Showing papers on "Contrast transfer function published in 1970"

A rapid method for obtaining electron microscope contrast maps of various lattice defects

Abstract: A fast and accurate matrix method is used for calculating and mapping electron microscopy contrast around lattice defects. As each contrast point is calculated separately the method can be used for defects of any geometry.

Electron microscope contrast of small atom clusters

Abstract: The contrast for gold due to single atoms, flat clusters of 3 and 7 atoms and a double layer cluster of 19 atoms is calculated as a function of defocus including the effect of spherical aberration. The exact complex scattering factor is used and the effect of the imaginary part of the scattering factor on the image contrast is investigated for both single atoms and clusters. The calculated contrast for the double layer 19 atom cluster is found to agree with an experimental image of a gold cluster on graphite. The calculations also investigate the effect of a phase plate upon the contrast of the clusters.

Enhanced contrast in electron microscopy of unstained biological material. II. Defocused contrast of large objects.

Abstract: SUMMARY A contrast minimum is observed when 88 nm diameter polystyrene latex spheres are underfocused, which is related to the wide-angle scattering peaks. Images due to scattered and non-scattered wave components are displaced due to objective-lens spherical aberration and defocus. Maximum overlap of these components produces a contrast minimum at underfocus, related to the spherical aberration of the particular lens used. Similarly, a high-contrast band at carbon-film edges arises from spherical aberration and defocus separation of non-scattered and wide-angle scattered waves. This band increases in contrast with film thickness and in width with lens defocus. These geometrical effects account for the well-known ‘blinking’ of contrast of large biological objects upon swinging through focus without an objective aperture, and for the general contrast increase of defocused large objects. Fresnel fringes account for only a narrow band of enhanced contrast at distinct edges and cannot account for contrast enhancement of large objects lacking distinct edges.

Magnetic phase contrast from thin ferromagnetic films in the transmission electron microscope

Abstract: A new technique for realization of magnetic contrast from thin ferromagnetic films in the transmission electron microscope is described. A phase shifting plate modifies the low angle diffraction pattern formed in the objective focal plane and leads to image contrast from areas of magnetic gradient.

Limiting Factors in Threshold and Suprathreshold Vision

Abstract: THE threshold contrast of object detail decreases with increasing size; the full curve of Fig. 1 shows the threshold contrast of circular object details as a function of their angular diameter1. To explain the form of this function some2–4 have proposed that noise is less effective in “obscuring” the larger details; others5,6 have suggested that the contrast loss in the optical and neural processes is less for the larger details. This fundamental uncertainty has not yet been resolved7. According to the first view, noise is sampled over the equivalent area of the object detail. Because a larger number (N) of random events would be integrated in a larger detail area, the relative standard deviation (N−½) would be less, and would in fact be inversely proportional to the detail diameter (Fig. 1, dotted line). According to the second view, the “obscuring” effect of the noise is constant for different diameters of object detail, and the form of the contrast threshold function results solely from variation in the contrast loss. The loss arises from imperfections in the optical and the neural processes. The contrast loss may be quantitatively described in terms of a contrast transfer function. For the present purpose, this is defined as the ratio of the subjective brightness contrast to the objective luminance contrast of the object detail, expressed as a function of detail diameter. Both contrasts refer to the centre of the detail in relation to its background.