Showing papers on "Contrast transfer function published in 1991"

Contrast analysis of cryo-images of n-paraffin recorded at 400 kV out to 2.1 A resolution.

Abstract: n-Paraffin was used as a test specimen for evaluating the relative merits of 400-kV versus 100-kV electron microscopy in recording data for electron crystallographic analysis of beam-sensitive materials. The parameter used for comparison, the relative contrast R, is the ratio of amplitudes from the computed Fourier transform of images and amplitudes from an electron diffraction pattern from the same crystal. R will thus be a measure of the contrast from an experimental image relative to that of a perfect image. Electron diffraction patterns and bright-field images were recorded at 400 kV at a specimen temperature of -167 degrees C. Using the flood-beam imaging technique the best R-value is 0.08 for all reflections in the resolution zone from 4 to 3 A. This value is equivalent to that found at 100 kV. In the resolution zone from 3 to 2A we have found R = 0.02. Using the spot-scan imaging technique, on the other hand, R was measured to be 0.42 for the reflections between 4- and 3-A resolution. This amount of relative contrast is 1.7 times that observed at 100 kV. Reflections at 3-2 A displayed an R-value of 0.05. Besides obtaining higher R-values when applying the spot-scan imaging technique at 400 kV, we observe a higher yield of images with isotropic diffraction and/or higher resolution reflections. Various contrast-attenuating factors, including the modulation transfer function of the photographic film and the cryo-holder, envelope functions for spatial and temporal coherence and lens and high-tension instabilities, the contrast transfer function and lastly the radiation damage effects, have been considered in interpreting the observed image contrast. Overall, use of 400 kV in combination with spot-scan does offer important improvements in contrast levels, which can be very useful in determining the three-dimensional structure from protein crystals.

Design and analysis of aspherical multilayer imaging x-ray microscope

Abstract: Considerable effort has been devoted recently to the design, analysis, fabrication, and testing of spherical Schwarzschild microscopes for soft x-ray applications in microscopy and projection lithography. The spherical Schwarzschild microscope consists of two concentric spherical mirrors configured such that the third-order spherical aberration and coma are zero. Because multilayers are used on the mirror substrates for soft x-ray applications, it is desirable to have a small number of reflecting surfaces in the microscope. In order to reduce the microscope aberrations and increase the field of view, generalized mirror surface profiles have been considered in this study for a two-mirror microscope. Based on incoherent, sine wave modulation transfer function (MTF) calculations, the object plane resolution of a 20x microscope has been analyzed as a function of the object height and numerical aperture (NA) of the primary for several spherical Schwarzschild, conic, and aspherical reflecting two-mirror microscope configurations. The ultimate resolution of an aspherical, two-mirror microscope appears to be about 200 a when using 100-a radiation. Better resolution can be achieved when shorter wavelength radiation is used.

Design and analysis of aspherical multilayer imaging x-ray microscope

Abstract: Considerable effort has been devoted recently to the design, analysis, fabrication, and testing of spherical Schwarzschild microscopes for soft x-ray applications in microscopy and projection lithography. The spherical Schwarzschild microscope consist of two concentric spherical mirrors configured such that third order spherical aberration and coma are zero. Since multilayers are used on the mirror substrates for x-ray applications, it is desirable to have only two reflecting surfaces in a microscope. In order to reduce microscope aberrations and increase the field of view, generalized mirror surface profiles have been considered in this study. Based on incoherent, sine wave modulation transfer function (MTF) calculations, the object plane resolution of a microscope has been analyzed as a function of the object height and numerical aperture (NA) of the primary for several spherical Schwarzschild, conic, and aspherical reflecting two-mirror microscope configurations. The ultimate resolution of an aspherical two-mirror microscope appears to be about 200 A when using 100 A radiation. Better resolution can be produced when shorter wavelength radiation is used.

An investigation into the performance of microscope objectives with near infrared light

Abstract: The performance of microscope objectives used with near infrared light has been examined by scanning a grating across the focused image of a point source. Objectives have been assessed under different conditions by varying wavelength, tube-length and coverslip thickness corrections in order to find the conditions for maximum correction of spherical aberration. Computations of light intensity around the focus have been convolved with an amplitude grating so that the amount of spherical aberration could be estimated. Results showed that significant reductions in the amount of spherical aberration could be achieved by altering the tube length.

Polarization aberration effects propagated in optical systems

Abstract: The propagation and imaging of polarized light through optical systems described by a polarization aberration expansion is treated by combining a scalar operator calculus with the Jones calculus. The martrix-operator framework provides a means for handling diffraction and propagation in optical systems containing polarization aberrations. An expansion for the polarization aberration function of an optical system, similar to the expansion of the wavefront aberration function into defocus, tilt, piston, and higher-order terms, is analysed. These polarization aberration terms introduce phase changes in the diffraction image proportional to the first and second derivatives of the non-polarization aberrated image structure.

Contrast transfer function of 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.

The Achievement and Limitation of HV-HREM

Abstract: Since the adoption of 500KV electrons was found to be effective for the high resolution of atomic level at Kyoto University, a 1000kV electron microscope was constructed to obtain a resolution better than 0.12nm especially for organic specimens by adopting a highly sensitive imaging recording system which is also used for the digital image processing.