Showing papers on "Contrast transfer function published in 1999"

Relativistic calculations of intensity distributions in elemental maps using contrast transfer functions.

Abstract: The interpretation of highly resolved elemental maps is not straightforward: one has to consider the quantum mechanical nature of the scattering process as well as the influence of the microscope. Existing calculations of the contrast in elemental maps are based on a non-relativistic approach, while in most of the currently installed electron microscopes, the electrons penetrate the specimen with relativistic energies >/= 200 keV. Therefore, we have recalculated the intensity distribution in elemental maps based on a fully relativistic theory. Using the concept of contrast transfer functions, the simulations account for lens aberrations as well as the defocus. Surprisingly, the results exhibit considerable deviations between the relativistic and non-relativistic calculations even in the region of low acceleration voltages such as 100 kV. These differences increase with increasing acceleration voltage and are strongly dependent on aperture and energy loss. Quantitative simulations and evaluations of highly resolved elemental maps should therefore make use of a fully relativistic theory.

A simple channelling model for hrem contrast transfer under dynamical conditions

Abstract: The application of electron channelling theory to dynamical exit wave calculations is briefly reviewed, and a comparison of channelling results with full dynamical calculations is presented. The channelling expression to the exit wave is combined with conventional imaging theory, and it is shown that a simple expression can be obtained for a dynamical contrast transfer function (D-CTF), which incorporates imaging aberrations and thickness-dependent dynamical scattering effects. The D-CTF can provide detailed insight into HREM images of a mixed cation oxide at thicknesses up to 200 A, whereby an approximate correction for non-linear effects is utilized in the larger thickness regime.

X-ray phase-contrast imaging study of soft tissue and bone samples

Abstract: Conventional radiography is based on absorption contrast and geometrical (ray) optics. After an outline of the relevant theory, this article reports results displaying both phase- and absorption-contrast, collected with a technique which utilizes a micro-focus x-ray source to achieve a high degree of spatial coherence, and relatively large object-to-image distances to enable (wave) interference effects (Fresnel diffraction) to occur and manifest themselves as phase contrast in the image plane. Both soft tissue (chicken knee) and hard tissue (finger bone) samples are investigated for a range of source sizes and object-to-image distances, encompassing conditions somewhat analogous to conventional radiography. Variation in image contrast and resolution as a function of these variables is observed and discussed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fusion neutron transient effects to charge coupled device camera images

Abstract: : A charge coupled device (CCD) camera's images were degraded by neutron-induced blemishes, called stars, while being irradiated with 14 MeV neutrons (n) from the Rotating Target Neutron Source. This thesis analyzed simulated images for a CCD camera operating during neutron irradiation. The simulated images were created to provide data to help determine how much shielding a CCD camera would require while being used a diagnostic tool during inertial fusion events at the National Ignition Facility. The simulated images were created from data obtained at the Rotating Target Neutron Source. The Contrast Transfer Function (CTF), autocorrelation function and visual comparisons were used as measures of the transient effects of the neutron irradiation on the simulated images. The CTF and visual image quality started to degrade significantly at neutron fluences around 10(exp 8) n/sq cm. The autocorrelation function determined that the average size of neutron-induced star was 4 x 4 pixels. Increasing neutron fluence produced image changes that could be explained by a variation in sensitivity across the camera face or (more likely) by increased Charge Transfer Inefficiency (CTI) with increasing charge loading of the CCD.

A simple channelling model for HREM contrast transfer under dynamical conditions

Abstract: Summary The application of electron channelling theory to dynamical exit wave calculations is briefly reviewed, and a comparison of channelling results with full dynamical calculations is presented. The channelling expression to the exit wave is combined with conventional imaging theory, and it is shown that a simple expression can be obtained for a dynamical contrast transfer function (D-CTF), which incorporates imaging aberrations and thickness-dependent dynamical scattering effects. The D-CTF can provide detailed insight into HREM images of a mixed cation oxide at thicknesses up to 200 A ˚ , whereby an approximate correction for non-linear effects is utilized in the larger thickness regime.