Showing papers on "Contrast transfer function published in 1972"

Molecular image resolution in electron microscopy

Abstract: In order to determine the ultimate molecular resolution attainable with a conventional electron microscope, the direct observation of hexadecachloro‐Cu‐phthalocyanine molecules was attempted. Since phthalocyanine derivatives are known to form crystalline films with columns of parallel stacks of planar molecules, the specimens were prepared by epitaxial growth on KCl cleavage face through vacuum evaporation so that the column axis was directed almost normal to the thin‐film surface holding an orientation suitable for the observation. The molecular orientation was determined by Patterson synthesis based on the laser optical transform of the electron diffraction pattern obtained from the individual crystallites placed on the microgrid mesh. The direct observation was carried out with the 100‐kV electron beam incident on the specimen along the column axis. The crosslike images arrayed in a centered rectangular net were clearly resolved, well representing the molecular shape of phthalocyanine with the configur...

Electron Microscopy of Biological Specimens by Means of an Electrostatic Phase Plate

Abstract: A new electron microscope imaging method has been developed that is especially suited to the study of thin biological materials. It involves the use of an electrostatic phase plate - a device which creates a more or less uniform difference in optical path between the un­scattered and scattered waves by means of its electric field. This phase plate functions in an analogous manner to the absorbing bright contrast phase plate of light microscopy. The contrast effects and aberrations peculiar to the method have been examined and are discussed in terms of their likely influence on the image’s representation of the object structure. Analysis of electron micrographs of some biological test specimens, whose structure is relatively well known, confirms that this representation, to a resolution of ca . 0.85 nm, is a particularly faithful one. In the analysis the resolution limit was determined by the degree of specimen preservation, and a real limit, determined by the degree of spherical aberration in the objective lens, of ca . 0.5 nm is expected. A special property of the imaging method, as distinct from the conventional bright field method, is that it emphasizes the detail within the biological material itself, but reduces the contrast from the surrounding film of stain; negative staining remains necessary only because it helps to preserve the morphology of the specimen during irradiation. Evidence is presented that this property enables the method to display information about the specimen that it would not be possible to detect with the bright field method.

Possibility of a phase contrast electron microscope.

Abstract: In-focus phase contrast has been demonstrated in the electron microscope using an arrangement analogous to that of the Zernike phase contrast light microscope. Thin carbon films with a central hole were placed in the back focal plane of the objective lens so that the scattered electrons were selectively phase-shifted by the film. The maximum phase contrast was obtained when the film thickness was adjusted to give a retardation of about pi/2 to the scattered electrons and appears to be due to the elastically scattered electrons. The observed contrast was about one-half that calculated taking into account the scattering of both the object and the phase plate and making the assumption that the inelastic scatter was incoherent. Improved phase contrast should be obtained if the nonscattered intensity is reduced by a beam stop and if phase-shifting can be accomplished by a small electrostatic lens rather than by a film. An objective lens ofthe smallest available spherical aberration is required. The in-focus phase contrast arrangement may provide useful contrast for thick (>2000 A) unstained objects in the l.0-MeV microscope. A combination contrast mode is recommended for conventional (100-kV, microscopes where amplitude contrast and enhancement of phase contrast are provided by filtering out the inelastic scatter.

Deconvolution in two dimensions with particular reference to the electron microscope

Abstract: The solution of the two-dimensional convolution integral is presented for the case when the experimental results are subject to error. In particular, a resolution kernel whose Fourier transform has zeros is considered. This corresponds to resolution functions obtained in high-resolution electron microscopy, where the image intensity is modified by lens aberrations (spherical aberration and defocusing). The method for deconvolution is applied to computer-generated results subject to (i) random errors in the range ±20% and (ii) photographic noise in the range ±20%.

Spherical Aberration in Long Wavelength Holography

Abstract: Images reconstructed from long wavelength holograms have aberrations as a result of large construction to reconstruction wavelength ratios. For many practical recording geometries, these aberrations of which spherical aberration is often the largest may seriously degrade image resolution.