceleration voltage.

Image resolution and image contrast in the electron microscope. I. Elastic scattering and coherent illumination

Abstract: The present work evaluates the effect of lens aberrations on image resolution and image contrast in the transmission electron microscope for the elastic component of the electron beam; coherent illumination of the specimen is assumed. The effects of lens aberrations on the image resolution are evaluated in terms of a resolution function in preference to the use of the transfer function. Although no precise figures can be given for image resolution, it is found that the condition for maximum contrast corresponds to the best resolution in the image. The fundamental limit in resolution, due to diffraction at the objective aperture, should be avoided by increasing the objective aperture size with a subsequent correction for the increased lens aberrations. For coherent illumination these corrections can, in general, only be made in bright field microscopy and not in dark field microscopy.

Third-Order Aberration Coefficients of Electron Lenses. II

Abstract: In the standard treatments of aberration coefficients of electron lenses, deviations from perfect imagery are expressed as power series of the ray coordinates in the object and aperture planes. The resulting aberration coefficients depend on the object and aperture positions, and a complete description of the aberrations of an electron lens would require a doubly infinite set of aberration coefficients for each voltage ratio of the lens. Hawkes has carried out a general treatment of the third-order aberrations of electron lenses which is independent of object and aperture positions. Six quantities are sufficient to specify the third-order aberration properties of an electron lens. We have derived equations for these six quantities in the form of integrals, involving derivatives of the axial potential no higher than the second, and using our previously calculated potentials have computed aberration coefficients for the two-tube electrostatic lens.

Some aspects of image deconvolution in electron microscopy

Abstract: A method for two-dimensional deconvolution is applied in the correction of a computer-generated transmission electron microscope image of a planar object subjected to lens aberrations (spherical aberration and defocusing), in an attempt to assess the viability of resolving molecular structure in the transmission electron microscope. The object considered is a phthalocyanine molecule examined in bright-field microscopy at incident electron energies of 100 and 200 keV, and for different objective lens defocus values. The results are presented in the form of intensity profiles along the y axis for the aberrated image in the absence of experimental error, and an image affected by (i) random error and (ii) sinusoidal error of varying frequency. The effect on the deconvoluted result of using an incorrect defocus value in the deconvolution procedure is examined. The results derived for the object structure differ in detail from the known molecular shape; these differences arise from the necessary assumption of a linear relationship between the image intensity and the object structure. Further, the effects of the lens aberrations extend out to 5 nm from the centre of the molecule, and the present results were restricted to a radius of 1?5 nm from this centre.