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Contrast transfer function
About: Contrast transfer function is a research topic. Over the lifetime, 934 publications have been published within this topic receiving 26533 citations.
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TL;DR: This publisher's note contains corrections to Opt.
Abstract: This publisher’s note contains corrections to Opt. Lett.44, 5141 (2019)OPLEDP0146-959210.1364/OL.44.005141.
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.
TL;DR: The spatial contrast response describes the relationship between the contrast in a pattern of vertical stripes, and the amplitude of the visually evoked response at various spatial frequencies for a given temporal frequency (pattern reversal frequency); it indicates how particular spatial frequencies are attenuated in the visual system.
Abstract: Visually evoked potentials were used to determine the spatial contrast response function of the visual system and the visual acuity of the pigeon. The spatial contrast response describes the relationship between the contrast in a pattern of vertical stripes, whose luminance is a function of position, and the amplitude of the visually evoked response at various spatial frequencies for a given temporal frequency (pattern reversal frequency); it indicates how particular spatial frequencies are attenuated in the visual system. The visually evoked responses were recorded using monopolar stainless steel electrodes inserted into the stratum griseum superficiale of the optic tectum; the depth of penetration was determined on the basis of a stereotactic atlas. The stimulus patterns were generated on a video monitor placed 75 cm in front of the animal's eye perpendicular to the optic axis. The spatial contrast response function measured at 10% contrast and 0.5 Hz reversal frequency shows a peak at a spatial frequency of 0.5 c/deg, corresponding to 1 degree of visual angle, and decreases progressively at higher spatial frequencies. The high-frequency limit (cut-off frequency) for resolution of sinusoidal gratings, estimated from the contrast response function, is 15.5 c/deg, corresponding to a visual acuity of 1.9 min of arc.
Patent•
10 Feb 2011
TL;DR: In this article, an aberration corrector corrects the electron beam for at least a spherical aberration, and a specimen holder is provided to hold a specimen in the path of the beam.
Abstract: A transmission electron microscope includes an electron beam source to generate an electron beam. Beam optics are provided to converge the electron beam. An aberration corrector corrects the electron beam for at least a spherical aberration. A specimen holder is provided to hold a specimen in the path of the electron beam. A detector is used to detect the electron beam transmitted through the specimen. The transmission electron microscope operates in a dark-field mode in which a zero beam of the electron beam is not detected. The microscope may also be capable of operating in an incoherent illumination mode.